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Western Historic Radio Museum
 

NATIONAL COMPANY, INC.


"MOVING COIL" COMMUNICATION RECEIVERS


Includes NC-100, NC-101X, NC-80X, NC-81X, NC-100A family, NC-200,
NC-240CS, NC-240D
Dept of Commerce & CAA Airway Communication Receivers
Military Versions - RAO family, RBH, NC-100ASD, R-115, R-116

History of Design and Production, Moving Coil Catacomb Details,
WWII Versions, Post-WWII Production, Serial Numbers, Restoration Write-ups

by: Henry Rogers WA7YBS-WHRM



The 1936 NC-100X artwork from the NC-100 instruction manual

Besides National's masterpiece, the HRO, another series of receivers produced by National Company, Inc. were very popular and accounted for a lengthy production history. The NC-100 Series "MOVING COIL" Receiver production spanned from 1936 up to 1949. Another masterpiece? Certainly the design of the "MOVING COIL" method for band change used in the NC-100 receivers impressed the commercial operators of the late thirties with modified versions being ordered by the Bureau of Air Commerce and the CAA for use in airports around the country (even as late as post-WWII.) The NC-100XA version impressed the U. S. Navy who ordered special versions with low Local Oscillator radiation that became the famous RAO family of WWII receivers. Even post-WWII buyers could still purchase the descendants of the NC-100 in the modernized NC-240D receivers. This web-article details most of the various members of the NC-100 family and it became an extensive family (over sixty different variations) that grew as National up-graded and improved the receiver over its 13 years of production history.   - Henry Rogers - March 19, 2012
 

NATIONAL COMPANY, INC.

"MOVING COIL" COMMUNICATION RECEIVERS
 

 

NC-100, NC-100X



photo above
: The "MOVING COIL" NC-100 Receiver from 1936 (sn 334-D) showing the original "art deco" design of the aluminum overlay front panel. Note the relocated pilot lamp and the hole plugs covering the unused crystal filter control holes when compared to the NC-100X (header photo.)


History of the Design and Production
- Many radio engineers of the thirties believed that the absolute, best sensitivity and stability of a communication receiver's RF amplifier, First Detector and Local Oscillator could only be achieved by using "plug-in" coils. This type of approach eliminated problems of lead length, shielding and stability along with isolation of unused tuned circuits - problems that were commonly found in broadcast receivers using conventional rotary bandswitches. Plug-in coils were a hassle, no doubt. Handling three individual coils for each band change, storage of the unused coils and how to remove B+ when changing coils all added to the counter-belief that plug-in coils were archaic. James Millen, National Co.'s General Manager and Chief Engineer, was one of the designers that insisted the best receiver performance was achieved using plug-in coils. But, how to effectively eliminate the valid arguments against plug-in coil use in a new product?   

Certainly, National was having fabulous success with the HRO receiver, which had been in production since early 1935. It was well-known that the HRO's legendary performance was in-part due to its plug-in coil sets. The HRO wasn't hassle-free though and Millen realized that for the SWL (Short Wave Listener) and intermediate-level hams, in other words, those who didn't have the experience or couldn't afford the $200+ HRO receiver, there had to be a design that would provide the excellent performance of plug-in coils without all of the hassles and expense.


photo above: The chassis of the NC-100

The NC-100 Features - National's mechanical engineers offered a solution that solved most of the negatives of plug-in coils and retained most of the advantages. With the use of a movable cast aluminum coil box called a "catacomb," all of the coils would be mounted in individual shielded compartments with short contact pins mounted in molded insulators on top of the catacomb. A large band selector knob on the front panel of the receiver would turn a rack and pinion gear mechanism that would move the coil catacomb into place, thus engaging the proper coil set pins into short, fixed position, spring-contacts mounted under the tuning condenser in insulator blocks.

The mechanical action simulated plugging in a three coil set for each band with the ease of turning a knob while keeping all of the unused coils isolated and shielded. "Switching noise" was eliminated by routing the RF and IF amplifier screen voltage through the foremost pin split-contacts of the LO coil section. The split-contacts were not soldered together but rather had the screen voltage wires connected to each of the two halves of the contact and when the coil pin, which wasn't connected to anything inside the coil catacomb, engaged in the two halves the circuit is completed and the screen voltage was then routed to the RF and IF amplifiers.   >>>


photo above: Under the chassis showing the Coil Catacomb

>>>  To keep the costs down by keeping the physical size of the catacomb relatively small only three sets of coils were used per tuning range. A single RF amplifier provided pre-selection and good reduction of images up to about 15mc. A separate Local Oscillator reduced the noise associated with the typical "inexpensive" Converter stage and a separated Mixer stage accounted for the three tuned circuits that were necessary in each coil set. Due to the nature of physically moving a metal box underneath the receiver's chassis the catacomb width dimension was limited by the chassis width and five tuning ranges were what could be fit into a metal box half the width of the chassis.

 A finely finished round metal rod that ran the full length of the chassis was mounted at the rear of the chassis to act as a rear bearing for the catacomb. The front of the coil catacomb had a "track" machined into it and the band change shaft, which also carried a pinion gear, protruded beyond the pinion gear and rode in this "track," thus supporting the front of the catacomb. The rack gear was mounted to the front of the catacomb at the proper level to engage the pinion gear to allow moving the catacomb via the band changing shaft. The "detent" action was actually caused by the 15 coil pins engaging the 15 contacts which provided the positive feel of the catacomb "locking" into position. Five marked holes in the front panel allowed viewing a white "flag" that was mounted to the catacomb and indicated which tuning range was selected. When shipped, the coil catacomb was screwed to one side where a guide pin was located to prevent any damage due to rough handling. When the receiver was installed, this screw had to be removed to "unlock" the coil catacomb.   

Another portion of the design involved the PW gear drive used on the NC-100 series. The famous HRO PW gear drive used a "precision worm" (PW) gear to drive a large split-gear with two ganged variable condensers flanking each side of the gearbox. The NC-100 series was only going to require a three-gang tuning condenser and this was going to be centrally located on the chassis running front to rear with the power supply of the receiver on the left and the receiver circuitry to the right. This required the gear box to drive the condenser from the rear of the box. As with the HRO, an elliptic hub was required to make the PW-D work but inside the NC-100 gear box were several changes. Gone was the large split-gear, replaced with dual driven gears, one of which was spring-loaded for anti-backlash, driving via reduction gearing a large condenser drive gear. This approach eliminated the spring loaded drive of the HRO gear box and replaced it with a much easier to operate gear box.


photo above: NC-100 Commercial  Rack Mount has .54-30mc coverage with no circuit modifications - photo: eBay

Since the NC-100 was intended for the SWL or intermediate-level ham, a different type of signal strength or tuning indicator was utilized. RCA had just released their "Magic Eye" in 1936 (everything RCA offered in 1936 had "magic" tied to it, "Magic Brain was their radio front-end, "Magic Voice" was a special sound chamber for their console radios, etc.) The "Magic Eye" was a cathode ray tuning indicator tube that glowed a mysterious green color and, as signals were tuned, the "eye" would open and close a "pie-shaped" shadow. Since the "Magic Eye" required a license from RCA to use in a design, not too many manufacturers incorporated it into their designs in 1936 (Zenith waited until 1938 and Philco never did use the tuning eye tube in any of their designs.) Since a cathode ray tuning indicator was part of the NC-100 design it was going to require strong and consistent AVC voltage. This required the incorporation of an Amplified AVC stage in the receiver. Since nearly all hams were running on CW in the thirties and their receivers were always operated with the BFO on and the AVC off, the use of an amplified AVC stage implies that National was designing the NC-100 more for voice reception of AM signals from Shortwave Broadcasters. IF is National's standard 456kc. 12 tubes were used in the NC-100 circuit.

Finally, there was the Push-Pull Audio output stage using a pair of 6F6 tubes driving a 10" Rola electro-dynamic speaker. Even an optional 12" electro-dynamic speaker was available. Again this great audio section implies the reception of voice and music, not the "dots and dashes" of International Morse. Since the intended market for the NC-100 was not necessarily hams, the receiver was introduced with a fabulous "art deco" front panel that featured geometric-linear black panels and black octagon control nomenclature layout combined with the natural aluminum finish along with a red highlighted central panel for band in use indication. The PW-D combined a gray index dial with a red number dial along with a green pilot lamp and the green "eye tube" (when in operation) finished off the striking "art deco" design of the receiver. The NC-100 and the NC-100X were introduced in August 1936 and were available from dealers in September 1936, with the initial pricing at $105 and $127 respectively.

Like several of the National receivers, the NC-100 had the potential for commercial use. When supplied as a Commercial Receiver the NC-100 would be a rack mounted receiver. This could be accomplished by added brackets to the sides of a standard table model, by building special racks that adapted to the standard table model (as in the Highway Patrol photo below) or by supplying the receiver with a special rack mount front panel that usually didn't have the "art deco" aluminum overlay but was rather painted black wrinkle. Commercial versions usually don't have extensive circuit modifications that the later Airway Communication Receivers employed. The Airway Receivers are detailed in a section below, "Airway Communication Receiver Versions."
 

photo above: The Ohio State Highway Patrol made use of the NC-100X receivers in their communications. This photo is from around 1940 since the NC-100XA (center) has the Noise Limiter circuit only found on the later versions. Note the National 10" loud speaker on the floor.               photo from  www.msprg.com

 

NC-101X

National was aware that the NC-100 didn't really meet the needs of the average intermediate-level ham who operated mostly CW so they simultaneously released a version of the receiver with a crystal filter, the NC-100X (shown in the header photo.) The NC-100X incorporated a Crystal Filter circuit that was identical to that used in the HRO. However, it was a "stop-gap" until National could release the "ham bands only" version of the NC-100X receiver, the NC-101X. NC-101X production starts with the second production run of the NC-100 Series (run- E, probably October-November 1936.) The NC-101X version seemed to be more of a ham's receiver, appealing to both CW operators and the few AM phone operators that were around in the late thirties. The art deco tri-color panel of the NC-100 was replaced with a black wrinkle finish panel with engraved nomenclature. The tuning-eye tune was retained with a fancy "eyelid" bezel that was also finished in black wrinkle. The B&W artwork to the left shows the NC-101X as advertised in QST.

The NC-101X tuned 160M, 80M, 40M, 20M, 10M amateur bands using 400 of the 500 divisions of the PW-D. This was exactly as the HRO band spread with the exception of the 160M band which the HRO didn't band spread at all. This band spread action gives the operator the impression that the particular ham band being tuned will just go "on and on." The 400 divisions equates to a linear dial over 9.5 feet long (see PW-D section below for details on the micrometer dial mechanics.)  >>>

photo left:  B&W artwork for the NC-101X as it appeared in QST ad April 1937

>>> The Crystal Filter was included on all versions working with the National standard IF of 456kc. The Crystal Filter is the same circuit as was used on the HRO receiver. The Phasing control has a cam-operated switch incorporated into it to remove the Crystal Filter from the IF however the Selectivity control remains in the circuit and must be "peaked" even when the filter is switched off. Push-pull audio (6F6s) was used to drive the 10" Rola electrodynamic speaker that was usually included (there was a 12" optional speaker also available.) Note in the B&W artwork (above) from the National Co. advertisement from the April 1937 issue of QST showing the NC-101X with the light grey Index dial, similar to the NC-100X. The earliest versions of the receiver may have had this PW dial but most NC-101X receivers will be found with the black (actually olive-black,) HRO-type, PW-D installed.

To solidify the image that the NC-101X was a ham receiver, the "magic eye" was soon replaced with a true S-meter (sometime around run-J, mid-1937.) The earliest S-meters used a white scale with 0-9 S-units printed in black. These meters were illuminated since the green pilot lamp had been eliminated to allow for the installation of the S-meter switch. By mid-1938, a light yellow meter face with 0-9 S-units in black and "db over" in red scales was being installed. The change appears to have happened around run N or P (see SN log.) Both meter types were built by Marion Electric. The later yellow meter scales darken considerably with exposure to UV light and the red numbers will tend to fade to the point of invisibility.

With the introduction of the NC-100A direct-read dial in June 1938, National began to also produce the NC-101XA - a ham bands only version with the direct-read dial. Interestingly, the ham purchaser could select either the NC-101X with its PW-D type dial or the NC-101XA with the "A" type direct-read dial for the same price of $129. In November 1939, a Noise Limiter circuit was added to all of the NC-100 series receivers. The Noise Limiter circuit required changing the 6C5 detector tube to a 6C8 tube duplex-triode to allow both the second detector and noise limiter functions. These late-version NC-101X receivers will have the Noise Limiter control installed between the RF Gain control and the Band Change knob. Since just tube types were changed, the total number of tube used in the NC-101X (with S-meter) was eleven.   >>>

Oct-Nov 1936  to  May-June 1937


photo above: The early version of the NC-101X with the eye-tube showing how the NC-101X appeared from its introduction in October 1936 up to around June 1937. The receiver is SN 181-G from early 1937.

 

May-Jun 1937  to  Apr-Jun 1938


photo above: This is how the NC-101X appeared from mid-1937 up to about mid-1938. This is SN 141-N, dating from early-1938. Note that the S-meter scale is white. Since the S-meter is illuminated it serves as the "power on" indicator. The green pilot lamp was removed to install the S-meter "on-off" switch. Front panel engraved nomenclature remained unchanged.

May-Jun 1938  to  Oct-Nov 1939


photo above: NC-101X as it appeared from mid-1938 to late-1939. Note that these receivers have no significant changes other than the yellow scale S-meter installed. This photo was not "flashed" and therefore shows the typical "black" appearance that the PW-D dial has in normal light.    Photo from eBay.

 

Nov-Dec 1939  to  Mar-Apr 1940


photo left: This is the very late production NC-101X with the Noise Limiter circuit. These receivers use different tubes from the proceeding versions with the 6C5 detector tube being replaced with a 6C8 duplex triode (det/nl) and the 6J7 AVC tube being replaced with a 6F8 duplex triode (that also added an audio preamplifier stage.) Note that the Noise Limiter control is between the RF Gain control and the Band Change knob. The pointer knob is original for the Noise Limiter control since the National bar knob is too large for this area of the panel. The "NC"diamond-logo was relocated to the upper right side of the panel. This NC-101X is SN 40-X dating from early-1940. The NC-101X with NL was produced from Nov 1939 up to about April 1940.

>>>  In October 1940, National introduced the NC-200 and this model offered the user both general coverage and bandspread coils in one receiver thus effectively eliminating the need for either the NC-101X or the NC-101XA receivers. Demand for the NC-101X had been dropping significantly since the introduction of the "A" version. In fact, the last NC-101X receivers are built on the NC-100XA chassis with all of the holes pre-punched for the "A" direct-read dial present but not needed or used. National ads for the NC-101X last appear in QST in April 1940 and the last discount dealer ads in QST are in October, 1940. It's very likely that the last few NC-101X receivers were built long before the summer months of 1940 and the discount dealer ads were actually for selling receivers they had in stock (since they probably weren't selling that fast anyway.) However, the production of the NC-101XA direct-read dial receiver continued on for a short time, probably until the NC-200 was in full production.

Need the calibration curves for the NC-101X?    NC-101X CALIBRATION CURVES    
 
 

NC-80X, NC-81X


In October 1937, National introduced a scaled-down, cost-reduced version, "moving-coil" receiver, the NC-80X. To keep costs low, the receiver used an AC-DC power configuration that eliminated a costly power transformer. The receiver didn't employ an RF amplifier stage so the catacomb only had two sets of coils per band. To compensate for the lack of RF preselection, the NC-80X used three IF amplifiers and moved the IF up to 1560kc which National claimed put the image frequencies "so far apart they can be effectively rejected without a preselector." An improved Crystal Filter was provided and a single 25L6 was used as the audio output amplifier. An external speaker (with output transformer) was required. The slide-rule dial featured a faux-mirrored scale to eliminate parallax and early versions use a two-speed tuning control. Like some other radio receivers of the time period, the tuning knob would tune at a 16:1 ratio when tuning to a station and then when the direction of rotation was reversed the tuning rate would be 80:1 for approximately half a revolution of the knob in either direction allowing "fine tuning" of the desired station. Later models used a large pulley that had a rim that was pinch-wheel driven for the tuning condenser while the pulley was used for the dial indicator string movement.  >>>

>>>  A band indicator was provided on the right side of the dial that pointed to the band in use. Four tuning ranges were used for coverage of .55mc to 30mc and the receiver used 10 tubes. There was a gap in the tuning coverage between 1.5mc and 1.7mc to allow for the IF of 1560kc. The gap is placed between the top of the first band and the bottom of the second band.

National also introduced a "ham bands only" version of the receiver which was designated as the NC-81X. This receiver increased the number of tuning ranges to five so that 160M, 80M, 40M, 20M and 10M could be covered. This also required a different catacomb than the NC-80X used (but still used only two coils per band.)

A "B" version was available for either the NC-80X or the '81X that was battery operated and eliminated the rectifier tube from the circuit. There was also a power transformer available to convert the operation to AC (essentially a chassis-mounted isolation transformer.) Mounting holes and "metal-grommeted" holes were provided on the chassis for mounting the optional power transformer.

The advertising artwork shown bottom-right possibly represents the earliest versions of the NC-80X. Later versions have vertical nomenclature panels at each side of the front panel as seen in the photo left. The initial selling price for either the NC-80X or the NC-81X was $88 but this was soon increased to $99. The receivers were available from October 1937 up into late-1939.

NC-80X production evolved rapidly and most of the receivers sold were somewhat different than the advertising artwork. The dual-speed tuning was the first to be eliminated and replaced with single speed tuning system. The bezel was dark brown plastic and the dial cover is also transparent plastic. The bezel had a cutout slot that ran along the bottom and contained six "clips." These clips could be slid along the slot and the "points" lined up on the dial for "marking" the location of favorite stations. The tuning knob was weighted to give the tuning a flywheel effect. Since the NC-80X was an AC-DC circuit, it was necessary to have the chassis isolated from the cabinet. This was accomplished by mounting the cabinet to the chassis using several natural rubber grommets. The corners were further isolated using heavy cardboard. Since it was possible to have a voltage potential difference between the chassis and the cabinet, the top lid doesn't lift up - it's held down with screws.

photo above: Chassis of the NC-80X. Note the mounting holes and metal grommets for the optional isolation transformer. Note large pulley is a combination rim drive for the tuning condenser and pulley for dial indicator string.

The "moving coil" catacomb contained two coils per band. These coil assemblies were identified as W, X, Y and Z for the frequency coverage and 1 (mixer) or 2 (LO) for the function. The cover for the bottom of the catacomb was sheet aluminum held in place with five screws. The catacomb used an economical "chain drive" instead of the expensive "rack and pinion" drive of the NC-100 series. Remote Standby (BSW terminals) was provided. An adjustable wave trap wasn't shown on the schematic but was installed in the receivers.

The NC-80X shouldn't be considered an "entry level" receiver. With ten tubes, three IF amps and a Crystal Filter, it was actually a moderately advanced design that used every opportunity to reduce production costs to a minimum. The end result was a circuit that performs very well but the mechanics seem "flimsy" and "cheap." During the Depression, it wasn't unusual for hams to convert AM BC radios to use for their ham receivers. Two of the large receiver companies, National and Hallicrafters offered less-expensive receivers that were for beginning hams or shortwave enthusiasts. National had the SW-3 and Hallicrafters was offering a couple of entry level receivers with the Sky Buddy being particularly popular. The Sky Buddy lacked an RF amplifier, it had only one IF and no Crystal Filter (selling price was around $30.) Certainly, the performance of the NC-80X was a major step-up from the typical entry-level receiver but at a selling price of $99 it was certainly a much more expensive "step."


photo above
: The coil assemblies for the NC-80X with lowest frequency coils to the right. See the write-up on the NC-80X further down this page for a photo of the chain drive system that is just visible in this photo.


photo above: NC-80X artwork  from QST ad October 1937 issue

 

NC-100A, NC-100XA, NC-101XA and Late-NC-100 Versions


photo above: NC-100XA SN: 58-R, ca. late-1938. Dial pointer indicates Band B is selected.

The use of an indirect-readout device such as the PW-D probably accounted for more than a few complaints to National from casual users who had to constantly refer to the manual for a graph that provided frequency versus dial readout correlations. The competition's direct-readout dials also could have factor that resulted in National revamping the entire NC-100 line to replace the PW-D micrometer dial with a direct readout, illuminated tuning dial. The suffix "A" was added to all NC-100 receivers that had the new dial installed. The introductory ad appeared in the June 1938 issue of QST. For sometime, the NC-101X was available with either the micrometer dial or with the "A" version dial as the NC-101XA. After all, the NC-101X was a ham receiver and many hams enjoyed the correlation exercises involved with using the PW-D.

One of the interesting features of the new "A" version dial was its articulated pointer. When the band was changed, the dial pointer would automatically increase or reduce its apparent length so that its red tip would line up with the selected band's tuning scale, thus indicating the "band in use." This required the pointer "lifter" mechanism to track the tuning while maintaining the proper length of the pointer. A dial cord that was anchored to the band changing shaft and then routed via a pulley system to pivot against the tuning condenser drive shaft so that the action of tuning the receiver didn't affect the apparent length of the pointer while it tracked around the dial. The pointer-lifter mechanism and the entire articulation system seemed overly complex for the simple task of a band select indicator. Small wonder that during WWII this feature was eliminated and replaced with an indicator dial mounted to the band change shaft. A logging scale was included to allow for precision frequency resetability that was comparable to the PW-D dial's accuracy. Initially, the dial cover was a pane of glass. Most later military versions had the glass replaced with plexiglass. >>>

>>>  In addition to the new tuning dial, the cabinet itself was increased in height from the nine inch height of the NC-100 to ten and a half inches of the NC-100A. The eye-tube was replaced with a "behind the panel" S-meter that was installed on all versions now. A push-pull switch allowed disabling the S-meter when CW was being received, just like the HRO receiver. With the S-meter was installed, the total tube count of the NC-100A circuit was reduced to eleven tubes since the cathode ray tuning eye tube was no longer used. 

A Crystal Filter was added to the NC-100A with the designation changed to NC-100XA. The Crystal Filter is the same circuit that was used on the NC-100X and the NC-101X receivers (and the HRO receiver) in that a cam-operated switch on the Phasing control places the Crystal Filter into the circuit. Selectivity is not a stepped switch but rather is a variable condenser that is infinitely adjustable while the Phasing control allows some adjustability of the "notch" within the passband. When the Crystal Filter is switched out, the Selectivity control must be "peaked" for maximum signal response since the variable condenser is still in the IF circuit.

The Tone control on early NC-100A versions used a large inductor in the circuit. Push-Pull 6F6 tubes were used in the audio output with the output transformer located on the external speaker. The speaker included was a 10" diameter Rola and the speaker-cabinet assembly was designated as MCS-10.

With the NC-100XA, some of the control locations were moved with the addition of the Crystal Filter (still at 456kc.) The S-meter switch was moved from adjacent to the meter to the lower part of the panel. Different nomenclature was required so the small panels were changed to reflect the additional controls needed on the "XA" version. The selling price for the NC-100XA was $147.00 from Allied Radio in 1939. Earliest reported "A" version is a NC-101XA with SN 130-M which would date from around May or June 1938. 

photo above: NC-100XA artwork from introductory ad in June 1938 QST

photo above: NC-100XA  SN: 58-R  Top of chassis. Note screw terminal Ant-Gnd used for a short time in 1938.

Throughout the NC-100A production, the circuit went through several minor changes. Later in production, an adjustable Noise Limiter circuit was added (Nov. 1939.) The NL circuit changed the 6C5 second detector to a 6C8 duplex triode to provide the second detector function and the Noise Limiter function (keeping the tube total at eleven tubes.) The 6J7 AVC Amp was replaced with a 6F8 duplex triode to provide AVC amplification and also to add an audio preamp (1st AF Amp.) Also, with the NL addition, the location of some of the controls was rearranged. The Audio Gain control was changed to a standard grid input on the new 1st AF amp and the Tone control was changed to an RC type control which eliminated the audio choke that was used in the earlier versions. Some of the NC-100A and NC-100XA versions featured a "weighted" tuning knob that added a fly-wheel affect to tuning the receiver. The weighted knob wasn't used on the military versions.

Production of the NC-100A and NC-100XA probably lasted until the NC-200 was fully in production (Oct. 1940.) However, National was supplying the USN with the RAO and RBH receivers and would soon be supplying the US Army Signal Corps with the NC-100ASD receivers so the NC-100A series production essentially didn't stop and continued on through WWII with these military versions of the NC-100A and NC-100XA receivers. The NC-100A was advertised in the 1945 Radio Amateur's Handbook in the National advertising section in the back of the book. All items shown have a disclaimer that "priorities" were required until released by the War Production Board. Most likely National was just showing what might be available after WWII ended. However, while National offered the HRO-5 and the NC-240D versions after WWII, no NC-100A versions were produced for the post-war civilian market.

 
Later NC-100 Receivers - Although not advertised extensively, the NC-100 did go thru evolutionary changes that followed the NC-101X and, to a certain extent, the NC-100A circuit changes. The first item to go was the art deco aluminum overlay found on the early NC-100 receivers. This overlay had set the new NC-100 apart from other "black wrinkle boxes" and certainly made the receiver "stand out." But, economics probably dictated its demise. At the same time, the eye tube was eliminated and replaced with an S-meter. Both changes were probably included as part of the NC-101X upgrade to the S-meter and should date from around mid-1937. From mid-1937-on, the NC-100 had a black wrinkle-finish front panel, a black PW-D and an S-meter. Most likely, the S-meter scale changes follow what happened with the NC-101X. Very late NC-100 receivers will have a Noise Limiter control along with the corresponding changes in tube line-up necessitated by that upgrade. It's likely that the NC-100X also followed this evolutionary progression. Since the crystal filter wasn't used on the NC-100, most (but not all) versions have black wrinkle-finish "hole plugs" installed into the crystal filter control holes (these hole plugs are also found on the early NC-100 - see 334-D photo in NC-100 section above.) As mentioned above, National built both "A" direct-read dial and "non-A" PW-D versions into 1940, either usually selling for the same price.

photo right: A late-version NC-100 receiver with S-meter, Noise Limiter and black PW-D  -  Photo from eBay

 

NC-200



photo above
: NC-200 from QST ad in November 1940 issue

In 1940, many of the communications receiver manufacturers decided that their models needed some modernization. In most cases this was more cosmetic than anything to do with the circuitry, which usually was in a constant "up-dating" mode anyway. Gray seemed to be the "hot" color for gear and many manufacturers were offering their products in either gray or black. National decided to go for a color change in a two-tone gray scheme with the dark gray in wrinkle finish and the lighter gray in smooth finish. Two wrap-around chrome bars finished the trim. Additionally, some substantial circuit changes were incorporated into their new model, the NC-200 (introduced in October 1940.)

Along with the cosmetic changes, the NC-200 presented another change in the catacomb design (the first was the NC-80.) By reducing the size of the chambers for the coils and trimmers, another band could be squeezed in. The NC-200 featured ten tuning ranges - six general coverage and four amateur band spread ranges. Essentially, the NC-200 offered the combination coverage of the NC-100XA and the NC-101XA in one receiver. With the NC-200, the operator had general coverage from the 490kc up to 30MC and separate band spread coverage of the 80, 40 20 and 10 meter ham bands. The 160M band was "band spread" covered on the NC-101X (or XA) but it was tuned as "general coverage" on the NC-200. National also combined the function of tuning and band changing into one control. By pulling outward with the tuning knob the tuning dial is disengaged and the pinion gear is the engaged into the rack of the coil catacomb allowing the band to be changed. Pulling and turning the tuning knob one revolution selects any of the general coverage coils. Rotating the knob about a quarter of a turn within the one revolution selects the band spread set of coils between the four upper general coverage bands. The NC-200 replaced the articulated dial pointer and went with a separate dual flag-type indicator that pointed to the scale-in-use thru slots on the tuning dial. >>>

>>>  Some tubes were changed with the most obvious being the push-pull 6F6 audio output tubes being replaced with 6V6 tubes in push-pull. A closer look shows that the interstage transformer coupling used in the NC-100 Series was replaced with a phase inverter circuit that repurposed part of the 6F8G tube. Single-ended tubes like the 6SK7 were used as the RF amplifier and one of the IF amplifiers. The 80 rectifier was replaced with an octal 5Y3 tube. Total tube count was 12 tubes in the NC-200. The receiver used a Marion Electric S-meter that had a square front. The S-meter toggle switch from the NC-100XA series was eliminated and the new S-meter switch combined with the RF Gain control. Only with the RF Gain fully advanced did the switch actuate and allow the S-meter to function. AVC had to be on for the S-meter to indicate signal strength. The IF frequency was changed to 455kc from National's "standard" 456kc and the Crystal Filter was upgraded to a stepped-switch Selectivity control with variable Phasing control. Six steps of increasing narrow bandwidth could be selected with the new Crystal Filter.

The tuning dial had a large plastic cover that was more durable than it looked. There are some reports of NC-200 receivers that have an original glass pane dial cover rather than the plastic type commonly seen. Note in the artwork shown above that the side dial cover metal mounting clamps aren't present implying the dial cover is mounted by some other method and could be a glass cover (like the proceeding NC-100A receivers were.) However, most NC-200 receivers were fitted with the plastic dial cover. No logging scale was provided on the NC-200 probably because the dial accuracy was specified at 1%. The tuning drive system was a departure from the NC-100 gearbox with the NC-200 using a large diameter fiber disc that was rim-driven by a pinch-wheel on the tuning knob shaft. Though there appears to be a gearbox in front of the tuning condenser it doesn't have any gears inside and is only used as a bearing for the main tuning shaft from the rim-driven disc. This tuning drive system resulted in a very smooth feel to the tuning action and a good reduction in the tuning knob rotation to dial-tuning movement. The tuning knob itself was weighted via the large metal skirt which imparts a "flywheel effect" to the tuning action.   >>>

photo above: NC-200 SN: C-536  Top of chassis

>>>  The Noise Limiter was the same type used on the late-NC-100A receivers. Later versions of the NC-200 will have a "Pick Up" jack on the lower right front panel for a phono input directly to the audio section of the receiver. Battery operation was possible using the seven pin auxiliary socket on the rear of the chassis. The matching speaker housing was updated from the old square black wrinkle finish box that had the 10" Rola electrodynamic speaker inside to a more rectangular box with two tone gray paint and chrome bars. The speaker was now a 10" Jensen PM speaker with the output transformer mounted on the speaker frame.  

It appears that the NC-200 receivers were built within three production runs, C, D and E. Run C was probably started in September 1940 and appears to have produced around 1000 receivers. Serial numbers as high as C-951 have been reported. Run C probably lasted until the "slow-down" that usually happened in the Spring. Run D probably started in mid-1941 and also produced a high quantity of receivers. Highest D-run serial number reported is D-700. Since exclusive WWII war production was going to start in April 1942, it's likely that Run D ended before that. Run E seems to be WWII production of the NC-200 variants. This puts the production of pre-war NC-200 receivers at around 1800, more or less. The pre-war NC-200 receivers aren't encountered as often as other models and should be considered relatively scarce.

During WWII, the NC-200 band spread was eliminated for the military and the receiver designated as the NC-200FG. Push-Pull audio was retained. A logging dial was added. Late during WWII, tube upgrades resulted in the new designation of NC-240.

Silver Anniversary NC-200

photo above: The Silver Anniversary NC-200 with the serial number C-536. Note that the knobs and the meter housing are a medium brown bakelite rather than the standard black bakelite. Also, note the gold-tint to the control nomenclature plates and the skirt of the tuning knob. Even the special "Silver Anniversary" NC-diamond insignia had a "gold-tint" applied to it. The two-tone paint scheme of dark gray wrinkle finish combined with smooth finish, semi-gloss light gray was standard for the NC-200.

If any of National's "Moving Coil" receivers has a confused origin, it's the Silver Anniversary NC-200. Whose anniversary was it, anyway? Well, it wasn't National's as many collectors think.

The Silver Anniversary NC-200 was announced in the December 1940 issue of QST. This QST was a "special issue" with a silver cover and articles galore on the origins of ham radio, the ARRL and QST. It was all to celebrate QST's 25th "Silver" anniversary. The back-inside cover of the magazine had a full-page National advertisement introducing a special version of the NC-200, the Silver Anniversary NC-200.

The ad states that this special NC-200 was "Dedicated to amateurs on the twenty-fifth anniversary of their own QST. A toast to QST, the ARRL and the Amateur!"

National's ad stated that each Silver Anniversary NC-200 would have a special NC diamond insignia that had "SILVER ANNIVERSARY" embossed around the perimeter of the diamond. Although not mentioned in the ad, each matching Silver Anniversary NC-200 speaker also had the "Silver Anniversary" diamond.

Additionally, each Silver Anniversary NC-200 was to be fitted with special brown bakelite bar knobs, a brown bakelite tuning knob and a brown bakelite S-meter case.   >>>

>>>  To complete the special finishes, the tuning knob skirt, all of the control nomenclature plates and the special NC-diamonds were finished in a gold tone.

It appears that National didn't remain consistent with the Silver Anniversary trim throughout production, however. Note in the three Silver Anniversary NC-200 receivers shown that C-536 has a beige color dial, D-499 has a cream color dial and the unidentified receiver has an off-white color dial. I also have a Standard NC-200 sn: D-700 and its dial is light beige. National sometimes was inconsistent with their paint mixes and some National parts always seem to have some minor variability in color tint. Note in the next section below (NC-200FG/SC and Other WWII Variants) that the dial on this NC-200 USCG receiver is beige.

D-499 has the brown S-meter but the knobs are black. Since D-499 must have been towards the end of Silver Anniversary production, this probably indicates that National must have run out of brown knobs (and didn't want to make or order more) but still had brown S-meters. Intermixing various parts was common at National as the company always wanted to "use up" all remaining stock, if possible. Note that the unidentified receiver has the brown knobs and brown S-meter but doesn't have the Pick Up jack.

The gold tone is very light on D-499 but is most noticeable on the tuning knob skirt. With the unidentified receiver it's difficult to tell whether the gold tone is present since the nomenclature plates also appear somewhat oxidized. The tuning knob on this receiver is not original.

All of these minor variations show that probably the only consistent Silver Anniversary identification is the NC diamond insignia that always had "SILVER ANNIVERSARY" embossed on its perimeter. >>>

photo above: This beautiful condition Silver Anniversary NC-200 is D-499 and is owned by Jim WA2MER. Note that this receiver has the brown S-meter but the knobs are black. The gold tone is very light on this receiver and is most apparent on the tuning knob skirt. The dial is a cream color. Also note that this receiver has the "PICK UP" jack on the lower right. Note the NC diamond insignia has the embossing around the perimeter.   photo: WA2MER
photo left: An unidentified Silver Anniversary NC-200 in rather sad shape. Note the brown knobs (the tuning knob is not original.) Also, the off-white dial. Since this receiver doesn't have the "Pick Up" jack, it must be earlier than D-499.         photo:  from eBay

photo right: The "Silver Anniversary" NC diamond insignia. There's still a little gold tone left on this example that is mounted on C-536's matching speaker.

>>>   Just how long the Silver Anniversary versions were produced is unknown. Of the two SNs reported, the span is from C-536 up to D-499 or 963 serial numbers. It's unknown if the Silver Anniversary models were the only type of NC-200 that could be purchased for awhile or whether the buyer had a choice of the "standard finish" NC-200 or the Silver Anniversary. There are three other serial numbers logged between C-536 and D-499 and they aren't listed as Silver Anniversary models but that can't positively indicate that those receivers are standard finish models either (it just means that the specific information wasn't included with the serial number reporting.) Receiver D-700 is a standard finish NC-200 but whether that indicates that no more Silver Anniversary versions were being built by that serial number is also unknown.

In an effort to determine the quantity and other production information of the Silver Anniversary NC-200 receivers, we'll need some serial numbers from surviving receivers that are in collections. Our assumption is that the Silver Anniversary NC-200 was serialized within numbers used in Run C and into Run D. What we don't know is if all serial numbers between C-536 and D-499 are exclusively used for Silver Anniversary models or whether the serial numbers were intermixed with the standard NC-200. A good quantity of serial numbers should reveal whether the Standard NC-200 and the Silver Anniversary NC-200 were produced during the same time period. Please report your Silver Anniversary NC-200's serial number (or any NC-200) using this e-mail link:

 SILVER ANNIVERSARY NC-200 SERIAL NUMBER

 

 


photo left:
This is the December 1940 QST ad that was on the inside-back cover that introduced the Silver Anniversary NC-200 receiver. It's obvious from the text that the Silver Anniversary NC-200 was for QST's twenty-fifth anniversary and really had nothing to do with any National Co. anniversary.

 

NC-2-40D aka NC-240D, NC-240CS


photo above:  An early NC-240D with matching speaker (SN: F-478) from 1946  - Note the location of the Band Spread scales (in red) nearest the center of the arc of the scales, similar to the NC-200 dial scales (the large red NC diamond below the logging scale is also a "holdover" from the NC-200 dial.) The logging dial is behind the tuning dial hub. Also, the round S-meter housing. The matching speaker shows the new style grille cloth (original cloth shown.)

NC-240CS - The NC-240C and CS were built through the later part of WWII. The amateur band spread function was eliminated in all of the WWII versions. The NC-240CS had a 200kc to 400kc band in place of the lower section of the AM BC band. It appears that the NC-240CS versions built during WWII maintained the push-pull 6V6 audio output stage. Just after WWII ended, a few NC-240CS receivers were sold to the post-war civilian market as "ham receivers" but National quickly designated the "CS" as a "commercial" receiver as soon as the ham band spread version, NC-240D, became available. In 1947, there was a contract for the NC-240CS to be was used as an Airways receiver. It was identified as the RCR (see RCR in Airport Receiver section below.)

NC-240D - By 1946, National had quickly returned the band spread option to the NC-240 and added the suffix "D" to distinguish that this receiver was the latest version. The 200kc to 400kc band was replaced with AM BC band coverage. National also added pedestal-type feet to the receiver cabinet and to the speaker cabinet. The NC-240D sometimes will have an additional T or R suffix, depending if the receiver was a table model or a rack mount style. Many of National's advertisements and manuals show the receiver model as "NC-2-40D" however it seems to have also been shown as NC-240D in some National literature. Early NC-240D receivers will use a letter prefix serial number while later versions went to the seven digit numeral serial number. 

As with the post-WWII HRO-5 and its many "build variations," the NC-240D receivers also vary depending on what WWII surplus parts National had on-hand (and wanted to "use up.) Many of the NC-240D receivers will have chassis that were punched for earlier or different versions of the receiver with the chassis having several large unused holes. By 1947, most of the WWII surplus parts at National had been used up and the receivers produced after that are all standard parts and similar in appearance. The 12-tube line-up replaced the two older dual triode tubes (6C8G and 6F8G) with the newer version tubes (6SL7G and 6SN7G.) Early versions of the NC-240D will have a 6SJ7 as an AVC amplifier while later versions use a 6V6 for the AVC amplifier. Also, the 80 rectifier used in some WWII versions went back to the 5Y3G.  >>>

>>>  Later versions of the NC-240D will have the square S-meter (as shown in photos below.) Early versions will have the Band Spread scales located nearest the center of the semi-circular arc of the dial scales. Later versions will have the Band Spread scales located above and alternating with the General Coverage scales. The logging scale was relocated on later versions to below the dial pointer hub.


photo above: The 1948 NC-240DR, in this case, a rack mount version. The post-war models returned to the amateur band spread feature that was lacking in the NC-200FG and the NC-240CS. Later versions of the NC-240D will have the square S-meter as shown on this receiver. Note the alternating band spread and general coverage dial scales used on later receivers.  Also, relocation of logging dial.  Photo eBay.

 The NC-240D was sold from 1946 up to 1949 and was the last of National's Moving Coil receivers. Selling price was usually around $225.


photo above: The 1948 NC-240DT or table model version. This receiver is from run 183 which is the latest reported production run for the NC-240D receivers and shows how the last versions appeared.

 

Airport Communication Receivers and Airway Communication Receivers

Built for: Department of Commerce, Bureau of Air Commerce or Civil Aeronautics Authority

photo above:  The Air-Ground Radio Station at Kansas City AP around 1936  -  from Sterling's Radio Manual 3rd Ed.

The continuing improvement of airport to airplane radio communications along with improved radio-based navigation equipment was on-going through the 1930s. At the time, the Department of Commerce and the Bureau of Air Commerce were in charge of airports, airport communication and air navigation. The first National receiver specifically for airways communications was the RHM, a superheterodyne (National's first) supplied in 1932. The RHM evolved into the AGS receiver that was also used at some airport installations. Additionally, the AGS evolved in the RHP, RHQ and the AGU - all based on the RHM circuit but using ganged, plug-in coils rather than individual plug-in coils (a set of three coils were required for each frequency tuning range.)

From 1935 up to around 1937, the HRO was favored by many airlines but the HRO had numerous accessories that required additional storage be provided. Each HRO came equipped with four coil sets, a separate power supply and a loud speaker. Custom installations usually were able to integrate the HRO and its accessories into the airport communications equipment racks.

photo left: This is the data plate used on the first of the Airport receivers, the RCD, from 1937. Note that the RCD is specified as a "Communication Receiver" only. The Bureau of Air Commerce - Air Navigation Division was under the Department of Commerce in 1937. Note that this data plate is attached to the front panel with drive screws (pins) rather than standard screws.       
photo by: Warren Anderson

Starting in 1936, National began supplying the standard NC-100 (with its art deco front panel and no modifications) to various airports (see photo above of the KC AP in 1936.) It's likely that the "commercial" NC-100 versions were also supplied to some airports in 1936.

Starting in 1937, National began supplying somewhat modified NC-100 receivers for use at airports for tower communications to local aircraft and for aeronautical communications which was the "ground to air" radio communication that supplied non-local aircraft with weather, navigational information and messages. There was also an aeronautical point-to-point communication system that was CW only. National's first "modified for airport use" NC-100-based receiver, the RCD, designated as "Communication Receiver." The RCD was essentially a rack mounted NC-100X with a frequency coverage that was altered to remove the AM BC coils and replace them with coils to cover 200kc to 400kc. The remaining catacomb coils were not changed and allowed 1.3mc to 30mc coverage in four tuning ranges. These initial NC-100-based airport receivers used a 3/16" thick aluminum panel that was black wrinkle finished along with retaining the Crystal Filter and the cathode ray tuning indicator. The RCE receiver that followed had several improvements which became standard for National Airport receivers although the RCE was still built for the DOC-BAC. The RCE removed the eye tube and the crystal filter used by the RCD and added a squelch control.


photo above: Data plate from the RCE SN: 302 receiver dated 11-19-37

By 1938, the U.S. Civil Aeronautics Authority, the CAA, had taken over the responsibility for airports and air communications. By this time, the Airport receivers had even more additional circuitry added to further adapt them to airport communication requirements. The CAA designated these newer receivers as "Airway Communication Receiver" as shown in the RCF-2 data plate shown in the photo to the upper-right. The RCF designation was probably assigned to another piece of equipment (probably USN,) thus the suffix "-2" added to the RCF-2 receiver.

The standard CAA Airport versions used 12 tubes, had no Crystal Filter and no carrier level indicator. The audio output was changed from Push Pull tubes operating an output transformer mounted on the electro-dynamic speaker of the "civilian" models to a single audio tube with an output transformer internal to the receiver which allowed PM speakers to be used.    

The power supply was slightly modified to include an extra filter choke since the field coil of the electrodynamic speaker wasn't available for that function. The typical CAA receiver used a 3/16" thick aluminum front panel painted black wrinkle finish with engraved nomenclature. This description also applied to the earlier RCE receiver built for the DOC-BAC but not for the earlier RCD.   >>>
>>> A gray painted PW-D was standard for all Airways receivers. The IF was usually 457kc. Later versions will have a two-position selectivity control and, later still, a crystal-controlled fixed-frequency function. For fixed-frequency operation, the receiver had to be tuned near the crystal frequency minus the IF for the RF amplifier stage and the Mixer to be tuned correctly. If the fixed-frequency desired was, for example, 4495kc, then the crystal required was 4495kc + 457kc = 4952kc crystal frequency. This put the fixed-frequency LO operating higher than the tuned frequency which is the normal configuration.   

Most CAA early versions will have a squelch added that is referred to as the Interchannel Noise Suppressor, or I.N.S., which was activated by a front panel toggle switch. The I.N.S. circuit used a 6J7 tube that was operated from the 6J7 AVC tube and when the AVC bias voltage was being driven negative by lack of a signal, the I.N.S. tube would bias off the 1st AF Amplifier tube (6C5) which reduced the audio output to a very low level. Although the I.N.S. could be adjusted to "full squelch," National recommended that the I.N.S "suppression" be set to allow a very slight background noise to be just audible and then when a desired signal was received the I.N.S. would provide a "normal" audio level. The I.N.S. is adjusted with the two potentiometers that are mounted at the rear of the chassis directly behind the tuning condenser. Typically, the eye-tube of the standard NC-100 or other type of carrier level measuring device was not used on the CAA receivers but at least one RCF-2 example has turned up with a National S-meter that appears to be a factory installation.


photo left: A close-up of the data plate on the RCF-2 sn 13 showing that National specified the RCF-2 as an "Airway Communication Receiver." The receiver was built the for General Electric Supply Corp. of Washington, D.C. to supply to the CAA. Note the contract date of December 4, 1939.

The audio output was rolled off at 3000 Hz by using an in-circuit audio filter that is between the output of the first AF amp and the input of the 6V6 audio output tube. National felt that the necessary voice characteristics that affect intelligibility are all contained in the audio frequencies below 3000 Hz. As with military versions of National receivers, the P-P audio was replaced with a single-ended audio output tube and an internal output transformer provided 600 Z ohm output along with a Hi-Z audio output (20K Z ohm.) The phone jack on the front panel is a 600 Z ohm output. Some versions had an internal relay that operated on 6vdc (supplied externally) to disconnect the speaker but not affect the headset output. A single loud speaker was supplied and also a rack mounted dual speaker assembly was sometimes supplied. Some versions also had remote control available for RF and AF Gain functions. Some versions had a dual fused AC line input while others have a HI AC or LO AC primary on the power transformer which is selected by which fuse clips are used in a dual fuse holder. Some receivers had both. All versions used oil-filled paper dielectric filter capacitors and have two filter chokes.

With the RCK-N versions, the I.N.S. circuit was replaced with the C.O.N.S. circuit, or Carrier Operated Noise Suppression. This was an improvement that operated a relay that silenced the receiver if no carrier was present. Additionally, the pilot lamp would illuminate when a carrier was present. The RCK-N was built for the U.S. Navy during WWII. It covers 200kc to 800kc in two bands and 2.5mc to 23.5mc in the other three bands. The IF on the RCK-N was changed from the typical 457kc to a higher frequency of 1560kc to allow complete tuning from 200kc up to 800kc. The designation RCK was also used for a piece of VHF four channel receiver used by the USN, thus the suffix "-N" to specifically identify this Airway receiver.

The RCL added a switch that allowed selecting either a Broad or Sharp selectivity. Later, it was found that the selection process slightly changed the IF center frequency. When crystal-controlled, fixed-frequency operation was installed the Broad-Sharp switch was removed.


photo above: Data plate from RCK-N  SN:182 from the WWII-era built for the USN

After WWII, many of the earlier version receivers (RCL and RCK versions) were modified into the RCP and the RCQ versions. The modifications were to add a selectable crystal-controlled fixed-frequency operation. These receivers also had modifications to the AVC and the addition of a series noise limiter (that was always on.) Also, the Broad-Sharp selectivity switch was removed due to IF instability problems. RCP modifications date from around 1945 and the rework was done by Schuttig & Company. The RCQ modifications date from 1948 and the rework performed by National Electrical Machine Shops, Inc. (NEMS.) The last of the National Airport Receivers was the RCR dating from 1948. It was essentially an unmodified NC-240CS from the late-forties.

Since Airport communications and electronics, in general, were evolving rapidly in the early-1950s, it's likely that the life of most of the National Airport and Airway receiver didn't last past the late-1950s. Many were repurposed into other functions, perhaps at airports and maybe other locations. Eventually, most were sold off to surplus vendors or scrap dealers.

Details on Some National "Moving Coil" Airway Receivers

RCE   SN: 302

The RCE was the second of the "Moving Coil" Airport receivers built by National. The contract is from November 1937 and the data plate indicates that the U.S. Department of Commerce and the Bureau of Air Commerce were to be the end users. The RCE eliminated the Crystal Filter and the Tuning Eye-tube and added several other features that were to become standards for the Airport (Airway) receivers. First was the I.N.S. circuit, the Inter-channel Noise Suppression circuit that was essentially a Squelch circuit. Additionally, two output impedances were supplied, a 600Z ohm and a 20K Z ohm output. Other features were the ability to silence the remote speaker via an internal relay that was powered externally.

The receiver uses a separate bottom cover and a full top dust cover. The upper dust cover is made out of aluminum. This type of dust cover is only used on the RCE. Later receivers had a slide-on full cover that interfaced with chassis-mounted side panels.

I mechanically restored this RCE because it had a serious breakage in the gearbox and a completely rusted tuning condenser. I was able to use a complete tuning condenser and gearbox assembly from a "parts set" NC-100 receiver. The restoration is described in the "Restoration" section further down the page in this web-article. Chassis photos are in that section.

RCF-2   SN: 13

Shown in the photo left is the RCF-2 Airway Communication Receiver SN: 13. This receiver is in excellent original condition but is missing its "slide on" dust cover but it does have the two side panels. The RCF-2 is the first Airway receiver that was built after the CAA took over airport operations and regulations from the DOC-BAC. Note that the data plate indicates that General Electric Supply Corp. was the contractor with National supplying the receivers to GES. Contract dates from December 4, 1939.
 

With the RCF-2 receiver, the locations of the RF Gain control and the Audio Gain control were interchanged with the RF Gain now on the right side and the Audio Gain now on the left. This would be when compared to the standard NC-100 receiver. This location continued up thru the RCQ receiver but was changed back for the RCR (probably because it was based on the NC-240CS receiver.) 

The photo to the right is the RCF-2 sn 13 chassis showing some of the differences the "Airport Receivers" have when compared to the standard NC-100 receiver. The small cover with the handle on top (next to the power transformer) protects the relay that is used for remote silencing of the speaker. Note the extra tubes to the left of the gear box. These are for the "I.N.S." (6J7) and First AF Amplifier (6C5) functions. Also, note the single 6V6 audio output tube next to the 80 rectifier tube and the relocation of the audio output transformer to the other side of the tuning condenser. The antenna terminal insulator is the later polystyrene type and next to the antenna terminals are the two potentiometers that adjust the "I.N.S." threshold and suppression.

Note that the power transformer cover appears to be mounted opposite of normal. Usually, National always had the "NC" logo right-side up when viewed from the front panel. The RCF-2 has two audio output impedances, a 600Z ohm line and a 20,000Z ohm line, both are accessed via screw terminals at the rear apron of the chassis.

RCK-N  SN: 182

photo left: This is the RCK-N  SN:182 from the WWII-era. These receivers were built for the U.S. Navy and cover different frequencies than the standard National Airway Receivers. Note that the nomenclature indicates that the lowest frequency band is 200kc to 400kc followed by 400kc to 800kc. The three highest frequency bands cover 2.5mc up to 23.5mc. To allow full coverage between 200kc and 800kc required that the typical IF of 457kc be changed to 1560kc. Note that there is a gap in the tuning from 800kc up to 2.5mc to allow for the 1560kc IF. Audio output is 600Z ohms.  

The switch identified as "C.O.N.S." is an updated version of the older I.N.S. control on the RCE and RCF-2 receivers. The C.O.N.S. squelch control operates a relay that silences the receiver when no carrier is present. Additionally, the pilot lamp will only turn on when a carrier is present. It was also possible to manually silence the receiver with a remote switch. The acronym C.O.N.S. stands for "Carrier Operated Noise Suppression."

As usual, this RCK-N is missing the side panels and the dust cover. The "red" band indicator flag is not standard, it should be white.

If you have a local AM-BC station operating on 1560kc, you might experience signal leakage into the RCK-N's 1560kc IF. The larger the antenna system, to a certain extent, the stronger the 1560kc leakage will be. Some receivers had a wave trap incorporated into the antenna line to allow "nulling" 1560kc but the RCK-N doesn't have any wave trap. A large, untuned end-fed wire will be very susceptible to AM-BC leakage, while a "tuned" antenna will be less responsive. Smaller antennae will also be affected less. In severe cases, an external wave trap can be placed between the antenna feed line and the receiver. Also, it is possible to align the IF to 1555kc or 1565kc to place the IF off of the AM-BC frequency. We don't have any local AM stations on 1560kc, so there was no leakage experienced. The 200-400kc band is good for NDBs while the 400-800kc covers the lower part of the AM-BC band. The HF bands cover 80, 40 and 20 meters. 17 and 15 meters are also covered. This RCK-N was already re-capped when I got it. I did an IF alignment and started using it. The performance is very good on 80M and 40M using a tuned "ham" antenna (135' CF Tuned Inv'd Vee.)

RCQ  SN: 288, SN: 242

Shown to the left is the RCQ receiver SN: 288. RCQ receivers were modified from earlier RCL or RCK version receivers in 1948 to add a crystal-controlled fixed-frequency function. Note the additional toggle switch that is identified as TUNE-ABLE and XTAL. The XTAL position selects the fixed-frequency mode.

After WWII, fixed-frequency became more and more necessary because it helped to eliminate tuning errors during operation and eliminated the possibility that the receiver might "drift" off-frequency.  Since many of the receivers were operated with a squelch circuit, frequency drift would go unnoticed unless the receiver was in fairly constant use. The crystal-control fixed frequency kept the receiver "on frequency" over long periods of inactivity. Even though the LO is crystal-controlled, the tuning dial must be set to the correct receive frequency so the RF amp and Mixer stages are "in tune." A dial-lock was added to keep the RF and Mixer correctly tuned for fixed-frequency operation.

Note the "vented" top cover on the power transformer. This is commonly found on Airway receivers.

Airport communications functions required fewer (if any) frequency changes and most set-ups had dedicated receivers for each frequency needed. The small paper label indicates the fixed- frequency is 4494kc but the crystal installed is 3448kc which would then provide crystal-control of 2993kc, so it's likely that the crystal was changed and the label wasn't.

To calculate the crystal frequency necessary just add 455kc (IF) to the desired receive frequency. For example, if 2993kc was the desired receive f, then 2993 + 455 = 3448kc for the crystal frequency.   Note that the IF was changed from 457kc to 455kc with the modifications of the RCQ.

Note that there is a "butch plate" attached to the front panel that has the nomenclature "TUNE-ABLE" and "XTAL" engraved on it. This butch plate is covering up the original engraving that was "BROAD" and "SHARP" for the original selectivity switch. This indicates that this RCQ started out as a RCL receiver. The hole at about "2 o'clock" by the PW-D was to mount the dial-lock assembly. It's missing on SN: 288 but is present on the RCQ shown in the photo to the right. The RCQ shown to the right is SN: 242. The panel has been repainted gray. The power transformer cover is missing and, of course, the side panels and dust cover are long-gone. I did get an original RCQ manual with SN: 242, however. 

There was a tube change in the RCQ receiver with the 6C5 detector tube replaced by a 6H6 duplex-diode.

Another note is that the RCQ is referred to as a "Communications Receiver" instead of a "Communication Receiver." The same reference is used on the RCP tag shown further below.

Shown to the left is a close-up of the data plate from the 1948 contract RCQ receiver SN: 288. All of the RCQ and RCP receivers were originally earlier versions of National Airport receivers (RCL or RCK) that were modified to update the receivers to perform to the specifications required by post-WWII airports. The RCQ modification added a crystal-controlled fixed-frequency function to the receiver. The RCQ was the last of the Airways receivers that utilized the NC-100 basic design.

RCQ SN:288 is not restored. In fact, it's in "as found" condition. I have an entire "packet" of information that came with the receiver that indicates a former owner was thoroughly engrossed in the rebuilding of this receiver without any actual documentation or without using any National parts. Actually, it's not too bad, an after-market audio output transformer and several circuit mods. Dust cover and side panels missing, as usual.

NOTE: On SN: 288, the IF cans are stamped "RCK" implying that this RCQ was built by modifying a RCK-N receiver. This would have required a different front panel, a different coil catacomb and different IF transformers. It seems likely that NEMS consolidated many of the parts necessary to rebuild either RCK or RCL receivers into RCQ receivers. RCK-N front panels couldn't be used because of the frequency coverage nomenclature would be incorrect for the RCQ. The RCK-N IF transformers couldn't be used since they are for 1560kc. The coil catacomb would have required standard coils be installed or it was probably easier just replace the RCK-N coil catacomb with a catacomb from a RCL receiver.

So, even though the IF cans on SN: 288 are marked "RCK," inside the cans are standard 455kc transformers. The front panel and coil catacomb must have come from an RCL receiver.

The RCQ manual states "This receiver (RCQ) was originally a Type RCK or RCL receiver." Probably a more accurate statement would be that "parts from both RCL and RCK receivers were used to build RCQ receivers."

NEMS - Note that the contractor for the RCQ was National Electrical Machine Shops, Inc., otherwise known as NEMS. NEMS formerly was known as NESCO, or National Electrical Supply Company but the name change dates from around 1938. The company NESCO dates from the late-nineteenth century. NESCO was supplying SE-1420 receivers to the U.S. Army in WWI. There was always some confusion between National Electrical Supply Company and another company called NESCO (National Electrical Signaling Company) that was associated with Reginald Fessenden but Fessenden's company was gone before 1920. In the early 1950s, as NEMS, National Electrical Machine Shops, Inc. formed a partnership with Alan Clarke and became "NEMS-CLARKE COMPANY" building VHF receivers for the military, government and commercial users. Nems-Clarke was bought by Vitro Electronics in 1957 but Vitro kept the Nems-Clarke name on the receivers.

RCR   SN: 17

National built the RCR on contract number Cca26391, probably in 1948. Note that the contract number on the RCQ Airport Receiver above is Cca 26227 and the date is 2-5-48. Compare that to the contract number on the RCR Airport Receiver - Cca 26391. The contract numbers are 164 contracts apart which certainly dates the RCR receiver contract to mid-to-late 1948 (there's no date on the data plate.) The receiver is essentially National's NC-240CS Commercial Communications Receiver. The NC-240CS differs from the NC-240D in frequency coverage, lack of amateur bandspread and an amber tuning dial.
photo left: The data plate from the RCR receiver SN: 17. Note that the tag refers to the RCR as a "Communication Receiver" as the earlier Airways Receivers.

Inside the RCR is a standard NC-240CS using a 12 tube circuit with single preselection, two IF amplifiers, crystal filter, noise limiter and tone control. Frequency coverage is 200kc to 400kc on Band F and then the remaining five bands, E thru A, cover 1.0mc to 30mc. Only a portion of the AM BC band is covered and no amateur bandspread coverage is provided. Audio output impedances provided are 8.0Z and 500Z. Additionally, 10,000Z can be used by paralleling the internal output transformer primary with a speaker-mounted audio output transformer with a hi-z primary. This allowed users to "plug-in"  the standard National NC-240-type table speaker, if desired. >>>

>>>   Differing from the preceding Airport Receivers, the RCR uses Push-Pull audio output amplifiers (a pair of 6V6 tubes) and doesn't provide a squelch control. Additionally, the RCR provides an S-meter and a direct-read dial, neither of which was used on the preceding Airport Receivers. Also, the receiver is painted a very dark gray wrinkle finish rather than the black wrinkle of the earlier receivers.

This particular RCR is serial number 17. It was used by a CAA official who was, at the time, located in Honduras, Central America. Apparently, when returning to the USA, the official brought the receiver back with him. It was used very little and is still in excellent condition.

RCP tag and a "One-off" Version of the RCF-2

photo above: Data plate from a post-WWII "Modified" Airport receiver, the RCP. Note that the date of the contract is Oct. 30, 1945. Schuttig and Company of Washington D.C. performed the modifications for the RCP receivers. The philip's head screws are not original.        photo: from eBay

photo right: RCF-2 Airway Receiver that was probably modified by National to have an S-meter. Although the first Airport receiver, the RCD, did have a tuning eye tube, most receivers didn't have any sort of carrier level indicating device. This receiver is an exception and it has all of the indications that the rework was carried out at National. The S-meter circuit wiring uses matching cloth-covered wire and it is professionally installed within the original wiring harness. The function of the toggle switch to the right of the band change knob is not known.   -  owned by Mike Everett W4DSE

Restoration and Repair of National Airway Receivers

Due to the continued use of the National Airway Communication Receivers by airports over a long period of time it's common to find the receivers in poor condition with missing parts and with several modifications that probably aren't documented. Modifications were usually to upgrade the earlier receivers to later versions and these modifications were professional in quality and normally the work was performed by well-known companies. These mods are well-documented since professional companies performed the rework and each receiver was redesignated as either a RCP or RCQ. But many receivers had mods installed "on site" and these may not have been well-documented. Many Airways receivers fell into the hands of hams that modified the receivers to their needs without regard to providing any documentation. Many repairs didn't used OEM parts (National parts.) Most ham mods were poorly installed and seldom helped performance.

It also appears that the data plates were sometimes removed on the receivers for various reasons. Removal of an original data plate would have been necessary when the receiver was upgraded to a newer version by a professional company but always a new data plate was installed to correctly identify these receivers. Some "unofficial" mods might have removed the data plate because it was "in the way" of installing the upgrade. Sometimes data plates were removed by later "ham owners" just to make the receiver not look like it was military or commercial surplus.

Since the Airway receivers were in commercial use, normal maintenance-type repairs were generally performed by airport technicians. It's common to find non-OEM parts used for these types of repairs. Sometimes, when considering removing a vintage repair that used a non-National (OEM) part, one should consider that the repair was part of the receiver's history. Possibly the quality of workmanship would determine whether the non-OEM part should be replaced. "Hamster" mods or sloppy repairs should be corrected to a workmanship level that would be consistent with what professional technicians were capable of.

It's very common that most of the Airway receivers found today will be somewhat incomplete. Dust covers especially seem to almost always be missing. Sometimes even the side panels were removed. It was probably done by airport technicians who felt that the dust cover and side panels retained heat and caused heat-related failures. Of course, one would think that proper ventilation would have been provided in the racks for all of the airport electronic equipment. Most of the time the original frequency chart is missing. Usually, the chart frame will still be mounted but, if it's missing, many of the frequency chart frames used are identical to the HRO chart frames found on the HRO coil sets. Luckily, nearly all of the mechanical parts and circuit components are interchangeable with the standard NC-100 series receivers. For example, I needed a complete tuning condenser and gear box to rebuild the RCE receiver shown above in this section. I was able to find a complete gear box and condenser removed from a standard NC-100 and it fit into the RCE perfectly. The RCF-2 shown above had a defective BFO coil and a replacement was taken from a "parts set" NC-200 receiver. It's lucky that most of the Airway receivers used standard National parts that are available on several different types of National receivers.

See the RCE restoration profile in the "Restoration Section" further down this page.

 

WWII Versions

RAO Series, RBH Series, NC-100ASD, R-115, R-116

WWII had started in Europe and the U.S. military knew that much of their radio equipment was obsolete and needed to be replaced. The U. S. Navy ordered the NC-100XA which became the RAO series (along with several similar versions, e.g. RBH and others.) The second version of the RAO added a second RF amplifier for reduction of Local Oscillator "leakage radiation" on the antenna with the added benefit of reducing images and maintaining decent sensitivity. By mid-war, the U. S. Army Signal Corps had ordered NC-100ASD receivers with 200kc to 400kc coverage instead of the AMBC band. By the end of WWII, thousands of RAO receivers had been produced by National and contactor Wells-Gardner.

U.S. Navy RAO & RAO-1 - National began supplying the U.S. Navy with the NC-100A as early as 1940. Designated as RAO, the first versions are somewhat similar to the standard NC-100XA receivers. Initially, the RAO circuit used was the standard NC-100XA although it's likely that a 500 Z ohm audio output transformer was incorporated into the circuit along with power supply modifications. This was standard procedure for Navy receivers. In fact, the civilian NC-100A receiver used a field coil type speaker and push-pull audio output, neither of which the Navy would have wanted on their receivers. Consequently, the RAO and the RAO-1 should have single-ended audio output along with an extra filter choke to take the place of the "speaker field coil used as a choke." Early versions probably use National's 456kc IF but by the RAO-2 the IF was changed to 455kc.

RAO-2 - Long before WWII began, the Navy wanted minimal radiation from the receiver's Local Oscillator appearing on the antenna. This was primarily to allow the receiver to be used in the presence of other shipboard radio equipment without interference. There was also the remote possibility that an enemy could "Direction Find" (DF) the radio's position from receiving the LO signal. It would be possible to also discern at what frequency the receiver was tuned. In the late-thirties, many of the Regenerative Medium Wave receivers used on commercial ships could easily be received at a distance of five miles or more - and this was from other commercial ships, not the enemy! The Navy believed that if the Germans put their minds to it, they could probably receive and DF inadvertent LO leakage radiation from some types of superheterodyne receivers up to 100 miles away.  >>>

photo left: 1944 RAO-7 sn:10 built by National Co., Inc.  Note the molded skirts on the bar knobs unique to the RAO-7 & 9.

>>>  Consequently, the Navy came up with a specification of <400pW as the maximum of LO energy could appear on the antenna. This assured that the receiver would not cause interference and it would be impossible to receive the LO leakage radiation at any distance away from the ship.

Beginning with the second of the numbered suffixes, the RAO-2, National added an extra RF Amplifier with an additional coil catacomb and tuning condenser housed in a bolt-on rear chassis and bolt-on cover. The added RF Amplifier upgrade probably dates from before WWII began for the US. A mid-1941 contract for the USCG R-116 receiver uses the added RF amplifier and it's likely the same time period was involved with the RAO-2. The extra RF Amp provided the isolation necessary to keep the LO radiation on the antenna below the designated level of <400pW. The second RF amplifier gain was set at unity-gain, so the overall receiver sensitivity is similar to the single RF stage NC-100A although with the extra coil set (TRF stage) the image ratio is improved and LO isolation from the antenna is increased. This additional RF amplifier section increased the depth dimension by about four inches and increased the weight significantly, running the scale up to about 75 lbs for most of the later RAO receivers (see photo of the underneath of the RAO-3 chassis below.) Tube total in RAO-2 (and later) is eleven. The LO tube was changed from a 6K7 to a 6J7.

The Crystal Filter circuit is same as was used on the NC-200 receiver in that a stepped-switch Selectivity control that provided five positions of increasingly narrow selectivity and the variable Phasing control to adjust the "notch" position within the passband.

The dial scale background was changed from silver to off-white paint and the articulated dial pointer was eliminated. Instead, a rotating dial mounted to the band change shaft indicated which tuning range is selected. RAO-2 and some RAO-6 receivers had an ID from National of NC-120 on the control panel for the Crystal Filter (along with the National Co. identification and the "NC" diamond insignia.)

The early RAO receivers (pre-RAO-7) are 17.5" wide, with an integral panel-cabinet-chassis construction that requires major disassembly if more than tube replacement, alignment or minor repair is needed. These early versions of the RAO used a cradle-type shock mount that is seldom found with the receiver today. Note in the photo to the right that the RAO receiver and the RBL receiver both are mounted in their proper cradle-type shock mounts.  



photo above
: This snapshot of a couple of Navy shipboard radiomen at work shows the typical receivers used during WWII. Far left is an early RAO receiver on it's shock mount. Next to the RAO is the RBL longwave receiver. The larger receivers are the RAK and RAL regenerative receivers. Also, to the upper right is the LM frequency meter for accurately determining either transmitter frequency or received frequency. Below the order binders and just visible is a Scott SLR type shipboard entertainment receiver.



photo above
: Wells-Gardner RAO-3 on its original shock mount that's marked "RAO-3"
 

RAO-3, 4 & 5 - Most WWII equipment was built under contract and not all RAOs were built by National. Wells-Gardner Company (Chicago, IL) was the second contractor for the RAO series, building the RAO-3, 4 & 5. The Wells-Gardner versions were generally intended for shore stations and were not considered "heavy-duty military-type" construction by the Navy. All W-G versions have the extra RF amplifier for double preselection and all versions have S-meters installed. Though W-G used National Co. parts, where specified, many other components were strictly W-G manufactured or purchased parts. The IF transformers, power transformer, chokes and the chassis all appear to be W-G components while the knobs, dial, tuning condenser and coil catacomb were definitely from National and were even marked so. The Crystal Filter panel was marked with the WELLS-GARDNER logo and name. The RAO-3 operated on 115vac only while the RAO-4 and 5 had a dual primary power transformer to allow operation on either 115vac or 230vac. Interestingly, the W-G versions of the receiver had embossed "feet" on the bottom cover that allowed using the receiver on a table top without the shock mount assembly. The "feet" were specifically intended for shore use only and is also probably why so many W-G versions are found without the shock mount assembly today. Also today, many W-G RAOs show up without data plates. Oddly, it doesn't appear that they were removed - they were just never installed (all mounting hardware is present even the lock washers.) It's possible that W-G did have many of the RAO receivers unshipped when WWII ended and they merely "dumped" them on the surplus market, which would account for the large number of W-G RAOs found without data plates or shock mounts today.

The photos below show the chassis of the RAO-3 version of the receiver. The top of the chassis (left photo) shows that though many parts are from Wells-Gardner the main components are made by National. Note the small shielded box for the antenna input next to the First RF tuning condenser. This was one of the first attempts to reduce the LO leakage.

The photo to the right shows underneath the chassis of the RAO-3. Note how the 1st RF Amplifier section consists of a "bolt on" chassis and utilizes a smaller catacomb that contains the 1st RF Amplifier coils for the five tuning ranges. The mechanical coupling from the main catacomb to the smaller catacomb is through a slot in the rear of the main chassis and a matching slot in the smaller chassis. On top of the bolt-on smaller chassis is a single tuning condenser that is mechanically coupled to the main tuning condenser which can be seen in the photo to the right. The 1st RF Amplifier tube is housed in a small metal box that is mounted to the inside rear wall of the main cabinet with a shielded harness to make the connections to the tube socket. This small box also has a top cover for shielding of the entire box. To complete the shielding, the bottom of the chassis is covered by two plates as can be seen by the mounting flanges on each of the boxes. The RAO-3 shown in the photos was in poor condition and its clean-up and restoration has removed much of the silk-screened nomenclature on the various components. Power transformer cover is not original. See the photo below of the interior of the RAO-7 for the typical IDs found on chassis components in the RAO Series.

photo above: The top of the RAO-7 chassis showing the improvements to shielding and the addition of a panadapter output - the SO-239 connector.

 

RAO 6, 7 & 9 - National continued on with the Navy contracts building the RAO-6, 7 & 9 (the designation RAO-8 was apparently not used.) These receivers were of robust construction and had increased shielding to further allow their use with other shipboard equipment without interference. The audio output Z was changed to 600 ohms with the later versions. The RAO-7 & 9 eliminated the S-meter in favor of panadapter connections. The early RAO-6 receivers had S-meters but later production receivers did not. The later RAOs (7 & 9) simplified the maintenance of the receiver by designing the chassis so that it was easily removable from the cabinet (handles were added to the receiver front panel to assist removal.) The cabinet itself was redesigned for better shielding and easier mechanical construction by making it a one piece unit and, though some later cabinets did have lids, most did not. While the early RAOs used a separate shock mount system, the new cabinets mounted the shocks directly to the bottom of the cabinet further easing construction and maintenance. The RAO-7 & 9 receivers are physically larger than the earlier versions with full 19" rack width panels although the receivers are not specifically designed for rack mounting. Electronically, there is no difference between the RAO-7 and the RAO-9 with the exception that, in the RAO-9, the two RF amplifier inputs each have a RC filter installed between the grid and the coil selected. These RC filters were low-pass types to keep VHF radar from interfering with the receiver's operation onboard ship. 

The photo to the left shows the top of the chassis of the later RAO receivers, this one is an RAO-7. Note the large and completely shielded RF input box. The SO-239 connector to the left is for the panadaper input. Note that the separate chassis for the 1st RF catacomb is still in use. The side panels provide rigidity for the chassis since it is now mounted in a complete cabinet rather than the sheet metal "pieces" that make up the earlier type cabinet.

Performance - All of the versions of the RAO receivers have impressive sensitivity and freedom from images. Dial accuracy is usually quite good given that the resolution is limited. The tuning rate, which is standard for the National NC-100XA Series, seems fairly fast but it's still easy to tune in CW signals and SSB can be "fine tuned" using the BFO control, if desired. The audio output was modified for the RAO series to eliminate the P-P output in favor of a single audio tube stage that was primarily designed for CW reception using earphones (although an optional loud speaker is shown in the manual.) Audio output of the RAO, especially the later versions, will have the bass rolled off significantly since this enhanced the copy of CW signals. AM signals will be significantly lacking in bass response. Stability is very good with very little drift after a 15 minute warm-up. The RAO does provide a remote standby so it's easy to set one up for a vintage military radio station. As a station receiver the RAO will have great sensitivity, excellent selectivity (use the crystal filter,) good stability and fair audio. I've used both the RAO-3 and the RAO-7 as station receivers with good results.

 

The RBH Series - aka: NC-156 - RBH was the Navy designation for the NC-156 receiver, a 10 tube superhet that covered 300kc to 1200kc and 1700kc to 17mc in five bands based on the NC-100XA. To allow continuous coverage of the 300kc to 500kc range the IF operates at 1500kc (which is why there is a gap in the tuning from 1200kc to 1700kc.) The initial RBH receivers will have "NC-156" on the National Co. nameplate (part of the crystal filter panel.) The first RBH receivers date from around 1940 and will have the typical modified audio outputs, this is, single-ended audio output stage driving a 500 Z ohm output transformer. The later versions of the RBH receivers were modified for use at sea. All of the RBH series with number suffixes starting with RBH-2 have an additional stage of preselection added with a bolt-in chassis and cabinet to house the additional catacomb section for the coils and an additional tuning condenser for tuning the stage. This addition was very similar to the RAO receiver change and was for the same purpose of reducing the LO radiation from the antenna (<400pW on the antenna.) Like the RAO, the dial system changed on the later RBH receivers, eliminating the articulated pointer and painting the background white rather than silver. It's likely that Wells-Gardner was a contractor for some of the RBH versions. The last of the RBH receivers incorporated the same improved cabinet of the later RAO receivers. The early RBH receiver shown in the photo to the right unfortunately has had all of its Navy tags removed. Sometimes tag removal was a requirement for sales of surplus equipment but more often the new owner wanted visitors to the shack to think his receiver was a new purchase of civilian equipment and not surplus military gear.

Performance - When operating an RBH today, strong AM-BC stations around 1500kc might resonate with the RBH's 1500kc IF amplifiers and can cause strong heterodynes when tuning in stations or, if the 1500kc AM-BC station is particularly strong, it may dominate the IF system of the RBH. A tuned antenna will help with most IF interference however a 1500kc wavetrap on the antenna lead-in may be required to cure any serious problem.


photo above: USN RBH aka NC-156, first version with single preselection. These versions will also have the articulated dial pointer. Additionally, these versions will have the fiber tube dial lamp holder. The NC-156 designation is on the Crystal Filter control panel along with the National Company logo. Originally, this USN receiver had a data plate mounted in the upper right corner of the front panel and two data plates mounted on the top lid.

Miscellaneous U.S.N. Versions - Some of the early RAO series are also marked as NC-120 on the Crystal Filter panel. This generally appears on RAO-2 and some RAO-6 receivers only. NC-156 appears on early versions of the RBH receivers with this designation appearing also on the Crystal Filter panel. The NC-127 was a component receiver of a triple diversity set-up that was designated as NC-127D. The NC-127 is very similar to the RAO-7 receiver with additional outputs on the rear panel for diversity hook-ups. According to Raymond Moore's book, "Communications Receiver's," only one NC-127D was built and therefore probably only three NC-127s were built.
 


   photo above: B&W artwork from the NC-100ASD manual

 U.S. Army Signal Corps  NC-100ASD - The NC-100ASD was built for the Signal Corps around 1943. There was another version, the NC-100ASC, which may have been also designated as the AN/GRR-3. From available artwork it appears that the GRR-3 may have had a crystal filter thus being similar to the National NC-100XA though it may have tuned 200kc to 400kc and not tuned the AM BC band. It's likely that it had a 500Z ohm audio output and used a single audio output tube. The Signal Corps wanted a version of the NC-100A without a crystal filter (probably for ease of operation and reliability) that tuned 200kc to 400kc in addition to 1.3mc up to 30mc with this receiver designed as the NC-100ASD. Since the size of the catacomb was more or less fixed at being able to accommodate five sets of coils, National had to remove the AM BC coils in order to have the 200kc to 400kc tuning range. The articulated dial pointer was retained even though all of the later USN versions (RAO, RBH) had eliminated this feature due to it being overly-complex and somewhat delicate (dial string was used for actuation of the mechanism.)  The IF was National's standard 456kc. The Signal Corps had no use for Push-Pull audio output, so a single 6V6 is installed, thus the tube total for the ASD is ten. The LO tube was changed from a 6K7 to a 6J7. The audio bass is not rolled-off in the ASD (as the RAO's was) so AM reproduction through the matching speaker is normal sounding. A second filter choke was installed in the power supply to take the place of the electro-dynamic speaker's field coil (acting as a choke and electromagnet) as found in the civilian version of the NC-100A.
Since the receiver had to have a 500Z ohm audio output it was necessary to install a 5000Z plate to 500Z ohm audio output transformer inside the receiver (the civilian version mounted a push-pull output transformer on the loudspeaker.) This allowed the speaker to be connected with just a two conductor cable with no B+ on the speaker connections at the rear of the chassis. A matching speaker was included with the NC-100ASD and it was very similar to the standard 8" National speaker box but with the 500 Z ohm matching transformer connected to the 2.8 Z ohm voice coil-speaker. The matching transformer is mounted to the speaker frame. Also, the speaker cable is shielded to prevent RF pickup. There are small variations as the NC-100ASD production evolved. Early versions have different style 4uf oil filled capacitors with a crimp around the top while the later versions have no crimp at all. Later versions use all-Sprague paper capacitors while the early versions have some Aerovox capacitors in some places. The dial scale on late ASDs will have a thin coating of MFP applied to prevent oxidation which isn't found on the early versions. No special military tags are usually found on NC-100ASD receivers with the only receiver identification being the "NC-100A" that is located on the upper right-side nomenclature plate along with the National Company information. Most units will have Signal Corps acceptance stamp on the back of the cabinet. Probable contract order number is 9727-PHILA-43.
NC-100ASD Performance - The NC-100ASD is just about as close to the civilian NC-100A as the military got. Like all of the military NC-100s, the ASD has oil-filled paper dielectric filter capacitors, so many will work fairly well on all original parts (since Sprague paper caps and Centralab resistors were used.) Calibration is usually very good on the receivers that are in excellent condition and were stored carefully. Lack of a crystal filter may be a problem when QRM is heavy since the bandwidth is typically 8kc at 20db down. Images on band A could be a problem with strong signals since the image ratio is around 10:1 on this band. Generally, the NC-100ASD is sensitive enough and the audio reproduction good enough that many surplus receivers were used in ham shacks in the 1950s and 1960s. It seems that post-WWII surplus availability was very good and many ASDs were offered in NOS condition at a selling price (in 1946 from Newark Electric) of $115 and that included the matching speaker. Even today, it's not too difficult to find very nice examples of the receiver. Production seems to be around 1000 receivers based on serial numbers reported.

photo right: 1943 NC-100ASD sn: 194 with matching 500Z ohm speaker

 
U.S. Coast Guard - R-116 - The USCG R-116 contract actually dates before WWII began for the US with a date of May 15, 1941. Interestingly, the R-116 chassis is very similar to the RAO-2 chassis in that an extra RF amplifier section is added to the rear of the receiver. The reason for the extra RF amplifier was isolation of the LO radiation from the antenna. Of course, this contract dates the double-preselection upgrade to pre-WWII and certainly must have affected the RAO evolution also. The R-116 uses eleven tubes as the RAO does however parts of the circuit are similar to the earlier NC-100 since there is no Noise Limiter circuit. The frequency coverage is different however with the R-116 covering 1.5mc to 27mc in five bands. The band-in-use indicator is the same as the RAO receivers. The dial is similar to the RAO. The front panel is significantly different with circular nomenclature plates (similar to the NC-200) used for all controls. The dial bezel is dramatically simplified when compared to the RAO. The R-116 doesn't have an S-meter. Additionally, the bar knob-operated dual toggle switches found on the RAO were changed to single toggles on the R-116. The locations for the controls are quite different from the RAO. Receiver operates with a separate AC power supply or from a DC source such as batteries or other ship power sources. Serial number on the R-116 shown is 46 implying that a significant quantity of R-116 receivers were part of the contract.


photo right
: R-116 USCG Receiver     photo from: eBay

U.S. Coast Guard - R-115 - National released the NC-200 in October 1940. WWII had started in Europe but was still a little over a year away for the US. Though the NC-200 sold well before late-1941, by the time the USA entered WWII, National had already started planning for changes to be incorporated into the receiver to make it acceptable for the military. First, the amateur band spread function was eliminated. This receiver was initially referred to as the NC-200FG. Soon, the NC-200FG became the NC-240CS which was essentially the NC-200 without band spread and with 200kc to 400kc installed to replace the AM Broadcast band. The IF was the industry standard 455kc. A logging scale was added to the tuning dial. The square S-meter style was replaced with the round front type as used on the HRO receiver and the replacement of the 5Y3G rectifier tube with the type 80 rectifier tube.

Shown to the left is a seldom-seen variant of the NC-200, the R-115, built for the U.S. Coast Guard. It tunes from 200kc to 400kc and then from 500kc up to 18mc, thus including the AM BC band. The receiver controls appear to have been reduced to a minimum and there is no Crystal Filter, S-meter, Tone control, Logging Scale or Noise Limiter. Unfortunately, the data plates are missing. The bar knobs are not correct and should be National-type bar knobs.  


photo left:  R-115 USCG Receiver         photo from: Art Lebermann 

 

Serial Numbering on the NC-100 Series

Serial Number Formats Used - Serial numbers on early NC-100 receivers appear to be the same kind of format used on other National receivers at the time. This consists of a letter that identifies the production run and a number that identifies the specific receiver within that production run. When the NC-100 was introduced in 1936, the HRO receiver was still using serial numbers that consisted of a letter prefix followed by a number suffix. Probably to keep the two receiver type serial number formats obviously separate, the NC-100 serial numbers began with a number prefix followed by a letter suffix (the letter identifies the specific production run.) From the serial numbers reported so far, it appears that the NC-100 series serial numbers begin with the "D" suffix to identify the first production run. Interestingly, the HRO first production run also used the prefix "D" to identify its first production run. By 1938, the HRO had used all of the letter prefix serial numbers and then began to use a letter suffix combination, like the NC-100 had been using. 

It appears that all NC-100 series serial numbers are sequential in their assignment within the particular production run and those production run letters are also sequential starting with run "D" in 1936. It also appears that most production runs had quantities of around 350 to maybe 400 receivers within them. From the serial numbers reported, it appears that all NC-100 Series receivers use a letter suffix in the serial number with the exception of the WWII military versions.

The NC-80 receivers have their serial numbers stamped on the rear panel of the cabinet at the lower-center just below the headphone jack opening. Since the cabinet was painted black wrinkle after the stamping, the serial numbers are sometimes difficult to see. The NC-80 receivers use a different format with a letter prefix followed by numbers. From the one reported NC-80X serial number, it appears that National used the prefix letters not used by the HRO series (A, B & C.) Since the production was low on these receivers, it's possible that all NC-80 series receivers used the "C" prefix (since the reported serial number is C-745,) although this is just speculation.   >>>

>>>  The NC-200 serial numbers use a letter prefix followed by up to three numbers. It appears that production runs C, D and E comprise the production of NC-200 receivers. It seems unlikely that the NC-80X series, that also used "C" prefix serial numbers, shared serial number prefixes with the NC-200 since the former was produced from 1937 up to 1939 and the latter was introduced in late-1940. Since both model receivers used the same serial numbers, one has to assume that National used the receiver model and its serial number for identification.  

During WWII, serial number formats vary somewhat but usually follow the letter prefix format. Often times, the serial number is only found on the particular receiver's data plate. Normally, if the serial number is on a data plate it will just be a number with no letters. The NC-100ASD serial number uses only numbers that are stamped into the chassis between the 6C8 and the 6F8 tubes. Early post-WWII serial numbers are still letter prefix format using the prefix F for the production run in 1946. It seems likely from the serial numbers reported so far that run F began after WWII ended. Sometime in 1946, National changed the serial number format to a seven digit serial number that consists of a three digit production run ID followed by a space that is then followed by a four digit number that identifies the receiver. So far, runs 169 and 183 appear to be for the late NC-240D receivers.
 

WHRM Serial Number Log for NC-100 Series Receivers - In an effort to see what kind of serial numbering National used on the NC-100 Series, WHRM is starting up another serial number log. These logs are very effective at providing production quantity information and year of manufacture information after a fairly large quantity of serial numbers have been collected. When providing a NC-100 Series serial number for our log, please be sure to let me know the model of the receiver that the serial number belongs to. Also, note any characteristics that will help in determining the manufacturing year. Also, note any unusual production changes or professionally installed modifications. See the log below for some of the identifiers and details we're looking for.

After enough numbers have been collected it may be possible to date the year of manufacture from the serial number used. Please send your serial numbers from any NC-100, NC-100X, NC-101X, NC-80X, NC-81X, NC-100A, NC-100XA, NC-101XA, NC-200, NC-240D, NC-240CS, RAO, RBH, NC-100ASD and any Airway Communication Receivers to this e-mail:

     WHRM: NC-100 Series Serial Number LOG info
 

Serial Number Locations: Generally, on the early versions of the receivers the serial number is stamped into the chassis top usually in front of the antenna terminal insulator between the rear of the tuning condenser and the front of the antenna terminal insulator. The serial number will appear upside down if you look at it from the front of the receiver with the lid lifted. See the first photo to the right for NC-100 Series SN location as viewed from rear of chassis. When the larger rear mounting flange was used on the tuning condenser, the serial number was moved closer to the antenna terminal insulator and more towards the audio output tubes. See photo far right for location on the NC-200 Series receivers.

Sometimes serial numbers appear only on the data plate of the specific receiver. This applies to the Airport receivers with their serial numbers only on data plate mounted on the front panel. During WWII serial number locations vary. The NC-100ASD has the three-digit serial number stamped into the chassis between the 6C8 tube and the 6F8 tube.   >>>


photo above: SN location on NC-100, NC-101X, NC-100A and NC-101XA (pre-WWII receivers.) SN 40X is an NC-101X. This shows the later polystyrene Ant-Gnd insulator.


photo above: SN location on the NC-200 receiver showing that the placement of the stamped SN has been moved because of the larger mounting flange used on the newer tuning condensers.

 

>>> Generally, the RAO and other WWII USN receivers will have the SN located on the data plate only. Post-war, with the NC-240 series, the serial number is stamped on the top of the chassis on the right side edge midway back. Some early NC-240D serial numbers are located on the top of the chassis at the left rear corner. Note that with the later NC-240D, the serial number is a seven digit sequence that incorporates the production run identification into the first three digits and the receiver identification into the last four digits.

NC-100
- 334-D (thin dividers TC*, brass hubs), 341-F (rack mount, no alum overlay on panel, eye tube),

NC-100X - 217-D (reworked from NC-100 at Nat'l), 227-D, 257-D, 435-D (has original rack mounting brackets), 216-E (early TC),  253-E, 99-J,

NC-101X - 78-D(? - ham mod'd with meter), 62-E(mod'd at Nat. with S-mtr), 217-F (eye),
181-G (eye), 365-G (S-meter - upgrade?), 490-G (eye), 541-G (eye), 247-J (S-mtr), 130-M (S-mtr), 101-N (white scale S-mtr), 141-N (white scale S-mtr),184-N (S-meter, thin dividers TC, brass hubs TC), 211-N (yellow scale S-mtr), 40-X (yellow scale S-mtr, Noise Limiter, later style TC), 162-Y (yellow scale S-mtr, NL)
 

NC-80X, NC-81X -  C-745 (80X),
 

NC-100A - 145-T,

NC-100XA - 58-R (thin dividers TC, no NL),

NC-101XA - 130-M (no NL),

 

*TC = Tuning Condenser - early ones have thin metal dividers between sections and brass hubs
? = conflicting or unexpected data       mod'd = "Nat." is National rework, "ham" is "hamster mods"

NC-200 - C-265(CS), C-536(SA), C-859, C-951, D-327, D-499(SA), D-632(SA),
D-700(parts set), E-39

NC-240CS, NC-240D -  F-104(240CS), F-478 (240D), 169 0061(240D), 183 0045(240D),
183 0080(240D), 183 0145 (240D), 183 0326 (240D),

 

RAO - 10 (RAO-7), 1412 (RAO-3),

RBH -

NC-100ASD - 194, 402, 449(rep'd as A49), 426, 931, 948,

R-115, R-116 - 46(R-116),

 

Airway (Airport) Receivers - 15 (RCD), 40 (RCD), 42 (RCD), 101(RCD), 288 (RCE),
302 (RCE), 234 (RCF), 13 (RCF-2), 182 (RCK-N), 221 (RCK-N), 272 (RCP), 242 (RCQ),
288 (RCQ), 17 (RCR)

Custom AP Rcvrs - 531-G (NC-100 mod'd .2-.4mc on highest range [range A,] separate PS)

 

SA = Silver Anniversary NC-200

 

Production and Engineering Changes

1936

NC-100 and NC-100X introduced in August 1936

Aluminum sheet metal overlay for front panel is silk screened in black and red along with silver - used on both NC-100 and NC -100X

PW-D micrometer dial is bluish-gray on the Index Dial and red with white numbers on the Number Dial on NC-100. PW-D is light gray Index dial and black with white numbers on
Number Dial on NC-100X

Chassis is painted gray on both NC-100 and NC-100X

Power transformer top cover has four large ventilation holes

6E5 cathode ray tuning eye tube is used

Rack mount versions will not have aluminum overlay, panel is 3/16" aluminum with engraved markings, black wrinkle finish, eye tube used. Some versions may have single-ended audio output. Some rack mounted versions will use National supplied rack mounting brackets that mount to a standard table cabinet thus allowing rack mounting.

The first NC-100 and NC-100X receivers were built in production run "D"

NC-101X is introduced with ham band only coverage, black PW-D on most of production, eye tube on early versions (NC-101X introduced during "E" production run, probably Oct-Nov 1936)

1937

Contracts for Airport "Communication Receiver" from Dept. of Commerce, Bureau of Air Commerce Air Navigation Division. DOC-BAC receivers are the RCD and RCE

S-meter replaced eye-tube in NC-101X - changeover to S-meter seems to have occurred between runs J to L (runs six, seven or eight, or around mid-1937.) Some earlier NC-101X receivers seem to have been modified to replace the eye tube with an S-meter. Some of these upgrades were obviously rework performed at National and exhibit professional-level installations. Others appear to be "hamster" modifications. For example, NC-101X sn 247-J is a sixth production run receiver but has the later Marion Electric yellow scale S-meter installed instead of an eye-tube or the earlier white face Marion Electric meter. Since some hams sent their receivers to National for repairs or upgrades, it's possible that early production run receivers that have yellow scale S-meters may have been replacements that were installed by National during a repair. It's also possible that late-style replacement S-meters were obtained from National were installed by receiver owners. In general, on original receivers that haven't been reworked, expect to see the white face S-meters from around run J up to around run N. The yellow face S-meter should be installed on receivers from around run P up to around run X (probably early-1938 up to 1940.)

Power transformer top cover vent holes eliminated   

Installation of the S-meter switch eliminated the green pilot lamp so all S-meters were illuminated to provide a "power on" indicator. All stock S-meters were manufactured by Marion Electric.

NC-80X and NC-81X AC-DC receivers introduced. Optional power transformer available to convert to AC-only operation. Also a battery-operated version was available.

1938

NC-100A and NC-100XA introduced in June 1938. New direct-read dial, articulated pointer, illuminated S-meter, cabinet height increased to 10.5", weighted tuning knob on early versions.

Contracts for "Airway Communication Receivers" begin - the U.S. Civil Aeronautics Authority, the CAA, was formed in 1938. CAA receivers begin with the RCF-2 version.

NC-101XA introduced. Both versions (direct read dial or PW-D micrometer dial) are available up around mid-1940.

Some models will be found with fiber board, blade screw terminals Antenna and Ground connections. These probably replaced the problem-prone push button terminals found on earlier models.

1939

Noise Limiter circuit added to NC-100A versions. 6F8G (1stAF/AVC Amp) and 6C8G (Detector/NL) replaced the 6C5 (Detector) and 6J7 (AVC Amp) tubes. The Noise Limiter was also incorporated into the NC-101X receivers along with the tube changes. NL available November 1939. On NC-101X with NL, the control is between the RF Gain and the Band Change knob where the National NC diamond logo was engraved. The "NC" diamond logo is relocated on these NL receivers to the upper right corner of the front panel.

By late-1939, NC-101X receivers will use NC-100XA chassis but are built as NC-101X receivers. That is, all holes needed for the "A" direct-read dial are present on the NC-101X chassis although not needed or used. Usually, one side of the chassis will have "101-X" written in orange grease pencil to ID chassis so assemblers know not to install the "A" dial and "A" gear box.

NC-80X and NC-81X not produced after 1939. NC-80X cut-off may be 1938. Neither receiver sold well and examples are rarely encountered today.

1940

Tuning condenser design changed to replace large bakelite insulator plates with smaller round insulators, thin metal dividers replaced with thicker metal dividers that now support the rotor contact insulators, rotor plate hub changed from brass to steel, support rods dimensions reduced, two screw rear mounting flange. Cost reduction? HRO tuning condenser went through similar changes.

Antenna/Ground thumb-screw terminals with Polystyrene insulator replaces the old "push terminals" with fiber board insulator or the fiber board, screw terminals found on some models.

NC-101X production stops before May 1940. The direct-read dial version - NC-101XA - production may have continued until the NC-200 was incorporated into production (Sept-Oct.)

When the two-screw rear mounting flange was used on the tuning condenser, the serial number location had to be moved to the left side of the antenna terminal insulator (nearer to the audio output tubes.)

NC-200 introduced (October) - features both general coverage and band spread coils within the catacomb thus eliminating the need for separate "ham bands only" receivers like the NC-101X or NC-101XA.

NC-200 Crystal Filter changed from variable condenser Selectivity control to six-position switch with five selectivity positions and "off" position. Crystal at 455kc to match new IF frequency.

LO tube changed to a 6J5 triode in NC-200 Series. Many other tube changes in the NC-200, see tube line up in section below "Vacuum Tubes Used in All NC-100 and NC-200 Series Receivers."

Dec. 1940 QST ad for "Silver Anniversary" NC-200. Special NC diamond insignia on receiver and matching speaker. Also, receiver equipped with brown knobs and brown S-meter case. All round control nomenclature plates, the tuning knob's skirt and the Silver Anniversary NC diamonds are finished in "gold tone." The Silver Anniversary NC-200 celebrated QST's 25th year and wasn't associated with any National anniversary.

1941-45

RAO USN versions - RAO, RAO-1,2,6,7 & 9 built by National, RAO-3,4 & 5 built by Wells-Gardner. RAO-2 through 9 have double preselection. All versions have .54mc to 30mc coverage. Early versions have 500 Z ohm audio outputs, RAO 7 & 9 have 600 Z ohm audio output. RAO-2 thru 6 are 17.5" wide, RAO-7 & 9 are 19" wide. RAO-7 & 9 don't have S-meters installed but have an output for panadapter use instead. RAO-7 & 9 had unique molded skirt bar knobs. Gears used in the gearbox are changed from brass gears to cast pot metal gears for wartime material conservation. Design pre-dates WWII and the dual preselection upgrade may have been available to USN mid-1941. R-116 USCG receiver from May 15, 1941 contract has dual preselection incorporated identically to the RAO-2 receiver.

RBH USN version of NC-100XA with special frequency coverage of 300kc to 1200kc and 1700kc to 17.0mc, IF changed to 1500kc, single ended audio, 500 Z ohm output. Later versions, probably starting with RBH-2, added an extra RF amplifier similar to the RAO and for similar reasons. It's likely that the RBH series follows the same evolution that the RAO did.

NC-100ASD Signal Corps version, ca: 1943 - replaced AM BC coverage with 200kc to 400kc, single 6V6 audio output with 500 Z ohm output transformer. Serial number is stamped into the chassis between the 6C8 and 6F8 tubes. Serial number is three numerical digits. Highest reported SN is 948. The ASD may have been a replacement for NC-100ASC (aka AN/GRR-3) that might have been a "militarized" NC-100XA.

NC-200FG, R-115 (USCG), NC-240C and NC-240CS produced during WWII. These receivers do not have band spread function. The CS version had 200kc to 400kc band to replace the AM BC band. The R-115 has no crystal filter, no tone control, no S-meter and no logging scale. Covers 200kc to 400kc and 500kc to 18mc.

NC-100A, NC-100XA and NC-200 offered in the National section of the 1945 Radio Amateur's Handbook. None of these receivers, or any other items listed, were actually available during the war and each page of the catalog states "Priorities are required for all products in this catalog until otherwise released by the War Production Board." The intention of the catalog was to show what National was probably going to have available after the war ended.

In 1945, Schuttig & Co. modified RCL and RCK Airway receivers into RCP Airway receivers. The RCK-N built during WWII used a 1560kc IF and tuned 200kc to 800kc and 2.5mc to 23.5mc in five tuning ranges.

1946-49

NC-240CS sold to the civilian market in post-war 1945 as a 1946 model. In 1946, National designated the CS version as a "commercial receiver"

NC-240D returned the band spread function and was sold from early-1946 up to 1949. Note in SN Log that F-104 is a CS model w/o BS and that F-478 is a D model with BS. Both receivers are from the same production run "F" which indicates that this change was rapidly incorporated. Early NC-240D dials are similar to NC-200 dial with all four Band Spread scales near the center of the circular arc of the scales. Later NC-240D dials will have the Band Spread scales alternating with and located above the General Coverage scales on bands D, C, B and A.  

In 1948, National Electrical Machine Shops (NEMS) modified RCL and RCK Airway receivers (or parts from these types of receivers) into RCQ Airway receivers

In 1948, some NC-240CS receivers were produced as RCR Airway Receivers. The RCR is the last National Airway receiver produced based on a Moving Coil receiver design.

 

Coil Catacomb Details

The "Moving Coil" Catacomb - Just as the plug-in coil sets and the PW gear drive and tuning condenser are the "heart" of the HRO design, the "Moving Coil" Catacomb along with its N-PW gear drive and tuning condenser are the "heart" of the NC-100 Series. The Coil Catacomb is a two-piece cast aluminum box containing fifteen tuned coils and their respective trimmer variable capacitors. Each coil is inside a singular chamber that completely shields all of the coils from one another and from the rest of the receiver circuitry. Each coil assembly is mounted with two screws accessible by removing the catacomb cover. The coil insulator blocks each have five short, stubby pins that make the contact to the tuning condenser section to complete the tuned circuit. Each coil assembly is identified with a letter-number combination to assure proper installation. The insulator material is National's brown bakelite mixture called R-39, noted for its superior quality over standard bakelite. Robust in construction, the Coil Catacomb assured that the tuned circuit alignments stayed in adjustment and that receiver stability was solid and consistent of long periods of time. The engagement contacts are located in R-39 insulator blocks that are mounted under the tuning condenser. This allows the connections from these contacts to be very short and accomplished with TC wire. The contacts are dual entry in design allowing the Coil Catacomb pins to enter from either side. This allows the bands to be changed from any direction.
 The two-piece engagement contacts are insulated from each other and when the connections are soldered on the top side (under the tuning condenser) both sides of the upper part of the contact are soldered together. The exception is the foremost LO contact. Since the LO only requires four connections, the unused contact is wired to act as a screen voltage switch for the RF and IF amplifiers to eliminate noise during band changes on the early models of the receiver. Later models used this contact for the RF amplifier cathode return to "mute" the receiver during band changing. The almost "sealed" nature of the catacomb construction generally protects the coils and trimmer capacitors from damage. Nowadays, most catacomb coils are still in good condition even if the rest of the receiver has not faired so well.

NC-100 Coil Assembly Identification - Each coil assembly is marked with a letter and a number that identifies its frequency of operation and its function in the circuit. Coil assembly identification will either be engraved or ink-stamped, depending on when it was manufactured (see photo right.) Coil sets A through E are generally found in any early single-preselection catacomb receivers that tune .54mc to 30mc. Coil sets A through D and coil set H are found in National Airport receivers. Some NC-100ASD receivers will have G coils in place of H coils. NC-200 Series coil assemblies are similarly identified.

A = 14mc to 30mc    A1 =  RF AMP,   A2 = MIXER,   A3 = LOCAL OSCILLATOR

B = 6.5mc to 14.5mc  B1 = RF AMP,   B2 = MIXER,    B3 = LOCAL OSCILLATOR

C = 3.2mc to 6.5mc   C1 = RF AMP,    C2 = MIXER,    C3 = LOCAL OSCILLATOR

D = 1.3mc to 3.2mc   D1 = RF AMP,    D2 = MIXER,    D3 = LOCAL OSCILLATOR

E = .53mc to 1.3mc    E1 = RF AMP,    E2 = MIXER,    E3 = LOCAL OSCILLATOR

F = .49mc to 1.04mc  F1 = RF AMP,    F2 = MIXER,     F3= LO  - NC-200 Series only

G = 200kc to 400kc   G1 = RF AMP,    G2 = MIXER,    G3 = LO  - Some NC-100ASD

H = 200kc to 400kc   H1 = RF AMP,    H2 = MIXER ,   H3 = LO  - Airport Rcvrs, NC-100ASD

NC-101X Coil Assembly Identification - Since the NC-101X tunes ham bands only and band spreads each range, slightly different coils and trimmers were required to work with the "band spread" main tuning condenser. The identifications on the NC-101X coil assemblies use only numerals and are tied to the ham bands that are covered. Therefore, 16 denotes 160 meters while 8 denotes 80 meters and 4 denotes 40 meters, 2 denotes 20 meters and 1 denotes 10 meters. As with the NC-100 coils, suffix numbers 1, 2 and 3 denote RF (1,) Mixer (2) and LO (3). Thus, 16 1 is the 160 meter RF coil or 4 3 would be the 40 meter LO coil. As with the NC-100 coils, the identifications are stamped into the insulator-base. (see photo right.)

16 = 160 meters     16 1 = 160 RF AMP,   16 2 = 160 MIXER,    16 3 = 160 LO

8   =   80 meters       8 1 = 80 RF AMP,       8 2 = 80 MIXER,        8 3 = 80 LO

4   =   40 meters       4 1 = 40 RF AMP,       4 2 = 40 MIXER,        4 3 = 40 LO

2   =   20 meters       2 1 = 20 RF AMP,       2 2 = 20 MIXER,        2 3 = 20 LO

1   =   10 meters       1 1 = 10 RF AMP,       1 2 = 10 MIXER,        1 3 = 10 LO

NC-80X Coil Assembly Identification - The NC-80X (and the NC-81X) uses a smaller catacomb since the receiver doesn't have an RF amplifier section. The catacomb is driven into position using a chain drive mechanism rather than the rack and pinion used for the NC-100 series catacombs. Only two coil sets per band are used, those being a Mixer coil assembly and a LO coil assembly. Since the IF used in both receivers is 1560kc, both Mixer and LO coils are somewhat different than the NC-100 coils. Each NX-80X coil assembly is identified using the letters W, X, Y and Z for frequency ranges and the numerals 1 or 2 for function. The assignment of the highest letter to the highest frequency coil set departs from National's usual assignment of the letter A to the highest frequency coil sets. But then, the entire NC-80X coil catacomb is entirely different from the NC-100 catacomb.

As an example, coil assembly X2 would be for tuning range 1.7mc to 4.6mc and for the LO function. The identification is stamped in the base of the coil assembly.
 

W = .55mc to 1.5mc          W1 = MIXER       W2 = LO

X  =  1.7mc to 4.6mc          X1 = MIXER        X2 = LO

Y  =  4.3mc to 12.0mc        Y1 = MIXER        Y2 = LO

Z  =  11.4mc to 30.2mc      Z1 = MIXER         Z2 = LO

 

Shown in the photo to the right is the NC-80X coil assembly Y2 which is for the 4.5mc to 12.0mc band and is the Local Oscillator coil function. Note the wire loop inside the coil form that is used for the low-end of the dial tracking adjustment. The air variable trimmer is for the high-end of the dial tracking adjustment. Unlike the cast aluminum cover for the catacomb used in the NC-100 series, the catacomb cover for the NC-80X is aluminum sheet metal.
 

More Coil Assembly Identification Confusion - Coils sets employed in the later RAO and RBH receivers with double preselection will have four coil sets per band. They are numbered 1 though 4 to identify the function and with letters to identify frequency coverage. The RAO bands are A through E and cover 30mc down to .54mc. The RAO coils for the 1st RF Amp covering 14mc to 30mc would be A1 and the LO for the same band would be A4.
The RBH also has its bands identified as A through E but bands E and D cover different frequencies than E and D on the RAO. The RBH coil assemblies are also identified as A through E even though they are different from the RAO coil assemblies - they are identified just like the RAO receiver's coil assemblies. RAO IF is 455kc and the RBH IF is 1500kc which means the LO and Mixer coil assemblies are different. In fact, band D and E LO assemblies both have padder capacitors on the RBH coil assemblies. The RCK-N Airport Receiver has two LF bands, 200kc to 400kc and 400kc to 800kc. These coils are identified as "H" and "G" respectively, while the remaining 2.5mc to 23.5mc coils are "C," "B" and "A" although they cover slightly different frequencies than the RBH coils or the RAO coils. As far as any markings other than the letter-number ID, there aren't any.

Shown in the photo to the right are the RF amplifier stage coils for the RCK-N receiver. Note that the 200kc to 400kc coil is identified as H1. The 400kc to 800kc coil is identified as G1. The 2.5mc to 5.5mc coil is identified as C1.

Just to keep the confusion going, National decided to change the whole band ID concept of letter A designating highest frequency band with the WWII RBL Regenerative LW Receiver. Although not a catacomb receiver, it was generally paired with and looks very similar to the RAO receiver. With the RBL, band A is the lowest frequency band (15kc up to 30kc) and band E is the highest frequency coverage (about 270kc up to 600kc.)

On the NC-200 and NC-240 receivers, with their very different coil assemblies that included four sets that had a bandspread function, still the letter-number ID remains the same. That is, the highest frequency coverage band is ID'd as Band A and the associated coil assemblies are also ID'd as A1, A2 and A3. NC-200 and NC-240 coil assemblies have seven stubby pins to provide for the bandspread function on 80M, 40M, 20M and 10M. Since the NC-200 and NC-240D receivers have six general coverage bands, the coil assembly letters are A, B, C, D, E and F. The four bandspread bands are derived from coil assemblies A, B, C and D. The A, B, C and D local oscillator coil assemblies have three trimmers and an inductance loop for adjusting the tracking on both the general coverage band and the ham band bandspread.

National apparently only wanted the coil assemblies within the individual catacomb to be identified for proper placement for the particular model receiver. It was probably thought "why have several different ID combos when nobody is going to be replacing the coil assemblies anyway." If a specific coil assembly was damaged beyond repair a new one could always be ordered from National specific to the model receiver it was for. Nowadays, while it's possible to find receivers that have had some tampering to the coils within the catacomb, it is rare. Most catacombs are in good condition because of the almost "sealed" nature of the housing which results in very few problems with the coil assemblies.

NC-100 Coil Assembly

NC-200 Coil Assembly


Inductance Loop

The individual coil assemblies are mounted into the catacomb with two screws - see photo right. These are 6-32 thread and screw into the catacomb base for mounting (WWII receivers used philip's head screws.) The photo above-left shows how the NC-100 series coil assembly looks - note the five pins. The photo above-center shows the NC-200 style coil with its seven pins. When removing an individual coil assembly or a particular section (coil assemblies 1, 2 & 3 of any letter designator) make sure the catacomb is disengaged from the pin contacts by setting the band switch in between bands. Then loosen the two screws until they are no longer threaded into the catacomb base and then lift out the coil assembly. Reinstall in the reverse and then check engagement by operating the band switch. Early versions (pre-WWII) and post-war versions will use round-head blade screws instead of philip's.

The photo above-right shows the alignment inductance loop that are in most of the NC-100 series coils A, B, C and D for adjustment of the low end of the dial tracking. This not only is used on the LO but also on the Mixer and RF Amp coils. Moving the coil loop slightly will change the inductance allowing the low end to be set exactly. During alignment, by using a plastic rod to manipulate the loop, you can have the receiver operating and set the loop position exactly for proper low end tracking (this wasn't possible on National's HRO receivers.) High end of the dial tracking is adjusted with the air variable trimmer. The lower frequency coils E or G will have a padder compression trimmer to adjust the low end dial tracking on the Local Oscillator coil assembly.

The photo to the right shows the NC-200 catacomb with its six band coverage. Note that there are many more air variable capacitors for adjustment. The two lowest frequency bands have padder capacitors on the LO and the four highest frequency bands have ceramic trimmers for the ham bandspread adjustments. To avoid confusion during alignment, National assigned the same designator for trimmers that performed the same function on each band. For example, the four bandspread RF amplifier trimmers are all designated as C-54 and the four mixer bandspread trimmers are designated C-55. Sounds confusing but it actually works fine, as long as you have the alignment instructions and drawings to guide you. LO coils are at the top, Mixer coils in the middle and RF coils at the bottom. Note LO coils on the four HF bands have three variable caps each. One for General Coverage, one for Bandspread trim and one for Bandspread padder. GC low end uses coil loops.


photo above: NC-200 coil catacomb. Note there are six sets of coils.

 

PW-D Details

PW-D Micrometer Dial - The NC-100 used the same PW-D micrometer dial as the HRO receiver but with different paint colors. Initially, the NC-100 used a PW-D that had the Index Dial painted a bluish-gray and the Number Dial was painted red with white numerals. This rather dramatic color combination went well with the "deco" styling of the metal overlay panel of the NC-100. Interestingly, for the NC-100X, which was supposed to be the ham version of the receiver, the PW-D Index Dial was painted light gray and the Number Dial was painted black with white numerals. This seemed to be a more conservative appearance that would appeal to the hams. When the NC-101X version was released, National wanted the receiver to appear like the HRO, so the standard HRO Senior PW-D was fitted to that receiver. Some of the advertising photographs of the NC-101X show it with the light gray PW-D installed but it looks like most of the actual production used the black PW-D, like the HRO Senior. Though "black" describes the color of the PW-D index dial in normal room lighting, when in sunlight or any bright light, the index dials will vary in color from a gunmetal gray-black to and olive-bronze black. The colors are usually only noticeable in very bright light (like a "flash" photograph.) The gray PW-D with black Number dial was standard for all Airway Receivers.
PW-D Mechanics - Internally, all PW-D dials are the same and work by rotating the Number Dial on an elliptic hub. The Number Dial is gear-driven by the Index Dial that is also set-screw coupled to the main tuning shaft of the N-PW gearbox drive. There are two moving parts (Index Dial and Number Dial) and seven stationary parts that make up the micrometer dial. The Index Dial has five windows with ten index marks between each window. The Number Dial displays sequential numbers that increment by 10 as each window comes into view at the top of the dial. The Number Dial has 0 to 500 numbers incremented by 10 and each revolution of the Index Dial increments the numbers by 50 (five windows viewed with each revolution.) With ten revolutions total the numbers displayed are 0 to 500 incremented by 10. Since the Index Dial is 4.625" in diameter, its circumference is 14.53" and with ten revolutions the total span is 145.3" or just over 12 feet (12.1'.) On the NC-101X receivers, the ham bands each span 50 to 450 on the PW-D. This is 80% of the full 500 divisions thus the ham bands equate to a linear dial that is 9.68 feet long.

As to the two moving parts (Index Dial and Number Dial) and seven stationary parts - besides the Index Dial and the Number Dial there are two springs that hold the two dials together, three flat-head screws to mount the HRK-style knob to the Index Dial and a single set screw on early versions. During WWII the set screws were increased to two. Elegant in its simplicity, about the only thing that ever goes wrong with a PW-D is caused by technicians who carelessly disassemble the dial and then reassemble it incorrectly.

photo above: The early NC-100 PW-D

 

PW-D Micrometer Dial Servicing - NC-100(X), NC-101X, Airport Receivers

The PW-D Micrometer dial only has two moving parts and seven stationary parts. Elegant in its simplicity of design, it is very easy to work on if you know the "tricks." About the only thing that goes wrong with the dial is that it becomes noisy in operation and might have a rough feel when tuning. Synchronization might be a problem too - if the dial was reassembled incorrectly. The inner dial has gear teeth cast into its inner perimeter and also has the bearing that fits on the elliptic hub of the tuning shaft bearing housing. The outer dial has mating gear teeth that are cast into a rim projection and a hub that is set screw attached to the tuning shaft. By having the inner dial ride on the elliptic hub cast into the tuning shaft bearing housing, the gears of the two dials engage and as the outer dial is turned, by way of the mating gears, it turns the inner dial and since the outer dial is coupled to the tuning shaft, it also moves the tuning condenser. The elliptic hub is actually round but since the tuning condenser shaft exits the hub "off-center," the hub bearing surface appears as an ellipse to the tuning condenser shaft. This eccentric placement forces engagement of the inner and outer dial gears, thus driving the inner dial with the rotation of the outer dial.  >>> >>>  To disassemble the Micrometer dial, first tune the receiver to "250" then loosen the dial set screw. The dial should come off of the elliptic bearing hub and tuning condenser shaft with very little effort. With the PW-D dismounted, look at the back of the dial and, if the PW-D was synchronized prior to removal, you will notice that the three screws that hold the knob to the dial appear through cast holes in the inner dial. Also note that the two springs that hold the inner dial to the outer dial are pointing straight up. Remove the knob. Then remove the two springs - their ends fit into slots at the end of the shaft hub of the outer dial (where the set screw is.) Now the dials can be separated. Examine the cast gear teeth for condition. Usually they are in good shape and all that is present is old dried grease. Use a tooth brush and light oil to remove all of the old grease and then apply a light coating of Lubriplate (or any modern light weight grease) to the inner dial gear only. Position the inner dial to the "250" position with the cast holes lined up for the knob screws. Then install the two retaining springs. >>>
>>>   Before putting the PW-D back on the receiver, lightly grease the elliptic bearing hub (since the inner dial rides on this it should be clean, smooth and lightly greased.) The PW-D should already be set to "250" and the receiver condenser should not have been moved. Place the PW-D back on the receiver by carefully noting how the elliptic hub requires the inner dial to be slightly down from center. If you have assembled the PW-D correctly, it will just slip right on to the hub. Tighten the set screw and test the operation - it should be ultra-smooth and very quite in operation. If your PW-D dial doesn't just slip onto the elliptic hub then it isn't assembled correctly. It can go together two ways, one correct and one 180 out. If your PW-D doesn't just "slip onto" the bearing hub, read the section below "Correct Assembly of the PW-D Micrometer Dial - Details."

Around the beginning of WWII, a guide pin was added to the inner dial, located just above the inspection/grease hole. This pin limits the vertical movement of the inner dial and was probably installed to keep the gear alignment fairly tight and allow easier installation of the PW-D onto the elliptic hub. Later production dials will have this guide pin molded in the casting of the inner dial.

 

photo left: Two moving parts and seven stationary parts are all that are used in the PW-D. A second set screw was used during and after WWII - total of eight stationary parts then.

Correct Assembly of the PW-D Micrometer Dial - Details - When the PW-D is assembled correctly, it will just slip onto the elliptic hub. If you are trying to force the PW-D onto the hub, then the PW-D is assembled incorrectly. There are two ways that the outer index dial can interface with the inner number dial. One is correct and one isn't. Look carefully at the photograph to the right noting the location of the smaller round hole, the two oval holes, the "NATIONAL CO., INC." embossing and the location of the screws that are for mounting the knob. If you orient the reverse side of your assembled PW-D in this position and then look at the outer dial, you should see "250" centered in the window that is directly behind the "NATIONAL CO., INC." embossing.

To check your assembly,...position PW-D as shown in photograph to the right. You should see the following:

1. "NATIONAL CO., INC" embossing should be at 12 o'clock

2. Small round hole should be at 9 o'clock

3. Two oval holes should be at 2 o'clock and 4 o'clock

4. Note that all three knob screws are directly centered in the small round hole and the two oval holes

5. Turn dial over from right to left and you'll see "250" centered in the window at 12 o'clock (the window should be directly behind the embossing "NATIONAL CO., INC" on the back of the inner dial.)

6. Push the inner dial so that the "250" appears closer to the bottom of the window. You might hear a slight "click" as the inner dial gear teeth mesh with the outer dial gear teeth. You may only "feel" the inner dial mesh with the outer dial.

If your assembled PW-D meets this criteria, then it will slip right onto the elliptic hub. Sometimes even though everything is assembled correctly, the PW-D still won't fit onto the elliptic hub. With the PW-D removed, push the number dial into the gears of the index dial while very slightly moving the position of the number dial. You'll probably hear a "click" as the gears mesh correctly. Check that the "250" is still centered in the index dial window. Now the PW-D should slip onto the elliptic hub easily. Sometimes, instead of hearing  the "click", you'll just "feel" the correct meshing of the gears. You must have the gears meshed before the PW-D will fit onto the hub easily.



photo above
: Correctly assembled PW-D micrometer dial looking at the back side.

Dismounting a Stubborn PW-D (it's not the same as an HRO) - Sometimes removing the PW-D Micrometer dial proves to be a very difficult operation. Almost always, the difficulty is caused by excessive marring of the tuning shaft by the knob set screw(s.) If everything seems it indicate that the PW-D is not coming off of the shaft no matter what, this is the procedure necessary to remove the dial. First, you'll need a couple of right-angle screw drivers. I usually have to make them because everything commercial is too big to fit between the back of the front panel and the tuning shaft elliptic bearing hub screws. Make at least two that have different angles for the blade engagement. You'll also have to add a slight "S" bend to the shaft of the angled screw drivers. This applies only to the NC-100, NC-100X, and NC-101X because these receivers are in cabinets that have a non-removable lip that impedes easy access to the hub screws. If the receiver is a rack mount, e.g., Airport receiver, then there is ample access and a straight shaft angled screw driver can be used.

Now, using the right-angle screw drivers, loosen and remove the four screws that mount the elliptic bearing hub. You'll have to use a small dental mirror to see the screws for proper blade engagement. It's difficult but it can be done. The bearing hub is just slightly spring loaded with a small dual leaf spring so when the last screw is removed the hub will just slightly come forward. Now, remove the entire PW-D and the bearing hub and the main dial drive shaft out through the round hole in the receiver's front panel. Now, remove the knob from the PW-D assembly. This provides access to the front of the tuning shaft. Then, using a soft metal drift, gently tap out the shaft from the PW-D dial assembly.   >>>

>>>   You will have to reassemble the gear box and set the anti-backlash per the instructions in the "Servicing the NC-100 Series Gear Box" section above. Also, be sure to dress down the marring on the tuning shaft so the next time the PW-D has to be removed, it will be an easy job. 

With the NC-100A Series, the main tuning shaft diameter was greatly reduced - from 5/16" diameter to 3/16" diameter. The smaller tuning shaft diameter doesn't seem to have the marring problems that the large, early style, PW-D shafts experience. Perhaps the material is harder since the diameter was reduced. Also, you'll find that the NC-100A Series receivers don't need an elliptic hub bearing. Generally, tuning knob removal on the later receivers is easy and problem-free.

NOTE: When remounting the bearing hub note that "TOP" is embossed on one of the flanges. This flange obviously must be mounted up. This is important because it positions the elliptic bearing correctly for the PW-D dial.

 

Servicing the NC-100 Series Gear Boxes

The gear box used in the NC-100 Series of receivers was designated as the PW-N. This gear box is substantially different from the box used in the HRO receiver although both require an elliptic hub to allow use of the National PW-D Micrometer dial. The gear box is accessed by removal of the cast metal cap with the "NC" diamond embossed on it. Four fillister head screws secure the cap to the gear box. >>>



photo above
: Early style gear box cap

>>>  The NC-100 gear box uses a main tuning shaft to which is mounted  a small drive gear which simultaneously engages two larger gears that are mounted in bearings that are located in the back and front of the gear box housing. The left side gear is able to rotate on its shaft and is spring loaded so it acts as the anti-backlash gear. At the rear of each of the two larger gears are longer small gears that simultaneously engage a single large condenser drive gear. The thrust pin bearing of the main tuning shaft enters into a socket bearing that is in the end of the condenser tuning shaft (provides rear support and some thrust action.) A small dual leaf spring also presses against the flange on the main tuning shaft to reduce end play. With much smaller gears being employed and with the reduction used in the gearing, the NC-100 gear box has a very "light" feel and is much easier to turn than the HRO box.


photo above: Inside the early NC-100 gear box

In the early gear boxes that operate with the PW-D micrometer dial an elliptic hub is integral to the main tuning shaft bearing. Since the elliptic must be properly mounted, "TOP" is embossed on one of the mounting flanges so that correct installation is easy. Shown in the photo to the right is the elliptic hub used in the NC-100 versions as viewed through the port in the front panel with the PW-D removed.



photo above:
The Elliptic Bearing Hub as viewed thru the front panel port on a rack mount RCF Airport receiver.

Cleaning and Lubrication -  The early gear box has a cast metal top cap that is mounted with four fillister-head screws. When this top cap is removed, the gears are accessible. Most of the NC-100 gear boxes don't have an excessive amount of dried-up grease inside but they will need thorough cleaning and re-lubrication for smooth operation. I usually use WD-40 applied with an acid brush to dissolve and loosen the dried grease. The old grease and residual WD-40 is then removed with a "dry" acid brush and a cotton cloth rag. You probably won't be able to remove 100% of the old grease but it really isn't necessary. Apply a high quality modern light grease to all of the gear teeth. Check the operation which should be very smooth and easy to rotate the tuning condenser via the front main tuning shaft.

Setting the Anti-Backlash - If you have to remove the elliptic hub you will unload the anti-backlash spring located on the left side drive gear. The anti-backlash setting will determine how "lightly" the gear box "feels" when tuning. A good setting is about four or five teeth. You'll feel the anti-backlash spring load at about two teeth but you do need a bit more load for proper operation. Too much load will cause gear wear and a "heavy" feel to tuning. Too little load will cause backlash and a very light feel to tuning. You'll have to hold the right side gear in place and then, using your other fingers, hold the anti-backlash gear in place while you insert the main tuning shaft and elliptic hub assembly. Besides engaging both large gears, the end of the main tuning shaft has a small thrust end pin that fits into the thrust socket bearing located in the end of the condenser drive shaft, so be sure that the assembly is fully seated. You'll feel a slight pressure being exerted by the dual leaf spring while you hold the bearing hub in place. Be sure that the "TOP" flange is up and then install the four mounting screws.

NC-100A Versions - Gear Box Details - With the change to a direct readout tuning dial, the gear box used on the NC-100A versions required a few modifications. First would be the elimination of the elliptic bearing hub. Since a PW-D was not used there was no need for the elliptic part of the hub. The new tuning dial featured a tracking "band in use" articulated pointer that used a rather large mechanism to support the pointer. Note in the photos right and left that a large cast metal arm is mounted to the tuning condenser shaft. This arm is cast to have a bend that goes over the gear box and then is attached to the disk of the articulated pointer mechanism. This disk also "rides" on a bearing that is part of the main tuning shaft bearing hub (which isn't elliptical anymore.) To remove the cap, which is now a flat metal piece with the "NC" diamond embossed on top, the pointer mechanism arm must be moved to one side of the other of the gear box top (tune the dial to minimum or maximum frequency on any band.) When the cap is removed, it will be noted that the NC-100A version gear box is almost the same as the earlier gear boxes. The two reduction gears that are driven by the main tuning shaft have been moved slightly so that the right side gear is mounted somewhat higher than the left gear. Otherwise the gear box is identical to the early versions. Servicing is the same as for the early box.
NC-200 and NC-240D Versions - Although there appears to be a gear box in these receivers, seeing the photo to the left reveals that the tuning shaft merely passes through the box and drives the tuning condenser. The gear box housing is merely acting as a bearing for the tuning shaft. The tuning of these receivers works on the large diameter fiber disk which is rim driven.

RAO and Other Military Versions - The RAO-2 and later versions will not have the articulated pointer mechanism since this feature was eliminated to simplify the construction and maintenance of the receiver. The cast metal arm that carried the articulated pointer mechanism was removed. To further strengthen the dial itself a bracket is mounted between the top of the gear box and the back of the main dial. You will notice that the RAO dial will have two screws near the top of the dial that are for mounting the bracket. To access the gear box on an RAO will require removal of the tuning dial bezel. This will require removing the associated knobs and control mounting nuts (if used.) With the tuning dial bezel removed, you'll have access to the two screws that mount the top part of the bracket. The mating nuts are located on the backside of the dial. Once this part of the bracket is dismounted, then the two screws that mount the lower part of the bracket to the gear box cap can be removed. The gear box cap is held in place with two hex head screws that are on the sides of the gear box. Loosen the hex head screws to remove the cap. To see how this bracket is attached, reference the photo of the RAO-3 chassis above in the section "WWII Versions." Once the gear box cap is off the remainder of the RAO gear box servicing is just like the NC-100A gear box. The early RBH gear box is similar to the NC-100A while later versions are like the RAO gear box. The NC-100ASD gear box is similar to the NC-100A box. Later WWII gearboxes will have cast pot metal gears rather than brass gears.
 

Vacuum Tubes Used in All NC-100 and NC-200 Series Receivers

Unlike the HRO receiver that continued to use glass tubes with six and seven pin bases up into WWII, the NC-100 was introduced utilizing octal base tubes with the majority of those tubes being metal tubes. The eye-tube, the audio output tubes and the rectifier tube are the only glass tubes used and the rectifier and the eye-tube are the only non-octal based tubes.

It's interesting that James Millen published a QST letter in 1937 that seemed to condemn the 6.3vac glass tubes used in the HRO in favor of using the even older 2.5vac tubes and yet here was the NC-100 sporting the latest types of tubes available. Many speculate that over-stocking of older parts was the reason for the position that National had on the HRO, although maybe National didn't want to change the design of an obviously superior performer like the HRO. Eventually, in 1945, the HRO was produced with octal tubes and the performance wasn't diminished at all (HRO-5.)

NC-100,100X,101X:  RF = 6K7, Mixer = 6J7, LO = 6K7, IF1 = 6K7, IF2 = 6K7, Det = 6C5, AVC Amp = 6J7, BFO = 6J7, P-P AF Output = 6F6(2), Eye Tube = 6E5, Rect = 80

NC-100, 100X (w/NL):  RF= 6K7, Mixer = 6J7, LO = 6K7, IF = 6K7(2), Det/NL = 6C8, BFO = 6J7, 1st AF/ AVC = 6F8, AF Output = 6F6(2), Rect = 80

NC-100A,100XA:  RF= 6K7, Mixer = 6J7, LO = 6K7, IF = 6K7(2), Det = 6C5, AVC = 6J7, BFO = 6J7, P-P AF Output = 6F6(2) Rect = 80

NC-100XA(w/NL) 101X(w/NL,)101XA(w/NL):  RF= 6K7, Mixer = 6J7, LO = 6K7, IF = 6K7(2), Det/NL = 6C8, BFO = 6J7, 1st AF/ AVC = 6F8, AF Output = 6F6(2), Rect = 80

NC-80, NC-81X:   Mixer = 6L7, LO = 6J7, IF = 6K7(3), Det = 6C5, AVC = 6B8, BFO = 6J7, AF Output = 25L6, Rect = 25Z5 (not in "B" models - battery versions)

NC-200:  RF = 6SK7, Mixer = 6K8, LO = 6J5, IF1 = 6K7, IF2 = 6SK7, Det/NL = 6C8, AVC = 6SJ7, BFO = 6SJ7, 1stAF/Phase Inv = 6F8, P-P AF Output = 6V6(2), Rect = 5Y3

NC-240D:   RF = 6SK7, Mixer = 6K8, LO = 6J5, IF1 = 6K7, IF2 = 6SK7, Det/NL = 6SL7, AVC = 6SJ7 or 6V6, BFO = 6SJ7, 1stAF/Phase Inv = 6SN7, P-P AF Output = 6V6(2), Rect = 5Y3

NC-100ASD:  RF = 6K7, Mixer = 6J7, LO = 6J7, IF = 6K7(2), Det/NL = 6C8, 1st AF/AVC = 6F8, BFO = 6J7, AF Output = 6V6, Rect = 5Z3

RAO:  RF1,2 = 6K7(2), Mixer = 6J7, LO = 6J7, IF = 6K7 (2), Det/NL = 6C8, BFO = 6J7, 1st AF, AVC = 6F8, AF Output = 6V6, Rect = 5Z3

RBH:   RF = 6K7, Mixer = 6J7, LO = 6J7, IF = 6K7(2), Det/NL = 6C8, 1st AF/AVC = 6F8, BFO = 6J7, AF Output = 6V6, Rect = 5Z3

Airway Receivers:  RF = 6K7, Mixer = 6J7, LO = 6J7, IF = 6K7(2), Det = 6C5, AVC = 6J7, BFO = 6J7, 1st AF (Squelch) = 6C5, AF Output = 6V6, INS Control = 6J7, Rect = 80

Note on the 6F8G Tubes - It seems that the audiophiles and the eBay tubes sellers have discovered that the duplex triode 6SN7's predecessor was the 6F8G. Consequently, the price of the 6F8G has increased significantly in the past few years. Some eBay descriptions insist that the 6F8G "sounds" better than the 6SN7 (hmmm,...really?) There are even adapters being sold on eBay to adapt the 6F8G to operate in place of a 6SN7. (Incredible!) There must be millions of 6SN7s around since every late-forties to early-fifties TV was loaded with them. The 6F8 was only used in a few applications with the National NC-100XA, NC-200 and RAO receivers being the most often encountered. So why create a pseudo-demand for a tube just because it happens to be a dual-triode and can substitute for the ultra-common 6SN7? The mystery of the audiophile mind and the eBay tube sellers, I guess. Though 6F8Gs can still be found for less than $20 each some of the current prices on eBay are insane with some BINs averaging around $50+ per tube. It is unfortunate that the NC-100A/200/RAO does require that the 6F8G (1stAF and AVC amp or NL in the 200) be in relatively good operating condition. Very weak tubes will distort the audio on strong signals since the AVC can't control the front-end gain of the receiver. It's probably is a good idea to stock-up and have a few spare 6F8Gs on hand - just in case the 6F8G goes the way of many other low-demand but insanely priced "audio tubes."
 

NC-100A, XA, ASD, NC-80X, NC-101XA, RBH - Original Dial Lamp Assembly

Almost all NC-100A receivers encountered will have the original dial lamp assembly damaged or missing. Apparently the fiberboard tube that held the lamp sockets deteriorated at the point where it was held in place with the two moveable retaining clips. This resulted in the dial lamp assembly not being able to stay mounted correctly. Many NC-100A series receiver owners modified the dial lamp mountings using clip-on lamp sockets that were attached to the fiberboard tube retaining clips though other types of modifications were probably also used.

If you want to make a replica of the original NC-100A dial lamp assembly, a photo of an original example from an NC-100ASD is shown to the right. The dimensions are as follows:

Overall Length =  8.375"                Wall Thickness =  ~ 0.016"

Outer Diameter =  0.50"                 Slot Opening Width =  ~ 0.468"

Material is fiberboard that is painted white. The material has to be non-conductive, somewhat flexible and has to be able to withstand the heat from the two dial lamps. That probably would eliminate some types of plastics, especially the types of plastics available around 1938 to 1945. Suitable modern material is difficult to find. Fiberglass tubes are expensive. Most vendors don't sell small in quantities. The small diameter is difficult to find. 

The original lamp sockets were for screw-base lamps. I recommend using the #46 lamps that are listed in the manual as #40 lamps don't provide adequate dial illumination. Be sure to use the low current #40 lamp for the S-meter illumination since a #46 is "hot" enough to really discolor the plastic meter scale.  >>>

>>> When the correct type of fiberboard material is found the tube must be cut to have a full-length slot. As viewed from the ends, the tube should look like a "C" in shape. The dial lamp sockets and wire harness slide into the slot of the tube. You must use screw-base lamp sockets in order to have them slide into the fiber board tube. To mount the lamp assembly, swing out the mounting clips that are riveted to the receiver cabinet top lip. Then place the dial lamp assembly tube in position and swing the retaining clips to engage into each end of the tube. Then push the dial lamp assembly tube into position under the top lip of the receiver cabinet. Make sure that the slot of the tube is facing down so the illumination will be directed down onto the dial scale. You will feel the dial lamp assembly tube securely "fit into place" when everything is correct. Route the wire harness out the left side of the tube and tuck the wires under the cabinet top lip. The wire harness connections are to a dual tie point mounted at the meter retaining plate using the upper left screw. This tie point also has the connections for the S-meter lamp.

I made a dial lamp holder for a NC-100XA using 0.5" OD 0.46" ID semi-rigid, clear Nalgene teflon tubing. Teflon is rated for well-over 400 degrees F. Even though it was clear material, it still needed the slot cut to allow flexing for sliding in the lamp sockets and wiring. It mounts the same as the original and can't be seen when installed.

 

Power Transformers for the "Moving Coil" Receivers

The original power transformers were rated at 100 watts for single tube audio output receivers and 150 watts for receivers with Push-Pull audio output. Under normal operating conditions the transformers were more than adequate to operate the receiver for long listening sessions. However, it's not too rare to find that the power transformer in a Moving Coil receiver has been replaced or is, in fact, defective. It's unlikely that the original transformers were in anyway prone to failure and probably provided service for at least a decade without any problems. As the receiver components aged (especially the paper-wax capacitors) and the load on the transformer increased, if the receiver continued to be used for hours on end, a power transformer failure was probably inevitable and was probably caused by excessive heat build-up.  

Replacement Power Transformers - National used the same basic style power transformer in all of the moving coil receivers. There are two different HV windings used so, essentially, there are two types of power transformers used. The first type is the "NC 100" (part number 10728) that was used in all receivers that had Push-Pull audio output stages. This transformer is usually marked "NC 100 10728" on the bottom of the core. The photo to the upper right shows how the 10728 looks from the bottom. The second transformer was the "NC 100 F" (part number 11028) that was used in all of the single-tube audio output stage receivers. This transformer has "NC 100 F  11028" marked on the bottom. The photo to the lower right shows how the 11028 looks from the bottom.

Today, most restorers "harvest" a usable power transformer out of a "parts set" that's the same model as the receiver they are rebuilding. But sometimes, one has to use power transformer from a similar Moving Coil receiver. Generally, if you use a "NC 100" power transformer in any moving coil receiver with P-P audio, it will provide the correct B+ voltage. If you use a "NC 100 F" power transformer in a moving coil receiver with P-P audio, the resulting B+ will be too low.* Correspondingly, if you use a "NC 100" power transformer in a moving coil receiver with single tube audio, the resulting B+ will be too high. The 6.3vac winding and the 5.0vac winding are the same in both types of power transformers, only the HV winding is different.

Both transformer versions are the same physical size with wires for the primary winding and wires for the HV winding ends but not the CT. All other connections use terminals. Some transformers have three wires for the primary if the receiver circuit allowed selecting high line or low line for the primary. It appears that all transformers had the three wire primary and if not needed the low line wire was cut close to the core (probably by National assemblers.) You may have to adapt the core shield connection as this was used in some receivers and not in others. Some later transformers have a separate terminal for the core shield. Some schematics show the shield connected to the 6.3vac winding CT but this CT isn't connected to chassis in some receivers. However in these receivers, one side of the 6.3vac winding is connected to chassis to provide both 6.3vac to the tube heaters return and a low resistance connection to chassis for the shield.

Looking at the photos to the right, the upper wires are the primary connections. The three upper terminals are (l to r) 6.3vac-CT-6.3vac. The lower wires are the HV winding ends. The lower terminals (l to r) are 5.0vac, 5.0vac and HVCT. Both transformers types have the same connection layout.

*With low B+ the receiver will function and perform adequately however the S-meter won't function correctly and the audio will be slightly reduced.

Rewind? - Although rewinding the original power transformer is certainly one method of repair, it is an expensive one. The advantage is that all new materials are used in the rewinding so you are essentially getting a brand new transformer core with the original laminations and covers. Besides the expense, turn-around time can also be an issue.

Some Observations - Check your receiver's original power transformer connections. Some production wiring is slightly different from the published schematics. Most NC-100A and NC-200 receivers don't use the 6.3vac CT even though it's present. Some receivers use a capacitor input on the B+ filtering and some receivers use a choke input. The choke input receivers will have somewhat lower B+ but the proper manual will provide the expected B+ voltage for the specific receiver. Some transformers have an internal shield that is connected to a terminal on the transformer, however most don't have a terminal and rely on the shield connection to the 6.3vac winding CT or a chassis-connection of one end of the 6.3vac winding. Most 3-wire primary windings are for "HI" and "LO" line voltages, assume 120vac for "HI" and 110vac for "LO." Some receivers have a fuse-block mounted under the chassis by the power transformer that provided an AC line fuse and the ability to select "HI" or "LO" line voltages for the primary winding. These receivers utilize the three-wire primary. If 3-wire is used in a 2-wire receiver, use the "HI" 120vac connection. The unused ("LO") wire can be insulated and tucked into the apron area of the transformer mounting or cut short next to core (as originally done.)  RAO-7 and 9 use a very similar transformer but insulation on primary and HV wire is a different material and color. RAO 3, 4 and 5 were built by Wells-Gardner and use an entirely different (not National) power transformer. Reuse the original power transformer top cover and bottom skirt to preserve original appearance unless original pieces are damaged. Some Airport receiver transformers will have a vented top cover consisting of either four large holes on top of the cover or several small holes on two sides of the cover. When looking at the receiver chassis from the front, the NC diamond on the transformer top cover should be right-side-up. There was a 25 cycle version that can be identified by its height (it's much taller) and also it's ink-stamped "NC-100-25" on the bottom side of the core.

Test Before Installing - Original power transformers harvested from part sets should be electrically checked before installation. First, check the DCR of each of the windings. Also, check to make sure that the windings are isolated from each other and from the metal laminations. Then apply AC voltage to the primary and using an AC volt meter measure the potential at each of the windings. The voltages will read slightly high since they are not loaded. Leave the transformer primary connected to AC for about ten minutes and check that the transformer isn't getting hot. It should be cool to the touch with no load on the secondary windings. If the transformer passes these tests it will more than likely function fine in the receiver.

Expected (approximate - unloaded) secondary AC voltages with 120vac primary input

Two (Push-Pull) Audio Output Tubes (marked NC 100    10728)                                             Single Audio Output Tube (marked NC 100 F    11028)

HV = 390vac-CT-390vac (780vac across ends)    DCR ~ 320 ohms                                       HV = 270vac-CT-270vac (540vac across ends)    DCR ~ 200 ohms

FIL = 6.9vac (across ends)                                                                                                                   FIL = 6.9vac (across ends)  
                      

RECT = 5.7vac                                                                                                                                          RECT = 5.7vac 

If you want to quickly determine whether an installed power transformer is correct for the receiver or you particular needs, you can pull the rectifier tube and measure the DC Resistance at the plate pins of the rectifier tube socket. This will be the DCR across the entire HV winding. If you measure around 320 ohms, the transformer is a "NC 100" type. If you measure around 200 ohms, it's a "NC 100 F."

NC-100, NC-100X, NC-101X (xmfr marked NC 100  10728) - 2 or 3-wire primary, CT HV secondary, CT 6.3vac, 2-terminal 5.0vac - These are for receivers with P-P audio output

NC-100A, NC-100XA, NC-101XA (xmfr marked NC 100  10728) - 2 or 3-wire primary, CT HV sec., CT 6.3vac, 2-term 5.0vac  - These are for receivers with P-P audio output

Airway Receivers (xmfr marked NC 100 F  11028) - 3-wire primary with fuse block, CT HV sec., CT 6.3vac, 2-term 5.0vac, shield terminal. These are for receivers with single tube audio output

NC-100ASD (xmfr marked NC 100 F  11028) - 3-wire primary with fuse block, CT HV sec, CT 6.3vac, 2-term 5.0vac, shield terminal (60 cycle version marked NC 100 F, 25 cycle version marked NC 100 25) For single tube AF out

RAO-2, 6 - Tapped 3-wire primary, CT HV sec, CT 6.3vac, 2-term 5.0vac, shield terminal (for single tube audio output) Xmfr may be marked "NC 100 F" since same requirements.

RAO-7, 9 - Tapped 3-wire primary, CT HV sec (shown as 240vac-0-240vac on schematic although this may be indicated as measured with transformer providing full load,) CT 6.3vac, 2-term 5.0vac, shield terminal (primary and HV wires have different wire insulation material & color) For single tube audio output. Top cover and bottom skirt are plated not painted. Xmfr may be marked "NC 100 F."

RBH (xmfr marked NC 100 F  11028) - Tapped 3-wire primary, CT HV sec, CT 6.3vac, 2 term 5.0vac  Later versions may be more like RAO. All RBH receivers have a single audio output tube

NC-200 (xmfr marked NC 100  10728) - 2-wire primary, CT HV sec, CT 6.3vac, 2 term 5.0vac  For P-P audio output 

NC-240, NC-240D (xmfr marked NC 100  10728 )- 2-wire primary, CT HV sec, CT 6.3vac, 2 term 5.0vac  For P-P audio output

 

S-meters for Moving Coil Receivers

The earliest NC-100, NC-100X and NC-101X receivers came with a 6E5 tuning-eye tube, type. By mid-1937, the NC-101X was updated to have an S-meter. This meter has a bakelite case with a white scale. This meter was illuminated and was built by Marion Electric. In 1938, the S-meter was changed to an illuminated Marion Electric meter with light-yellow scale. These yellow scale meters are found on later versions of the NC-101X, all of the NC-100A, XA and NC-101XA, the NC-200 and NC-240D receivers. RAO receivers up through RAO-6 use this same meter as do the early versions of the RBH. In some receivers, the housing will be a metal round case with a very small flange without mounting holes. These meters were mounted with a separate metal mounting plate. These metal housing meters were used in all of the various NC-100A versions, including the RAO receivers. The bakelite case, round flanged meters were used in the NC-101X and some WWII and early post-WWII NC-240D versions. The bakelite case, square flange meter was used on the NC-200 and most versions of the NC-240D

What we encounter today when viewing a yellow scale Marion Electric meter is a very much darker scale than the original scales were. (See photo right for what the original meter scale looked like.) Additionally, the red printing was vibrant and easily read when the meter was new. Exposure to bright light, especially UV light, will darken the color of these meter scales to an amber color. This same light fades the red printing to the point of invisibility. There's also some thought that the internal lamp used for illumination causes some of the darkening of the plastic material. (Always use a low current bulb in this application. For screw base, 6.3vac 150mA, use #40. For bayonet base, same ratings, use #47.)

The reason the meter scale shown to the right hasn't darkened is because it's a NOS replacement meter that has always been "in the box." It was never installed in a receiver or exposed to any type of light and never had a lamp illuminated behind the scale. Compare this NOS meter scale's color to any of the receiver S-meters in photographs in this web-article to see just how much darkening of the meter scale happens over a period of half a century of typical use and exposure to light.

National Speakers for the Moving Coil Receivers

All of the Moving Coil receivers originally were sold with a matching speaker. There were a few optional speakers that could be ordered if the user wanted another type of speaker. The early NC-100 may have been equipped with a 10" electrodynamic speaker housed in a black wrinkle finish box the was essentially a wooden box with a metal box cover. This speaker housing has an embossed "NC" diamond logo on the top. The speaker opening is round with no grille except for the grille cloth. This style may have also housed the optional 12" speaker. This style speaker enclosure wasn't available very long and was replaced with the commonly-seen National square box that's black wrinkle finish (MCS-10.) This later version was only available for 10" speakers. The vertical bar grille has a cloisonne NC diamond emblem and the speaker opening has grille cloth that is a small herringbone pattern. The grille cloth was dark brown to black when new but many have now faded to a gold color. This box usually has a 10" diameter Rola electrodynamic speaker with the audio output transformer mounted on the speaker frame. This speaker was supplied with the NC-100, NC-100X, NC-101X, NC-100A, NC-100XA, NC-101XA.

The NC-80X and NC-81X used the standard 8" PM speaker box (MCS-8) that was also standard for the HRO receiver. The audio output transformer was mounted on the speaker frame. The military normally used a headset for audio reproduction but sometimes loudspeakers were necessary. The typical RAO or NC-100ASD speaker is the standard 8" PM speaker box but the audio output transformer is a 500Z or 600Z matching transformer. Sometimes the military speakers will have a shielded cable for the receiver connection.

The NC-200 was the first departure from the square metal box for National. The new speaker was a rectangular shape with two tone gray paint and two chrome bars. The speaker opening was square in shape and covered with a large herringbone dark gold grille cloth. The speaker was a 10" PM Jensen with the audio output transformer mounted on the speaker frame.

The NC-240 used the same cabinet as the NC-200 but with a solid gray wrinkle finish paint with the chrome bars. The same Jensen speaker was used and the grille cloth was a dark gold herringbone pattern. The post-WWII NC-240D added decorative metal feet to the box and changed the grille cloth to off-white herringbone pattern.

Airway receivers and some commercial receivers used rack mounted 8" speakers. Some airport set-ups used a dual speaker rack mounted assembly. 


photo top right: MCS-10 and MCS-8 speakers.                                                       photo bottom right: NC-200 style speaker (repro grille cloth)

 

Restoration Write-ups

If you read the following restoration write-ups "one after the other," you'll find them somewhat repetitious, more or less covering the same steps over and over. My intent is,  if you're working on a particular model of Moving Coil receiver, you can check out my write-up on that same model (or a similar version) and read what sort of problems I encountered. Most of these write-ups were written while the rework progressed to illustrate the order in which the work was performed. I've also included my mistakes which might help restorers avoid those types of problems with their rebuilds. 

 

Restoration of the Silver Anniversary NC-200  sn: C-536

This project started out as a "return to original" condition because of a "hamster-job rework" performed by a former owner. I hadn't really decided whether to leave the receiver in good cosmetic condition doing duty as a "shelf-queen," or whether to do a complete "museum quality" restoration job so the receiver would also be a reliable performer. Curious temptation for a "quickie power-up test" resulted in finding a defective power transformer and this was the deciding factor to go ahead with the full restoration. Also covered in this write-up is how to replace the plastic dial cover with a new repro dial cover. This method can also be used on the NC-240D receivers.

I acquired this Silver Anniversary NC-200 and matching speaker as a trade that involved a restored BC-344-N receiver. The NC-200 receiver was mostly in very good, original condition as far as the cosmetics were concerned (although somewhat dirty.) The original can electrolytic capacitors had be rebuilt by a former owner in a most unusual way using orange plastic pill bottles on the interior holding the new capacitors. The tops of both cans were missing and the metal back-covers from potentiometers were installed as tops instead. Under the chassis, three capacitors had been replace with modern "orange drops." Everything else was still original. The receiver had never been "powered-up" by the former owner.

My initial goal was to just re-do the electrolytic capacitors to appear original, re-install original Sprague paper-wax capacitors where needed and to do a cleaning. I thought I'd just have the Silver Anniversary as a "shelf-queen" because,...who needed another restored "moving coil" receiver anyway?...,...right?

Rebuilding the Electrolytic Cans - This required removal of the plastic pill bottles and new electrolytic caps and accurately trimming the cut that had removed the tops. I then looked in the junk box for some defective can electrolytics (easy to find) and accurately cut the tops off of two of the cans. Since both the original cans and the new tops were the same outside diameter, as long as the cuts were accurately accomplished the seam joining the two pieces together should be minimal. I installed a dual 10uf 450wvdc in the can that originally was a dual 8uf and I installed a 40uf 300wvdc in the can that originally was a 40uf at 200wvdc. The new tops were then mounted using epoxy with a cardboard tube inside (to hold the epoxy.) The finished caps looked nearly perfect and, other than the new caps inside and the new tops, they were essentially the original cans for the receiver.


photo above: The Silver Anniversary NC-200 as received.  Dirty, no NC diamond and an ultra-dark S-meter scale.

Cleaning - The only way to really clean the chassis is to remove the cabinet. The bottom is mounted with several sheet metal screws, including two on each side of the cabinet that must be removed. Then the back and lid assembly is mounted using sheet metal screws. The knobs and control mounting nuts have to be removed to dismount the front and sides assembly. Four 10-32 machine screws  and nuts are used to also secure the front panel to the chassis which also must be removed. The front and sides assembly then pulls forward and up to dismount. The chassis is now easy to access for cleaning. Like many older receivers, this one had been somewhere exposed to greasy or oily dirt. WD-40 removes this kind of grime easily. You'll usually find this dirt is an excellent preservative and the chassis paint will clean up nicely. Once the grease is removed, I then clean the chassis with Glass Plus to remove the WD-40 residue. Don't scrub the aluminum shields as this will tend to polish them. They should be matte finish. Lightly clean with WD-40 followed by Glass Plus. Use a soft paint brush to lightly remove the dirt then dab dry. This will preserve the matte finish and clean the aluminum surface. Most pre-war National equipment will have acceptance markings on the IF cans that consists of orange grease pencil "OK" or maybe initials. These orange grease pencil markings shouldn't be removed during cleaning. Cleaning the Wrinkle Finish - Use Glass Plus and a soft brass brush (sometimes sold as brushes for suede shoes - they are kind of hard to find.) Spray a heavy coat of Glass Plus on the wrinkle surface and then use a circular motion with the brass brush for several minutes. Don't scrub hard but use gentle to moderate pressure in a circular motion to clean the dirt out of the convolutions of the wrinkle paint. The original wrinkle paint is very tough and can take this treatment. Wipe off the dirty Glass Plus with dry paper towels. You'll have some dirt and a little color showing on the paper towels. Do one more application. The paper towel should be pretty light on dirt and color this time. Move on the the next surface and use the same procedure. When all sides have been cleaned, let the wrinkle finish piece dry completely. Then apply 10W machine oil with a clean cotton cloth and rub into the wrinkle finish. You don't need a lot of oil since very little covers a fairly large area. Wipe off the excess with a clean, dry cloth. The oil works much better than Armor All and doesn't leave the surface feeling "slippery." Also, the oil provides a nice luster and doesn't have the "glossy shine" that Armor All has.


photo above: The NC-200 chassis after chassis cleaning, rebuilding the metal-can electrolytics and installing a replacement power transformer (using original cover and skirt though.) Note the brown S-meter case.

Cleaning Knobs and the Meter Case - Soak in warm soapy water for about 30 minutes. Scrub with a soft tooth brush to remove the dirt and then dry. Now polish each knob with Wenol's or Semichrome. The meter case can be cleaned with Glass Plus and then polished with Wenol's. With this receiver the S-meter scale was extremely dark from having a high-current lamp installed for illumination. I swapped scales from another National (Marion Electric) meter that was only slightly darkened. Originally, these scales were light yellow and the red nomenclature was bright and easy to read but UV light and high heat illumination darkens the plastic and fades the red printing.

Replace Power Cord - I just couldn't leave a Kawasaki power cord installed on this receiver. I didn't even know Kawasaki made electrical parts. I looked around in the junk pile to find a vintage radio that still had it's original power cord. I found an old junk broadcast radio that had its original power cord and molded plug (and that was still pliable.) Later, with the second power transformer installation, I used an original NC-101X power cord from a parts set. This power cord was in great condition with the original molded plug.

Repair NC Diamond - I was really lucky that the former owner had kept the Silver Anniversary NC diamond after it had become detached from its mount. To remove the mount required that the cabinet be apart (which it was, for cleaning.) I used a jeweler's file to clean the back of the NC diamond and the top of the screw mount. I then used high-tensile strength epoxy to "glue" the diamond to the mount. I think originally the diamond was soldered to the mount and then the cloisenne NC and the plating was done. I wasn't sure that the heat of a solder mounting wouldn't damage the cloisenne so I opt'd for the epoxy which worked fine.

Re-doing the Gold Tone - This finish was almost totally worn off from years of use. It wasn't an electroplating but rather was a gold tone lacquer that wasn't very durable. I initially thought the gold tone was smoker's residue but tobacco deposits are water soluble and this toned lacquer, of course, wasn't. My "tip off" to the true nature of the gold tone came when I removed the large brown bakelite tuning knob from its skirt. There, protected by the knob, was the original color finish - a light gold color (about the color of beer.) I was lucky to have a special yellow toning nitrocellulose lacquer that had been mixed for me several years ago for duplicating amber lacquer finishes. I had found that diluted a little, this lacquer also duplicated MFP coatings (without the smell.) I had also used this lacquer to duplicate the gold finish on HRO crystal filter boxes, so I knew this would work fine for the gold tone necessary for the Silver Anniversary. Mixed about 3 to 1 with thinner and lightly sprayed, the yellow lacquer duplicated the color found under the tuning knob exactly. I also shot all of the nomenclature plates since the two Crystal Filter plates still had their gold tone present indicating that all of these plates were also gold tone.

Functional Test Finds Bad Power Transformer - The Silver Anniversary NC-200 was reassembled and then photographed for this website. I thought that was going to be the extent of the work performed. I had asked the former owner if he had ever powered up the receiver. He said that he hadn't but that the fellow he got it from had said it worked. Hmmm. I couldn't resist temptation. Since I had the original matching speaker, I connected up everything and carefully applied AC power. I watched the 5Y3 rectifier tube to see if any blue glow would indicate a severe overload but everything looked fine. In about 25 seconds, receiver background noise came over the loudspeaker. I connected an antenna and signals were coming in strong. I tuned around on all bands and everything was working fairly well. After about ten minutes, I though I better feel the power transformer. It was very hot. I felt the chokes and they were still cold. Hmmm. This indicated that there wasn't a high current flow thru the chokes as would be the case with a high leakage capacitor load on the B+. I turned off the receiver. The next day, I pulled all the tubes along with the three lamps and again powered up the receiver. After about ten minutes the transformer was again hot and beginning to smell. A sure indication that the power transformer had a problem. The removal of the rectifier tube removes the entire HV winding and the 5vac winding from that load on the transformer. With the remaining tubes and the lamps removed (the 6.3vac load) there was "no load" on the transformer. I also checked the HV winding and found that it seemed to be okay. The likely problem was insulation breakdown on the primary winding causing it to draw more current than normal and to overheat, even though the secondary voltages seemed correct.
First Replacement Power Transformer - I have several "moving coil" receiver spare transformers so I just selected what was the closest which happened to be from an NC-100ASD receiver. Only a slight modification is needed since the "ASD" uses a three-wire primary and the NC-200 uses a two-wire primary. National assemblers would cut the third wire when required since all of the power transformers originally came with the three-wire primary. All other connections are the same. These transformers are usually ink-stamped "NC-100-F" on the bottom part of the core. Since almost all of the "Moving Coil" receivers have the same power transformer (I thought) and same basic power supply, if the wires are carefully unsoldered from the "part set", then the wires should be long enough when installed in the new receiver. This was the case with the ASD to NC-200 swap. I couldn't use the top cover and the bottom skirt from the replacement transformer since they were rusty and pitted. I replaced these parts with the cover and skirt from the bad NC-200 transformer. While this swap did function, it isn't correct. More on this at the end of this article.



photo above
: The NC-200 Silver Anniversary after the capacitors have been rebuilt. It's difficult to tell that these caps have been restuffed because I coat the outer shell with bee's wax after the new capacitor has been installed in the original shell. The audio gain pot is not original but is from a National NC-100ASD parts set. Note that the tuning knob is brown bakelite.

Rebuilding the Original Capacitors - As long as I had gone this far, I decided to "re-cap" the receiver too. Since this was a fairly rare set, I opt'd to "restuff" the original paper-wax caps so the underneath of the chassis maintained its original appearance. In the past, I'd usually rebuild one capacitor at a time. Since I wasn't going to be able to do the wax-work at the bench, this time I had to come up with a way to do several capacitors at a time. This involved making a rough sketch of the capacitor locations first (with values and orientations noted.) I then would clip-out about seven or so capacitors and take them over to the rebuilding bench. This bench had the Steelcase waist paper basket, the heat gun, the hot-melt glue gun, the bee's wax and the soldering iron. This bench was in an area that allowed for better ventilation since the waxing can smell a little.

The Steelcase basket is used in melting the original caps using the heat gun. This is a messy job with a lot of excess wax blowing off of the old capacitor shell, so the metal waist paper basket works great for this. The old cap is heated, then the original insides are pulled out and dropped into the basket. The shell is wiped down with a paper towel to clean off the excess old wax. All seven (or however many) caps have this step performed first. Next, the new capacitors are inserted into the old shells and secured with hot-melt glue. Although metalized-film capacitors can be oriented in either direction, I still orient all the new capacitors in the same direction as I'm rebuilding the old caps. My method is, with the capacitor nomenclature right-side up, the left side of the cap is the outer wrap. It really doesn't matter but that's how I do it. Next, the cap ends are waxed with bee's wax and finally the other shell is coated thinly with bee's wax. I use the soldering iron to melt the bee's wax directing the wax drips into the cap ends until they are full. The seven (or so) caps are then taken to the receiver bench and installed. Then the next seven are removed and the process repeated again until all the caps have been rebuilt. The outer coating of bee's wax is the "key" to having the rebuilt capacitors look absolutely original. The photo to the left shows the NC-200 chassis after the capacitor rebuild. It's unfortunate that a former owner removed the original 10uf chassis mounted 1st AF amp cathode bypass capacitor. Without the original capacitor I had to leave the modern electrolytic in the circuit until I can find a parts set that can donate the original cathode bypass capacitor.

Weird Tube Problem - I tested all of the tubes and found several that were weak but none that were defective. I replaced the weak tubes making sure that the two metal 6V6 tubes were a close match on their respective Gm. National notes in the manual that if you don't use the metal 6V6 tubes and have to use the glass version that "Goat" tube shields should be installed since the glass audio tubes could oscillate.

As I was installing the now "tested" tubes, I dropped one of the 6SK7 metal tubes. Its fall to the floor was broken by it first hitting my foot, so I thought "no problem." I didn't bother retesting the tube.

It came as a surprise that when powered up, the NC-200 was silent. I touched the grid cap of the 6F8 which is the first AF amplifier and got a loud hum - okay, audio was fine. I touched the IF amp grid (6K7) and also got some hum. I touched the grid cap of the Mixer tube (6K8) and started to pickup a strong AM BC station. Hmmm. I pulled the 6SK7 RF amp tube (the one I had dropped) and the receiver started to weakly pickup the AM BC station. I installed another 6SK7 RF amp tube and soon the NC-200 started playing strong. Now,...I've dropped lots of metal octals on the floor before but I've never had one go bad from the fall. This 6SK7 didn't even hit the floor directly but still ended up defective,...weird.

Installing the Plastic Dial Cover - The dial cover on this NC-200 had "yellowed" quite a bit. I polished it with Wenol's which did lighten it somewhat but it still was difficult to read the red scales on the dial itself. Years ago, I used to order repro plastic dial covers but they all came with flat fronts, not convex fronts like the originals. As a result, I quit ordering repro plastic dial covers and just tolerated the cracked, split or yellowed originals. It had been so long since I had ordered a repro dial cover, I thought I'd try out Mark Palmquist at dialcovers.com. The NC-200 dial cover from Mark was excellent with the original type convex front and a perfect fit. If you need a plastic dial cover, try dialcovers.com. Mark has an online catalog but can also do custom dial covers.

To install the new dial cover requires the upper chrome strip be loose. From inside the cabinet, push the four front panel wire retainers thru their mounting holes. This will allow the chrome strip to be pulled forward-out from the front panel. The two sides of the chrome strip will slide in their wire retainers so you don't have to totally dismount the strip. Remove each dial cover side retainer strips that are held in place with two screws on each strip. Now pull the old dial cover up and behind the upper chrome strip. The new dial cover will have a protective plastic cover that should be left on the front but remove the back side protective plastic. Now insert the new dial cover behind the chrome strip and move it into place. You'll have to pull out the bottom chrome strip a little to get the edge of the new dial cover to slip behind it. Move the dial cover around until it is squarely in place and even on each side. Now mark the mounting holes with a fine point Sharpie. Remove the dial cover and, using a pin-vise, drill the mounting holes. Start with a .060" drill and finish the hole at no larger than .125" diameter. Now, reinsert the dial cover back in place in the same manner. When the four holes are aligned, push the upper chrome strip back in place. Be sure the protective plastic is slightly removed around the edges so it's not under the chrome strip. Once the chrome strip is tight, entirely remove the protective plastic. Then mount each side retainer strip with the mounting screws. Do not over-tighten the four side retainer strip screws as this can crack the plastic of the dial cover. This completes mounting the new dial cover. The same method can also be used for the NC-240 receivers.     

Test-run Reveals Another Problem - I ran the NC-200 on all bands over a period of a few days to "ring-out" any problems. I was then planning to do a full IF-RF alignment. After several listening sessions with no problems I started the alignment. When I got to the RF tracking on Band E, the receiver stopped operating and after a minute a small curl of smoke was noticed behind the catacomb. I shut down the receiver and began checking. It turned out that I had a direct short from B+ to chassis at the second IF transformer. I disconnected the four wires that connect the IF transformer into the circuit. I still had the short from the IF transformer B+ wire to chassis indicating the problem was inside the IF can. I removed the IF can from the chassis and checked but I still had the shorted B+ to the can. I removed the transformer from the can and, of course, then the short was gone. The problem appeared to be one of the rotor contacts terminals on the plate side trimmer was longer than normal and slightly protruded beyond the ceramic mount. Also, the aluminum mount for the ceramic trimmer mount was slightly bent causing the can to slightly distort when the mounting screws were tight and this caused the rotor contact terminal to press against the inside of the can. I put a slight bend on the end of each of the rotor contact terminals and straightened the aluminum mount. I then reassembled the can and with the mounting screws tight there was no B+ to can short. I then reinstalled the IF transformer back into the receiver. I checked the IF plate load resistor that apparently smoked a bit but it was still well within spec for value. As to why this type of mechanical short would develop after years in a non-contacting position is a mystery but it does seem to happen once in a while. IF Alignment - The IF was pretty far off. It seemed to be aligned to 445kc. I tested the crystal in the crystal filter and found it to be 453.6kc which indicates that the IF was probably 455kc. Although National had been using 456kc IF since the beginning of the NC-100 series, with the NC-200, the crystal filter was redesigned and the IF changed to the industry standard of  455kc. However, that's just the design IF, you actually use the crystal frequency for alignment. After the IF adjustment, the BFO control should be set to "0" and the BFO trimmer adjusted for zero beat with the IF.

RF Tracking - The RF Tracking alignment has a lot of adjustments but the trimmers and padders are logically placed to avoid confusion. If a metal screw driver is used for the adjustments there will be some interaction because of the steel blade. You can use a fiber tool but often the trimmers require a higher amount of torque than the fiber tool can provide without breaking. If this is the case, you can compensate for the steel blade by slightly "over-shooting" the peak and see if the adjustment is correct when the screwdriver is removed. The higher the frequency, the more interaction will be encountered. The General Coverage must be performed first, then the Band Spread. General Coverage uses coil loops for the low-end adjustments (variable L) requiring a fiber tool to move the loop around for adjustment. The two lowest frequency bands use padder variable capacitors for adjusting the low-end. Band Spread uses adjustable trimmers for high-end and adjustable padders for the low-end. When adjusting Band A and the 10M band spread, be sure to check that the image is 915kc below the tuned frequency. This assures that the LO is correctly adjusted to above the tuned frequency. The only problem I encountered was with the dial pointer. After I had finished the two low frequency bands, I noticed that the pointer seemed bent (to the right.) I had to remove the plastic dial cover, straighten the pointer, replace the dial cover and then realign the two low frequency bands again. 

Transformer Problems - After the NC-200 restoration was complete, I started using the receiver. It performed quite well but the S-meter didn't respond to strong signals and even a signal generator input could barely produce S-9. The sensitivity was good and the audio level seemed normal. When measuring the B+, I should have seen +230vdc but I would only measure around +150vdc. Obviously, something was wrong. I started to thoroughly investigate the power transformer types used in various Moving Coil receivers and discovered that there are two types of power transformers. I had installed a 11028 transformer from a NC-100ASD which is a single tube audio output receiver. The NC-200 original transformer was a 10728 for all Moving Coil receivers with Push-Pull audio output. The latter transformer has about 780vac across the HV winding while the former has about 550vac across the HV winding. To prove to myself that my research was essentially correct meant that I had to replace the "replacement" power transformer. I "harvested" a good 10728 power transformer out of a "parts set" NC-101X. After testing it, I performed the second transformer replacement. Upon power-up, the NC-200 B+ was at +230vdc, exactly as specified. The S-meter would go well-over S-9 on strong signals. The audio had more "presence" and seemingly more power. The sensitivity was slightly better (but only slightly.) It was amazing that the NC-200 had performed pretty well with the low B+ but now, with the proper B+, all of the circuitry functions to specifications.

Performance - The NC-200 is like having a NC-100XA receiver and a NC-101XA receiver in one package. You can use it as a ham bands-only receiver where it has great dial accuracy across each ham band. Or, just pull out the tuning knob, turn and select a general coverage band, push the knob back in and tune in your favorite SW BC or utility station. The dial accuracy is good on general coverage but the ham band spread is, of course, more accurate since the frequency span is covered is smaller. Sensitivity is comparable to any of the later moving coil receivers and the crystal filter works quite well. I'm lucky to have the matching speaker for this Silver Anniversary NC-200 and the sound reproduction is impressive from the 10" PM speaker. The audio circuit uses a phase inverter rather than an interstage transformer and the reproduction seems to benefit from that change. Lots of bass from the push-pull 6V6 tubes. Great audio when listening to AM BC or to some of the better SW BC stations. Some ham AM stations also sound fabulous. The band indicator flags are very useful and help you find the desired band with this ten-band receiver. My only complaint is that the dial is practically unreadable if you want to just use the dial lamps for illumination (with #44 lamps installed!) The black scales are difficult to read and the red scales are virtually impossible to read. A desk lamp or ceiling lamp is really necessary to adequately read the dial unless you're listening during the day and the window curtains are open. As a vintage station receiver it provides ample sensitivity and admirably copes with just about any QRM.
 

Restoration of the 1938 NC-100XA  sn: 58-R

I was beginning to wonder if this restoration project was ever going to be completed. It started out as a "museum-quality" restoration in 2012. The project then got stalled with our move to Dayton, Nevada, also in 2012. Eventually, in early 2017, I got started on this project again but this time with a somewhat less ambitious rebuild in mind. Maybe this was just to self-motivate because once 58-R was operational, a full-rebuild seemed inevitable. Details on catacomb removal, tuning condenser removal, wrinkle finish painting techniques, Crystal Filter details and more.

Initial Assessment - Why were so many receivers repainted this ugly shade of "robin's egg blue?" This old National NC-100XA wasn't the first receiver I'd come across with that kind of custom paint job. The former "restorer" went even further and painted the 10" speaker cabinet to match. The script "National" emblem must have come off of an old cash register. Why did so many "hamsters" mod their receivers to take an audio input from an external phonograph turntable? Was the audio from the communications receiver that good? How about being able to switch your dial lamps on and off? Really necessary? Of course not, but there was a time when modifying your gear was endlessly advocated by ham magazines such as CQ (especially if the gear was military surplus.)

I would have considered this receiver a "parts set" but the condition of the chassis was impressive. No corrosion with most of the correct parts and very stock. Some of the original wiring had been replaced with red-striped white vinyl insulated wire and there were the various extra holes "hamstered" into the cabinet and panel.

Under the chassis was pretty stock and, although there were several original wires replaced with the red-striped white vinyl insulated wire. Some of the original wires had black electrical tape used as insulation where the original insulation had deteriorated. The entire dial light system had been replaced with bayonet base sockets that clipped onto the original dial lamp assembly retainers. Unfortunately, the original can electrolytics had been removed. A quick check of the transformers revealed that they were in good condition so the possibility of a complete restoration of this early version NC-100XA was considered. I'd always liked this style National but had always come across the NC-100ASD versions made for the Signal Corps. This NC-100XA had push-pull 6F6 tubes in the audio output and the receiver was the early style "XA" without the Noise Limiter circuit. Inspiration enough to get started, anyway. 


photo above: The NC-100XA sn: 58-R "as received." Certainly considerable searching was required to find a suitable "National" emblem. The RCA jack allows for a phono input - a popular mod with the teenaged hams. Switched dial lamps and custom paint job - wow.

Disassembly - Since body work and a repaint of the cabinet was going to be required, those pieces certainly had to be removed. Since there was a lot wiring that had to be replaced, it would be much easier if the catacomb was removed. Additionally, the tuning condenser needed to be dismounted because of wiring problems underneath it. This gave me an opportunity to thoroughly clean and detail the tuning condenser. Another benefit of tuning condenser removal is easy access to the tray under the tuning condenser and the ability to clean the years of grime and contamination usually found there. With a moderate amount of disassembly, the ability to clean and detail the chassis becomes a lot easier. Chassis Cleaning - On this NC-100XA most of the dirt was greasy so I had to use WD-40 first to easily remove the grease and then use Glass Plus to remove the WD-40 residue. The gray paint used on the National chassis of this period is very glossy and when clean looks incredible. These receiver chassis were "show quality" in their finish with matte aluminum shields on the IF transformers, a chrome cover over the Crystal Filter's crystal along with the brass and nickel-plated hardware on the tuning condenser, all complimenting the gloss gray paint of the chassis and power transformer and choke covers. Thorough cleaning will result in the receiver looking almost as it did when it was new.
Removing and Servicing the Coil Catacomb - Unless your NC-100 Series receiver has been modified to have clearance holes for the rod bearing screws to be removed without removing the cabinet,...you'll have to remove the cabinet first. I don't advocate drilling extra holes because if you're contemplating catacomb removal, you'll probably want to check over a lot of other things too, most of which will be easier with the cabinet removed. "Cabinet removal" is a more accurate description of the process rather than "chassis removal." You have to remove the bottom cover, remove all knobs, remove the small panel that holds the glass dial cover, remove all control securing nuts, remove the large pan head screws from the front panel, remove the screws that secure the back panel and lid,...then remove the cabinet by pulling it forward.

Now you have to be careful about turning a NC-100A receiver over because the dial is higher than the components mounted on the chassis so damage to the dial could occur. Be sure to place some spacers that will elevate the receiver chassis off of the workbench when it's turned bottom-side up.

IMPORTANT NOTE: Once the receiver chassis is bottom-side up, set the band change to a point midway between two of the bands. This way the coil catacomb pins are disengaged.

On each end of the round rod bearing there is a large flat head screw that mounts the rod to the rear of the chassis. Remove each of the screws. The rod ends are countersunk and the chassis has a dimple formed to lock the rod bearing in place.  >>>

>>>  It will be fairly difficult to dislodge the rod but pull up on one end of the rod. You may have to use a tool to pry the rod off of the dimple (a large blade screw driver works okay.) Some of the early NC-100A receivers will have very deep dimples that lock the rod in place and these make rod removal difficult. Sometimes you can expand the chassis edge slightly to help pry the rod off the dimple. I believe on these early receivers the dimple was formed after the rod was installed and that's why they are so difficult to remove. Once one end is loose the other end will also be loose. Now lift up on the rear of the catacomb with the rod and gently pull back. This should disengage the pinion gear from the rack and also the band change shaft thrust bearing from its track in the front of the catacomb. Now the entire coil catacomb can be lifted out of the chassis. To reinstall, just observe that the catacomb is fit into the receiver in the same position it was removed. That way the dial indicator will be correct as the catacomb is moved into position for that band (on A versions.) If the dial pointer has shifted position just assure that as the pinion gear is mated to the catacomb rack that the catacomb is in the correct position for the position of the dial pointer. Also, before installing the rod, if the receiver is an early version and it was difficult to remove the rod, then use a file to slightly remove the depth of the dimple. Not too much needs to be removed but enough so future rod removal is easy to accomplish. You only need enough material in the dimple to form a chamfer for the flat-head screws to work against and a slight cone in the inside to help lock the rod in place.

Generally, it will be found that the top of the catacomb is dirty and greasy. It should be cleaned with a degreaser like WD-40 and then wiped down with Glass Plus to remove the WD-40 residue. Clean all of the short contact pins with De-Oxit and a clean paper towel. Check over all of the pins to make sure there is no physical damage. They are stout and seldom have any problems other than grease and dirt. The contact pins shouldn't need anything other than cleaning with De-Oxit. Don't grease them with Tuner Grease or anything like that. It will only cause problems in the engagement contacts and collect more dirt. De-Oixt is all that is needed. The rod bearing should be cleaned of the old grease and lightly lubricated with any type of modern, light-weight grease. The rack and the pinion gear should also be cleaned and lubricated with the same type of light-weight grease.

Dismounting the Tuning Condenser - Due to the mechanical design of the coil contact insulator support, a dirt and grease trap is formed by the tray that exists under the tuning condenser. A close look at almost any NC-100 Series receiver will no doubt surprise anyone who expected this area to remain uncontaminated over a period of sixty years or so. Certainly how bad this area is will depend on how the receiver was stored. If the rest of the chassis is dirty, you can be sure that under the Tuning Condenser is really dirty. Removal of the Tuning Condenser is not recommended unless you have a mechanical problem with the tuning condenser or an electrical problem under the tuning condenser. It is possible to clean this area with long handle small brushes and Q-tips with the tuning condenser installed. Also, low-pressure compressed air will help to remove loose particles of dirt and contamination. However, if the tuning condenser needs to be dismounted this will allow a thorough cleaning of not only the tray area underneath the condenser but an intense cleaning of the Tuning Condenser itself. You'll have already removed the cabinet for coil catacomb removal so the next step to remove the Tuning Condenser is to unscrew the four machine screws that mount the N-PW gear box to the chassis and remove the one or two screws that mount a rear bracket to the chassis. >>> >>>  Later versions, mostly military, will use two screws for the rear bracket. There are several TC wire connections to each of the three condenser sections. These are easiest to unsolder at the connection at the condenser rather than trying to access the insulator block. Some versions will have a heavy gauge TC wire that partially supports the grid-leak RC combination to the LO grid. This should be removed at the tuning condenser connection side. Most versions use ~20 gauge TC for the condenser to coil block connections. The grid leads are stranded wire with black cloth insulation (these don't need to be removed from the coil blocks.)

With all of the TC wires unsoldered, the Tuning Condenser can be removed. Depending on how and where the receiver was stored, you might find anything in the tray under the Tuning Condenser. Grease and dirt are the most common but bugs, wire pieces and other contaminates are also likely. Use any type of degreaser-cleaner to remove the greasy dirt and Glass Plus to thoroughly clean up after that.

photo above: This is the tray that is under the tuning condenser with the tuning condenser removed. Note the dirt, foreign objects, condition problems on the chassis and poor rework that was performed by a former owner. 

Cleaning the Tuning Condenser - The tuning condenser on any Moving Coil receiver will almost always be filthy with grease and dirt. The best method of cleaning is to use a WD-40 bath first to remove all of the greasy dirt. Next use Glass Plus or some other degreaser to remove the residual WD-40. I usually go over the metal parts with a soft brass brush. I just want to remove dirt and minor corrosion not scratch up the metal. A long handle paint brush works best for cleaning. If corrosion is very apparent on the plates of the condenser you can spray the plates with a heavy coating of Easy Off Over Cleaner. Sodium Hydroxide is the active ingredient in EOOC and it reacts with aluminum quickly to remove dirt and corrosion. After no more than one minute rinse the EOOC with cold water. Then switch to a hot water rinse for a minute to warm up the parts. Blow dry with compressed air or a heat gun on low to dry the remaining water. The tuning condenser will look like new but will need careful lubrication. Use De-Oxit on the rotor contacts and 10W oil on the bearings. Only use the EOOC if the tuning condenser can't be cleaned-up by less-harsh methods.
Remounting the Tuning Condenser - I usually install new TC wires so that I can bend them for the best routing to the connections at the Tuning Condenser. The original TC was about 20 gauge wire. Notice that the wire to some of the Tuning Condenser terminals was sleeved with "spaghetti" tubing to prevent any shorting of the TC against the metal parts of the chassis or the Tuning Condenser. You'll find that the wires easily can be routed to the terminals after the Tuning Condenser is mounted to the chassis. Be sure to check that the foremost LO contact is actually two connections and electrically separated before mounting the Tuning Condenser. This is the switch for removing the RF/IF screen voltage while the coil catacomb is being moved. 

You'll need to use a small soldering iron for the connections. Something like a Weller 25 watt Soldering Station works quite well. After all of the connections are soldered, try the Tuning Condenser and make sure that nothing contacts the rotor. Check to be sure no wires are shorting against the stator. If you used sleeving as original, there will be no problems. If you've already cleaned and lubricated the N-PW gear box, the operation of the Tuning Condenser should be very light and smooth.

Coloring Cloth Insulation - When replacing original wires that have deteriorated beyond the point of restoring, finding the correct color cloth insulated wire is next to impossible. I don't bother except that I look for the correct gauge and correct type, that is, stranded or solid wire. Once I have the correct cloth covered wire, I use Artist's Acrylic paint to mix up the correct color for the cloth insulation. Once I have the paint mixed, I put about a teaspoon into the fold of a paper towel that has been folded multiple times (so it's thick enough for what we're going to do next.) Now, pull a foot or two length of the cloth covered wire through the paint in the fold. You'll have to do this several times and twist the wire as you're pulling so the paint gets all around the cloth insulation. Now, with another paper towel, wipe off the excess paint and let the wire dry for about 20 minutes. Now you have the correct color wire to install in the receiver.
Wrinkle Finish Painting - The NC-100XA cabinet pieces had to be stripped and this required removal of the four panels that are riveted to the dial bezel and the main cabinet. Drill the backside of the rivet and then drive it out from the back using a small metal punch. 

With the panels removed, the cabinet pieces can be stripped. Use Methylene-Chloride stripper like JASCO Epoxy Paint Stripper. You'll have to do this outside and use Nitrile gloves that are covered by Platex gloves. Platex refers to the material not the brand name. Platex is not affected by Methylene-Chloride and protects the Nitrile gloves which are. The three pieces are the main cabinet, the back and lid piece and the dial bezel.

NOTEOCT 2016 - JASCO - I used to always use JASCO stripper but it has apparently changed its formula so the stripper has the consistency of Jell-o. It can't be spread easily and makes any stripping job tedious. Try using Klean Strip instead. It's methylene-chloride and works great.

After the pieces are stripped, the holes are filled with Bondo. After the Bondo has set-up it can be sanded flat. At this time prime the Bondo patches with a small paint brush using lacquer paint. Use several coats to assure that the Bondo is sealed. You must seal the Bondo before painting wrinkle finish or the Bondo patch will show through the wrinkle. Sealing the surface of the Bondo prevents that problem.

The next step is to paint the gloss black, which is on all of the interior parts of the cabinet. Mask the top edge that forms the lid lip and paint the lip part gloss black. Be sure to use gloss black lacquer paint. After the gloss has dried, you can set up for wrinkle finishing. See photo below for close-up of lid lip paint. >>>


photo above: The proper top lip treatment showing the smooth finish black where the lid sets and the wrinkle finish that is exposed when the lid is closed. Also note that the interior of the cabinet is smooth finish black.


photo above: The VHT Black Wrinkle paint job with the dial bezel setting in place and the four nomenclature plates setting in place. The nomenclature plates have not been restored yet. Only the far right plate needs to be redone.

>>>  I use VHT High Temperature Jet Black Wrinkle Finish available at many auto stores, like O'Reilly's here in Northern Nevada. You will need two aluminum bell reflector lamps with 100 watt incandescent lamps installed for overall heating of the paint. Also have a hand held heat gun handy for spot heating. Set up so you can paint the piece on a board so you can then place the board and the piece under the lamps. Have the lamps about 10" to 12" above the painted surface. Mask the gloss finish paint job that is on the lid lip pieces so that paint won't be wrinkled over. 

Now, apply three coats of wrinkle paint with about one to two minutes between each coat. Then place the piece under the lamps. I wait about 5 minutes or so and then begin using the heat gun to apply heat to the edges where the lamps don't heat up very well. Don't apply too much heat directly or you'll "gloss" the paint and it won't match the overall finish. If possible, apply the heat gun heat from the backside of the piece as this works much more affectively and doesn't "gloss" the paint. The wrinkle will start within five to ten minutes with the beginning pattern starting under the lamps. Continue to apply heat from the heat gun in a general area around where the wrinkling has started. This will help the wrinkle to migrate towards the edges of the piece to complete the wrinkle process.

When finished, allow the pieces to remain untouched for about an hour to cool down. The wrinkle paint is very soft when warm and easily damaged. After the cool-down, it's harder and can be handled gently but it's still very soft. Allow the wrinkle finish to "cure" at least over night. Curing for a week is better (I let this paint job cure for five years - boy, was it hard by then!) The longer you can wait before mounting everything back on the painted surface the harder the paint will become and less likely it will be to mar when mounting the parts.

Delayed Assembly - As with many projects around here, the NC-100XA restoration was put on hold for about four months. During that time, the wrinkle finish had time to really "harden up." If it's possible to wait a week or two before assembly, you'll find that the paint has become much more durable than if you try to assemble the receiver the next day. ~ July 2012

Major Delay - Our November 2012 move from Virginia City, Nevada to Dayton, Nevada has caused all of our restoration projects to be put on hold for several months while we get everything set back up in the new shop. We'll be getting back to this restoration as soon as possible.

More Delays - Here it is mid-2015 and I still haven't finished this NC-100XA. Restuffing caps, restringing the dial and reconditioning the small nomenclature panel plates remain unfinished at this time. I needed to re-evaluate the "museum quality" restoration of the NC-100XA and think about a less ambitious rebuild just to get restarted on this project.

Getting Started Again - Well, here it is January 2017. It was only about five years ago I stopped this project for some reason. Then came the Dayton move and more delays. Not that I've been inactive, I just wasn't interested in completing the NC-100XA. However, recently I had just finished a complete "museum rebuild" of a Silver Anniversary NC-200 "catacomb" receiver, so while "catacombs" were still in my "active memory," I decided I'd try to finish the NC-100XA.

I needed to rethink this project just to get up enough ambition to get started. I thought I'd make the project less work and then maybe I could finish it.

I had repainted the cabinet several years earlier so the wrinkle finish was about as hard as it was ever going to get. I decided to not refinish the nomenclature plates. These would be "touched up" to look decent but not perfect. I would still have to make the dial lamp assembly. I would still have to restring the articulated dial pointer. First though, I had to complete the repair of the wiring and get the receiver functional.  

Tube Set - I went through the tube boxes to find a complete set of tubes for the NC-100XA. These were tested and installed. What's nice about this early NC-100XA is that it doesn't use a 6F8 tube.

Power Supply - I installed new 10uf 450vdc electrolytics under the chassis. To make the top side of the chassis look original, I installed two old-style can electrolytics. These can caps had the wires cut short but they could be rebuilt at anytime in the future, if desired. There were a few wires still missing under the chassis in the power supply section so these were installed to complete the circuitry. I disconnected the B+ from the receiver circuitry and measured the DCR to see if there were any serious short-circuits. Measuring more than 20K ohms seemed a positive indication. I applied AC to the power supply and measured +300vdc without a load, another positive indicator. I used clip leads to connect a current meter in series with the B+ to load and measured 20mA of current flow, another good sign. With the B+ reconnected and a antenna wire on the Antenna terminal, AC power was applied. Within a short time, AM BC signals were coming in strong. I tuned in the local 50KW station and the audio through the original 10" Rola electrodynamic speaker sounded great. So, while the NC-100XA did function, I wouldn't operate it for extended periods since all of the paper-wax caps are still originals.

Re-stringing the Dial Pointer Lifter Mechanism - Shown in the photo to the right is a close-up of the dial pointer lifter mechanism. Note the wire spring and how it forces the pointer to the lowest position. The dial cable will raise the pointer mechanism with the rotation of the band changing knob and shaft (which also drives the coil catacomb into position.) Note that the entire lifter mechanism rotates and carries the dial pointer along as the receiver is tuned. Two separate dial strings appear to be required, one for the lifter-pointer to dual pulley and one from the dual pulley to the coil catacomb band change shaft, however the dial pointer movement is accomplished using only one continuous dial string. Details and photos are shown in the NC-100ASD section further down this page. Also, Bill Fizette has written up a procedure with drawings about re-stringing the Dial Pointer Lifter Mechanism. Bill's procedure is available on BAMA edebris. There was also a procedure published in Electric Radio a few years ago that is also available on BAMA edebris.

It's easiest to install the dial pointer lifter string before the cabinet is reinstalled. In fact, on some versions of the receiver you can dismount the dial itself for better access to the dual pulley. However, only some receivers allow dismounting the dial. Note in the photo that this NC-100XA has the dial riveted to the mounts. It's only slightly more difficult to string the lifter with the dial in place. Use thin dial string so it will fit thru the holes in the pulley (the 1mm diameter mentioned in the NC-100ASD rebuild article further down this page is the largest diameter string that can be easily used - thinner is better.) Once the string is installed and working you can install the dial (if it was removed) and fine adjust the pointer string with the tie-off knot. Apply a drop of glue to each of the two knots to secure. Once the dial pointer lifter is working, the cabinet can be installed.

Cabinet Completion - Three of the nomenclature plates were in good condition. One was severely worn from "fat finger syndrome." I was going to repaint all of them but decided that a good touch-up might be all that was necessary. Two of the nomenclature plates had "baby blue" paint overspray that needed to be removed.  I used 0000 steel wool and light-weight machine oil to gently remove the blue overspray. I then cleaned the plates with denatured alcohol. I use "thinned" jet black nitrocellulose lacquer applied with a fine brush to fill in the places where the paint was worn off. This was then rubbed down to match the rest of the original black paint.

To mount the four nomenclature plates I was going to use "fake rivets" which are 2-56 round head machine screws with the head ground down to look like a rivet. Then the plates are mounted with nuts on the back side. A check of the dial escutcheon showed that there wasn't enough clearance behind for the nuts. I would have to mount the two plates on the escutcheon differently. I used a small amount of epoxy to secure the two plates to the escutcheon. Then I installed the fake rivets but secured them in place with epoxy. This allowed the clearance to be minimal. The two nomenclature plates that mounted to the cabinet had ample clearance for using nuts on the fake rivets. The heads of the fake rivets were painted black as the originals were.

The S-meter is mounted using a mounting plate that is secured with 6-32 binder head screws and nuts. The meter has to be mounted before the cabinet is installed onto the chassis and before the escutcheon is mounted. There is a small tie strip that is used for the S-meter lamp and dial illumination wires that mounts to the upper left screw and nut that mount the meter plate.  >>>

>>>  It was necessary to replace two wires going to the AC power toggle switch assembly. One wire had damaged insulation and the other wire was barely long enough and didn't allow for a nice looking routing of the wires.

The "fit" of the cabinet and the clearance of the dial pointer to dial and the logging dial pointer to logging dial all has to be checked. Some adjustment will probably be necessary since the clearance is very tight on these moving parts. The dial can be adjusted with either the two mounting screws or by slight bending. The logging dial should fit fully onto the logging dial boss on the tuning shaft. There is also a boss on the logging dial hub that allows the proper spacing when the logging dial is fully seated. The logging dial pointer is part of the cabinet and should be very slightly bowed out so when the dial glass is mounted, the logging dial pointer is up against the backside of the glass. You have to do a little trial fitting to see that all moving parts are going to have clearance and not rub against other parts. The clearances are very tight so slight adjustments are the norm on these versions of the NC-100A.  

A glass pane is installed for the dial cover. I had the original so it was reinstalled. The edges of the glass are covered with tape to act as a cushion and to prevent chipping. The glass I used had the original thin paper tape still installed so it was not necessary to replace it.

When the cabinet is being mounted to the chassis there should be a long spacer that is between the back of the front panel bottom and the chassis. This spacer is secured with two 10-32 pan head screws, lock washers and nuts. There are two other screws above the S-meter switch and above the Phone jack. These are "dummy" screws and only serve to plug two holes that aren't used on the NC-100XA version.

Cabinet Assembly - The lower part of the cabinet is attached around its perimeter using 10-32 pan head screws that are black wrinkle finish. These mount through the chassis and are secured with lock washers and nuts. The back and lid assembly attach at the sides to the cabinet sides with sheet metal screws and attaches to the chassis on the back panel to the chassis, also with sheet metal screws. All controls are mounted to the chassis and protrude through the panel holes with no mounting nuts required except for the Tone control which mounts to the cabinet panel. Also, the B+ Power switch assembly and the AVC-MVC-BFO switch assembly mount to the cabinet panel. Be sure to install spacer washer on each of these controls before mounting the escutcheon. Once the escutcheon is mounted (with the glass dial cover in place) then the two switch assemblies can have their trim washers and nuts installed. All that remains is to install the knobs.


photo right
: The top of the chassis of  NC-100XA SN: 58-R. This particular receiver used two can electrolytic capacitors that were 8uf at 450vdc and then a third, physically smaller 8uf 450vdc electrolytic that was mounted under the chassis. Note the plug-in crystal on top of the crystal filter assembly. This type of crystal filter was used up to the introduction of the NC-200. The gold color was originally an irridite chemical treatment that didn't necessarily prevent corrosion and was more for appearance. The original gold was mostly worn-off and this is a tinted lacquer coating that looks very much like the original irridite. Also note the antenna-ground connection terminals are blade screws, not the push button connectors or the knurled nut type. This screw-type of antenna-ground connector was used for a short time in the late-1938 time period.

Initial Performance Assessment - Since I hadn't replaced the paper-wax capacitors, I wasn't expecting great performance. Since the tuning condenser had been dismounted, cleaned and remounted I wasn't expecting very accurate dial frequency indications. Since I hadn't aligned the receiver, I wasn't expecting great sensitivity. Well,...for most of this, I got what I expected. But, I was surprised that the NC-100XA did function and seemed very sensitive on the ham antenna. I was impressed with the audio when listening to SW BC stations that were playing music (not too many of those!) Not that there weren't some issues though. The Crystal Filter seemed to always be in the circuit resulting in a very narrow passband. Overall though, pretty nice performance for a receiver running on all original paper wax caps. Oops,.. - When changing bands, the articulated pointer worked just fine. However, it was operating backwards. Only Band C was correct since it's in the middle of the range and this was where I had done the listening on 75M and checked WWV at 5.0mc. When I picked up what I thought was an ultra-strong SW-BC station on 8mc, I realized that the station was actually a local Spanish-speaking AM-BC station around 1650kc. Fortunately, this kind of "stringing" problem is very easy to correct. I just moved the catacomb to the lowest frequency position, which then had the dial pointer indicating Band A (highest frequency.) I then untied the knot on the band changing shaft and pulled the dial string out. I routed the dial string around the band change shaft in the opposite direction and through the hole in the shaft. I then pulled the string until the dial pointer indicated Band E and tied the knot. A band change test showed the articulated pointer worked correctly and that the catacomb was in the correct position for each band selected.
Crystal Filter Problem - Of Course, this means Cabinet Removal,...again - As mentioned, the Crystal Filter seemed like it was always in the circuit. A quick check is to pull the crystal and measure the DCR between the two socket pin openings. This should show a "short circuit" when the Crystal Filter is out of the circuit. Another test is to operate the receiver with these two socket pins shorted. The receiver operated normally with the two pins shorted and measuring the DCR indicated that the cam-operated switch wasn't closing properly. I thought I might be able to access the switch if I removed one side-cover on the Crystal Filter assembly. The side next to the tuning condenser is accessible but the other side is blocked by the BFO assembly can. With the cover removed, the problem was obvious - see photo right. Wasps had at sometime in the past gotten into the filter assembly and built two "mud dabber" nests. To remove the nests completely and get the filter assembly clean and operational meant removing the entire Crystal Filter. I had just totally reassembled the NC-100XA, so I first tried to remove the filter assembly without taking the cabinet apart. It's not possible to remove the filter assembly unless the cabinet is dismounted. So, with the cabinet removed from the chassis a second time, the filter assembly is easy to remove. The second wasp's nest was against the side cover that couldn't be removed with the filter assembly mounted to the chassis because of the BFO assembly can. This second wasp's nest was what was preventing the cam-switch from closing. Complete cleaning with Glass Plus removed all wasp's nest debris and cleaning with switch with DeOxit and paper strips got the switch working. The filter assembly was given a coating with yellow-tinted lacquer to duplicate the original finish. Then the filter assembly was remounted to the chassis and the three wires resoldered to complete the installation. I thought I better check the IF cans and the BFO to be sure the wasps hadn't gotten into those holes but the clearance there is much smaller and not as attractive to "nest building" since nothing was found.

Crystal Filter Alignment - When aligning the IF section of the receiver, the active frequency of the crystal must be determined first. Use a RF signal generator input to the grid of the mixer tube. Use a modulated 400 cycle wave and adjust the amplitude for a medium level signal in the receiver. Place the crystal filter into the circuit and then adjust the frequency of the RF sig gen to around 456kc. Sweep the frequency of the generator and listen to the receiver. You'll hear a "thump" as you go past the active frequency of the crystal. You should have a digital frequency counter connected to the signal generator to measure the frequency exactly. In the case of this NC-100XA, the active frequency was 456.4kc. You'll have to remember the frequency because you will be changing the signal generator for part of the alignment. With the crystal filter out of the circuit, adjust all of the IF transformers for peak at the crystal frequency. Then switch in the crystal filter and set to 5 on the Selectivity and adjust the Phasing for minimum response. Now, adjust the RF sig gen to about 4kc lower than the crystal frequency (in my case from 456.4kc to about 452.4kc - it's not ultra critical, around 4kc is fine.) Now adjust the hex nut adjustment on the crystal filter box for peak. Then switch out the crystal filter. Return the RF sig gen to the crystal frequency (in my case, 456.4kc) and adjust the compression trimmer located in the small hole to the right side of the crystal for peak. This completed the IF and Crystal Filter alignment.

photo above: A mud dabber next inside the Crystal Filter box.

Re-cap? - Although the NC-100XA did function okay on all original capacitors, it really couldn't be left operating for long listening sessions. If I was going to use this NC-100XA as a station receiver then certainly the original paper-wax capacitors would have to be replaced. I really think this receiver is now in nice enough condition that it should be thoroughly completed and have the paper-wax caps restuffed along with replacing any out-of-spec resistors followed by a complete alignment.  So,...from "just finish the project" to a "museum-quality" rebuild in just a couple of weeks. I guess I just can't help myself. - February 16, 2017

 

 

 

photo left: 58-R reassembled. The rebuild included complete repainting of the black wrinkle finish. The nomenclature plates are original paint but the lower C.W.OSC - A.F. GAIN plate has been retouched with black lacquer that was blended and semi-polished to match. The articulated dial pointer is indicating that band B is selected and the frequency tuned is around ll.9mc. The logging scale shows "738" which could be used to accurately retune to the frequency. Since analog dials with wide frequency spans didn't have the resolution for accurate readout, the logging scale provided a means to very accurately retune a "logged" station. Even though the original dial spec was 1% accuracy, at 4.0mc that's indicating anywhere from 3.960mc to 4.040mc. The logging scale is much more accurate for frequency resetability.

Rebuilding the Original Sprague Capacitors - I used to restuff these types of capacitors "one at a time" which assures that the rebuilt capacitor is reinstalled correctly. But, this was much more time consuming than the method I now use. Now, I first make a good drawing of the chassis and all of the capacitors locations, connections and orientation. Each capacitor on the drawing is marked for value also. The drawing allows me to remove several capacitors at a time. These are taken to a workbench specifically set-up for doing the capacitor restuffing. I have a metal waist-paper basket, a heat-gun, a hot-melt glue gun, bee's wax and a soldering iron. Each capacitor in the group (from four and up to seven capacitors) has the internal original capacitor "melted out" using the heat-gun. I hold one lead on the cap with a pair of needle-nose pliers and hold the cap over the metal waist-paper basket while heating. When hot, the old cap is pulled out of the shell and dropped into the basket. I then wipe the shell with a paper towel to remove any dirt or wax and then the shell is set aside. This is repeated until all of the original caps in the group are out of their shells.

I use polyester dielectric tubular capacitors (so-called "yellow jackets") with a 630vdc rating. Modern caps are much smaller and need "padding" to fit into the shell. I use masking tape and wrap the cap until it fits into the shell without falling out (doesn't have to be tight.) All caps are oriented with the nomenclature right-side up then the left lead is considered the outer wrap. It doesn't matter on film capacitors but that's what I do. Install all of the correct capacitors into their proper shells. I then apply some hot-melt glue to secure the new cap inside the shell. I hot-melt glue all the caps in the group. Once the hot-melt has set-up, I fill the ends of the shell with bee's wax to cover the hot-melt glue, again, to all caps in the group. I use a soldering iron to melt the bee's wax and drip the wax into the ends of the shells. I then coat the outside of all the shells with bee's wax. This completes the rebuilding of the group of capacitors which are then taken back to the radio workbench and all capacitors in that group are reinstalled into the receiver. Then the next group of four to seven original caps are removed and go thru the same process. This is repeated until all of the caps are rebuilt.

I've found this method to be somewhat faster in the long run than the "one at a time" method. The drawing is used to double-check how the rebuilt caps need to be oriented when reinstalled. Also, as each rebuilt is installed it is "red checked" on the drawing so I know it has been rebuilt. This is necessary because it is very difficult to tell the "rebuilts" from the originals.

Rebuilding the Electrolytic Capacitors - Each can is carefully scribed around the perimeter of the can to make sure the cut is going to end up straight. Make two scribe lines spaced about .125" apart. Then the can is sawn between the scribe lines. Remove the old capacitor that's inside and discard it. You'll now have each piece of the can having a straight line to guide you in the next step. Using a file, remove the rough edge of the sawn cut and remove the material until it is exactly even with the scribed line. Do this to both can pieces. When finished, these two pieces will fit back together perfectly with just a fine line showing.

The older style electrolytic capacitor cans that are mounted with a large threaded end and a large nut will have two cloth insulated wires exiting out the nose of the threaded end. This actually makes installing a new electrolytic easy since there is no "can" connection required.

Solder two cloth insulated stranded wires to a new electrolytic capacitor leads. Use shrink tubing to insulate the solder joints and cover the entire capacitor with electrical tape. Feed the two wires thru the hole that goes thru the threaded bottom. Pull the wires until the capacitor is fairly tightly held. Apply five minute epoxy on the outside of the hole in the nose of the threaded end to hold the wires securely.

Using a heavy paper tube, coat the two inside pieces of the can with five minute epoxy. Also coat the paper. Fit the paper tube inside one piece and then fit the other piece of the can over the remaining paper. This allows the epoxy to "hold onto" something and makes the glue joint much stronger. Use masking tape to hold the two can pieces together while the epoxy sets up. Once the epoxy has set-up, the seam should be almost invisible. You can coat it with silver paint to hide it but usually it just looks like a fine line around the can. Install the rebuilt capacitors into the receiver to complete the job. 

Building a Repro Dial Lamp Assembly - Like most NC-100A-type receivers, 58-R was missing its original dial lamp assembly. This piece originally consisted of a fiber tube that was slotted. The lamp sockets were inside the tube with the slot facing down and the entire piece was held under the front lip of the cabinet by two "swinging" clips. The connecting wires were a twisted-pair that wired the two lamps in parallel and then connected to a tie strip that also had the wires for the meter lamp connected to it.

The whole assembly is very easy to make if you can find the right type of fiber tube. I could find fiberglass tubes that probably would work fine but were very expensive and could only be purchased in large quantities. I couldn't find anything like the original fiberboard tube. I couldn't use plastic because of the heat from the two lamps. However, I could use semi-rigid Nalgene teflon tubing since teflon is rated at over 500 F degrees.

I used a 0.5"OD piece of semi-rigid Nalgene tubing that cost about six dollars a foot. The tube still has to be slotted to allow inserting the lamp sockets and to allow the light to shine down onto the dial scale. Length of the Nalgene tube is 8.75" and the slot is about 0.325" wide (it's not too critical.) The dial lamp assembly tube is held in place with the two swinging clips under the front panel lip. Once in place the entire dial lamp assembly can't be seen because it's tucked under the front lip and, in this position, if the slot is facing down, the lamps shine directly onto the dial scale.

If I make another one of these, I'm going to try polycarbonate tubing. It's rated for about 250 F degrees which is probably a higher temperature than the surface of the illuminated lamps.


photo above: The finished NC-100XA SN: 58-R with its matching loudspeaker. Having the loudspeaker on top of the receiver cabinet isn't recommended but when table top space is limited there's no other choice. I've not had any audio feedback problems with this set-up.

Alignment - I had already done the IF alignment when I was working on the Crystal Filter so all that was left to do was the RF tracking alignment. The receiver seemed to track fairly close and sensitivity was very good which probably meant that the RF tracking wasn't too far off.

The AM BC band actually has a padder adjustment for the low end of the band making the LO tracking on that band easy to accomplish. The remaining four bands use an inductance loop for adjusting the L of the LO on the low end of the band. I use a fiber tool to push the inductance loop into position for proper low-end L. An air trimmer is used for the high end. The LC adjustments interact so a little "back and forth" is necessary to get the LO tracking correctly. All of the alignment adjustments needed were minor - just slight tweaking for best alignment. This receiver was just about the closest to being "already aligned" that I'd done in a long time. Dial readout after alignment was very accurate given its analog limitations. Original spec was 1% accuracy which was easily accomplished.

Performance - I think the NC-100XA has ample sensitivity up to about 20mc which is where most thirties receivers begin to show their age. On the frequencies we use most in the 75M and 40M bands, the sensitivity is excellent. Selectivity is pretty narrow on just the IF bandwidth. I'd guess it's around 8kc at 3db down. Most of the QRM can be dropped off of one sideband or the other in the AM mode. The crystal filter is very sharp, as would be expected. Most hams used the crystal filter in the CW mode but I find it does a good job in AM also. Just switch in the crystal filter and set the Phasing for a null and then tune the AM station "on the nose." Bandwidth will be very narrow but voice will still be intelligible. The best part of the NC-100XA performance is its audio. Natural sounding but with ample bass. Well-adjusted transmitters sound incredible. The audio works great for both BC or AM ham stations, even music sounds pretty good. The RLC Tone control works better than the later RC control.

The dial on this NC-100XA is the best illuminated that I've had. The silver dial was in really nice condition to begin with, which obviously helps. The repro dial lamp assembly that I made from a piece of semi-rigid Nalgene really reflects the light much better than the white-painted fiberboard original assemblies. The entire dial is well-lighted and easy to read, even with "lights out" listening.

I set-up the NC-100XA as a station receiver with an ART-13A transmitter to actually have some "on the air" time. The NC-100XA's ability to handle adjacent frequency QRM is impressive. Even activity 2kc away is easily dealt with by tuning to one sideband or the other. I haven't had to use the Crystal Filter (yet!) These days, when actually using a vintage receiver on the 75M band, that receiver's ability to cope with QRM is just as important as its sensitivity and audio quality. As the fore-runner to the famous USN RAO receivers, the NC-100XA provides excellent performance with somewhat of a military connection. When compared to the 80 pound USN RAO, the diminutive NC-100XA weighs in at only 60 pounds and also offers P-P 6F6s in the audio output. A very good performer that's easy to fit into any vintage station landscape.

 

Restoring the NC-100ASD

 This restoration is a combination of "museum quality" electronic restoration and some cosmetic rejuvenation. It involved using two NC-100ASD receivers to end-up with one receiver that functioned correctly with a nice, original appearance. At the risk of becoming repetitious, here's yet another rebuild write-up.

Sometimes shunned by collectors because of its "often-seen" status and military origins, I've always had a "soft spot" for the National NC-100ASD. Probably because my first exposure to an "ASD" was at a neighboring teenager's ham shack. That was WA6LHL, Dennis Antilla, and his station consisted of an NC-100ASD and an EICO 720 with Heathkit VFO and homebrew modulator. Dennis was an early "Elmer" of mine although he joined the Navy before I got my license (I was 14 at the time.)

I've owned a few NC-100ASD receivers since then but none of them were in very good condition. Most were actually "junkers." However, recently I saw an ASD for sale on Craig's List and since it was only 15 miles away in Carson City, I went over, inspected it and ended up purchasing it. While the chassis was in excellent cosmetic condition, the cabinet had a non-original hole to mount an ON-OFF switch for the dial lamps (hmmm, why was this so popular?) There was also a mod to replace the 6F8 tube with a 6SN7. Luckily, no extra holes were needed for that mod.

The Other"Parts Set"ASD - Many years ago, while I had the Western Historic Radio Museum open in Virginia City, I had a fellow stop by with a small pickup full of radios. He wanted to sell them for his neighbor. Among the radios was an NC-100ASD. This "radio-selling guy" turned out to be the famous Bob Wilkins. As a teenager I had watched Bob Wilkins host "Creature Features" on KTVU out of the SF Bay Area. When Wilkins retired he moved to Reno and was a neighbor of my old friend W7MS in Reno.

It turned out that the NC-100ASD I got from Bob Wilkins had spent a lot of time in a basement under a leaking water pipe. The rust had eaten through the top lid hinges and the chassis was a total mess. Amazingly, this NC-100ASD did work. It actually worked pretty well but the cosmetics were terrible and I ended up storing the receiver for some future need - like a "parts set." 

It was lucky that I did save the "Wilkins' ASD" because it could supply the front panel-sides cabinet piece and replace the same piece in the latest ASD with the non-original hole. A combination of cabinet parts would result in the best condition original ASD cabinet. I was to also going to find out as the restoration proceeded that I was going to need some very important parts that only this "parts set" could supply.

photo above: This is a shot of the Bob Wilkins' "parts set" chassis showing the corrosion that resulted from the receiver setting under a leaking pipe in a basement. This chassis is SN: 931 and the restoration chassis is SN:194. Note the difference in the filter capacitor construction between the two chassis. Other differences noted was the 1.0uf under the chassis capacitor was an Aerovox on 194 and a Sprague on 931. The dial scale was not coated with MFP on 194 but was coated on 931. Other differences noted were IF transformer wires were color coded in 931 and were not in 194. The original test sheet for 194 is dated August 1943. I would guess that 931 is probably a few months later. It's interesting that so many differences are noted in receivers that are within the same contract. 
Restuffing Original Paper-Wax Capacitors - I wanted to actually use this NC-100ASD set-up with an ART-13A transmitter for a vintage WWII station. This required that the receiver be reliable since it would be in operation for extended period of time. There really isn't an option when the receiver has paper-wax capacitors and long-term operation is desired. You have to replace the capacitors. However, I wanted to also preserve the "under the chassis" original appearance as much as possible. This leaves "restuffing the caps" as the only option.

In the past, I've melted out the old cap, hot-melt glued the new capacitor in the old paper shell and then filled the ends with brown sealing wax. I left the paper shell clean and somewhat "wax-free." This looked really nice but not authentic since the original capacitors were dipped in bee's wax. I decided this time to try a slightly different approach.

I still removed the capacitors "one at a time" to restuff. This avoids confusion or any difficulty remembering where the caps are to be installed and their proper orientation.*  I use a heat gun to apply enough heat to remove the old cap from the paper shell. I melt the excess old wax into a metal wastepaper basket (it is a messy process.) I hold one lead of the old capacitor with needle nose pliers and when everything is hot I pull the paper shell off with a paper towel acting as a "heat protector" for my fingers. I wipe the excess wax off of the shell and let it cool for about one minute. Next, the new cap is inserted and held in place with hot melt glue. Leave about 0.250" space on each end. Let the hot melt cool for a few minutes. Now this was where I changed what I had been doing. This time I end-filled the caps with bee's wax. I just use a soldering iron to melt the wax and fill the ends a "drop at a time." When the bee's wax is cool enough, I then melted more bee's wax over the outside of shell. When this was almost cool, I wiped the excess wax off with my fingers. The rebuilt cap was then installed into the circuit. This procedure was followed until all 26 paper caps were rebuilt.

Shown in the photo to the right is the end result. It's difficult to tell that the paper capacitors have been rebuilt. The coating of bee's wax is the final touch that really imparts an authentic appearance. In fact, at first glance it was so difficult to tell the caps that had been rebuilt from those remaining, I had to place a small square of white tape to each rebuilt cap. The white tape pieces (~ 0.250" squares) were removed after the job was finished.


photo above: The completed capacitor rebuilding is difficult to distinguish from original. The electrolytic capacitor in the upper left corner was also restuffed-rebuilt. This requires sawing the can apart (mark it for proper reassembly) then installing a new electrolytic. The can is reassembled with a paper sleeve inside to hold epoxy to insure that the bond has a lot of glue. Note the "600-900-1200" written on the catacomb. This was a portent to future surprises when aligning this receiver.
*   I don't use this "one at a time" method anymore. I use a capacitor location, value, orientation and connection drawing for reference to allow the removal of several capacitors at a time. These are then rebuilt and then reinstalled using the drawing for reference. Then several more originals are removed and go thru the rebuild and are then reinstalled. This process is repeated until all of the capacitors are rebuilt and reinstalled. I find this method less time consuming and allows for a separate workbench to be set up specifically for capacitor rebuilding.
Restringing the Dial Pointer on the NC-100A Receivers - The action of the articulated dial pointer is controlled by a dial cord that is tied to the band change shaft, then is routed up to a dual pulley, then strung around the tuning shaft, routed to the dial pointer mechanism pulley, then back over the tuning shaft and finally routed down to an anchor tie point on the receiver chassis. A single dial string is required that is around 20" long and is about 0.025" inches in diameter (actually most dial cord is now measured metrically, so slightly less than 1mm.) The length is much longer than necessary but gives you enough for the knots required at each end of the run (and then some.)

Note in the photo to the right that the dial cord wraps over the shaft in a clockwise direction when viewed as shown. You have to tie a knot in one end of the dial cord and then route it thru the hole in the shaft making sure that you end up with the cord going over the shaft as shown. Be sure that the coil box is set to the lowest frequency band (box all the way to the right as viewed from the bottom-front.) Route the dial cord thru the hole in the chassis as shown.

Next, look at the photo below-left and note that the dial cord exits the chassis hole and is routed up to the large sheeve of the dual pulley. Be sure you wrap the dial cord counter-clockwise (viewed as shown.) You will have to wrap the cord at minimum one and a half turns. You will see that there are two holes in the dual pulley that allow you to thread the cord thru the upper hole to a lower hole to allow the cord to wrap around the small sheeve of the dual pulley. The cord is only going to wrap over the small sheeve and then be routed to the top of tuning dial shaft. Looking closely at both bottom photos, you can see that the cord wraps over the tuning shaft and down to the small pulley that works the dial pointer lifter. Loop the cord around this small pulley and back up to the tuning shaft. Route the dial cord over the tuning shaft and down to the tie point that is mounted to the chassis.


photo above: Note how the dial cord is routed in this photo. The dial cord use here is "waxed twine" that is 1mm in diameter. Don't use any larger diameter than 1mm. Slightly smaller diameter works much better and is easier to route thru the two holes in the pulley.
Before you knot the dial cord test the operation of the dial cord by changing bands and noting that the articulated pointer moves as the band changing shaft is rotated. You'll have to hold the dial cord at the tie point with your fingers while testing the operation.

Where the dial pointer "points" is dependent on the mounting of the dial scale and the tension of the dial cord which is dependent on the last knot at the tie point. You can string the dial cord with the dial scale removed but final setting of the pointer position will require that the dial scale be mounted (at least temporarily.) Test how the pointer operates and if it points to the correct bands on the dial scale. When satisfied, tie the final knot at the tie point. Be sure to knot at least three knots and glue the knots with Duco cement.


photo above: Note the routing of the dial cord thru the two small holes of the pulley by observing the cord "loop" at the back of the pulley. Also, the missing paint on the chassis was touched-up after the photo was taken. See chassis photo below.
Reassembly - The RF Gain control is mounted in a bracket that requires the control to have an extension. You should be able to mount the pot with the front panel/sides part of the cabinet somewhat in position. Once you have the RF Gain pot mounted and its extension installed, then you can slide the cabinet into position. Next, install the remaining pots and switches with their appropriate hardware. Mount the S-meter using the S-meter bracket. This must be done before mounting the dial panel because the bracket screws are hidden when the dial panel is installed. With the S-meter mounted, then install the dial panel with its glass dial cover in place. Be sure you have a gasket around the glass perimeter to protect the edges from chipping. I use a 0.25" wide piece of friction tape to make a gasket and since its tape it stays in place while the glass and panel are being installed. Be careful when tightening the dial panel screws and be sure the glass is not binding or flexing anywhere. Proceed with the tightening if no binding is encountered. Next, install the mounting hardware for the controls that mount to the dial panel. Be sure to have installed washers on the controls to act as spacers between the cabinet in the dial panel. Only one washer per control is needed and they should be around .030" thick. The RF Gain control just exits the panel with no other hardware. Be sure to install the cabinet to chassis spacer which is a rectangular piece about 0.190" thick that is held in place by the two lower front panel screws. Install all of the other cabinet screws, washers and nuts and tighten. Be sure to install all screws. Position the rear panel/top lid piece of the cabinet in mount with the appropriate screws. All of the cabinet screws that hold the rear panel/ top lid piece in place are short sheet metal screws. Install all of the knobs and bottom panel. This completes reassembly.

photo right: This is the chassis of sn: 194 after cleaning. Note the older style filter capacitors. The tops of the filter capacitors were eventually painted silver so they didn't look so much like derelicts.
Preliminary Testing Reveals Other Problems - The NC-100ASD powered-up with no issues and seemed to be fairly well in alignment. Most stations were close to their proper frequencies on the dial. I then switched over to Band H, the 200kc to 400kc MW band, and heard AM BC stations - that can't be right! I tuned across the dial and found that 200kc to 400kc actually tuned .5 to 1.2mc. I removed the catacomb top cover and inspected the coils and found that an "H" set was installed but had been modified to cover the AM BC band by removing the adjustable padding capacitor. Probably, a former owner wanted to listen to AM BC and trimmed the LO coil assembly until the AM BC was tuned in. Although "H" is the correct coil set, ASDs were also fitted with "G" coil sets that also cover 200kc to 400kc. Fortunately, I had the "Wilkins" ASD parts set that could provide an original set of "G" coils. When removing or installing the coil assemblies just be sure that the pins are disengaged. Just switch the catacomb "between bands" and then the pins on all coil sets are disengaged. Once the swap was made, I checked the mechanical alignment by switching bands to see how the pins engaged. You just about can't get the coil assemblies mounted any other way than correct with proper mechanical alignment. Receiver operation on 200kc to 400kc tested "as expected" with the "G" coils installed.

A second problem involved audio distortion on strong signals. Very strong signals would "cut-out" as if the receiver was in  MVC and was being "over-loaded." The problem turned out to be a Russian metal octal "look-alike" 6K7 in the 2nd IF amp. The tube tested like a 6K7 and looked (at a quick glance) just like a 6K7. So I installed it into the 2nd IF position. The distortion problem was found by tube substitution. Close examination of the Russian tube showed that it was a "6X7" with the "X" actually being the Russian letter that looks like two "K" letters back-to-back. The nomenclature was all engraved and black so close examination was required to read everything (and that showed that it was all in Russian.) Anyway, a standard 6K7 cured the problem. >>>

>>>   A third problem involved strong images on band B. Normally, with a single TRF stage, one wouldn't expect to hear images until around 15mc. However, 10mc WWV was tuned at both 10mc and 9.1mc at about the same strength. The image on 15mc WWV was barely heard which seemed to indicate something was wrong with the B coil sets. The problem appeared to be caused by a defective Mixer coil assembly which had a damaged "ticker" coil - probably done when trying to adjust the inductance loop. Luckily, the parts set provided a complete set of "B" coils. After installation of the "B" coils, alignment of that band was necessary. After alignment the 10mc WWV image was barely perceptible at 9.1mc. 

Alignment - The IF section is straight forward and is aligned to 455kc. I just inject an RF signal at the Mixer grid that is modulated with 400 hz and use the S-meter as an indicator. The receiver will have to be operated in AVC with the RF gain at 10. Adjust the gain by using the RF signal generator's attenuator to have the S-meter reading about mid-scale. Adjust each IF transformer for maximum indication on the S-meter. Since there's no Crystal Filter, the IF is not ultra-critical but try to adjust as close to 455kc as possible since this will affect RF tracking somewhat.

RF alignment uses an air-trimmer for the high end of the band and the famous National "coil loop" in each of the inductors for adjustment of the low end of the band. I use a plastic tool to move the coil loop to adjust the low end and adjust the air trimmer for the high end until tracking is correct. With the catacomb receivers you can adjust the loop with the receiver in operation (unlike the HRO.) This means you can observe the S-meter (or AF Output Meter for non-A receivers) and adjust the low end inductance exactly (much easier than the HRO.) The ASD has a padder capacitor for low end adjustment on the 200kc to 400kc band. Once you have good tracking adjusting the LO section then adjust the Mixer and RF Amp sections at the high end of the range for maximum indication on the S-meter. You can check the low-end Mixer and RF Amp adjustment by moving the coil loop while observing the S-meter. Normally, these seem to be set correctly and the movement of the loop only confirms that they were in adjustment. This completes alignment.

Performance - I've set up sn:194 with one of my ART-13A transmitters as a vintage WWII military station. Granted, the ART-13 is an "aircraft" transmitter and wouldn't have been paired with a "ground" receiver like the ASD, but still, they are of the same vintage and go nicely together. In such a use, the ASD has to be able to receive reasonably weak signals through various types of QRM and propagation conditions. Although almost any receiver can provide decent reception when the operating frequency is in the 3.8 to 4.0 mc part of the spectrum when conditions are good, a usable receiver will be able to cope with all of the different types of reception problems and provide the user with Q-5 signals.

>>> The ASD is easy to set up as a station receiver. The remote stand-by uses a screw terminal strip on the rear chassis. Having the matching speaker helps the audio since the 500Z ohm matching transformer is mounted on the speaker itself. The eight inch Jensen does sound pretty good and with the 8kc bandwidth the signals won't have too many highs and thus will have a mellow sound. The ASD doesn't "roll-off" the bass like the USN RAO receivers do. The ASD audio is natural sounding with respectable bass response.

>>>  The ASD does have its limitations. The tuning rate is very fast as all general coverage NC-100s are. It makes CW and SSB difficult to tune until you develop the "micrometer finger" touch to your tuning ability. You can use the BFO to fine tune SSB or CW signals. Lack of a Crystal Filter leaves you with only the receiver's IF bandwidth to combat QRM. Bandwidth is around 8kc at 20db down. That's fairly selective but might require some fine tuning to either sideband on AM if QRM is encountered. Also, one might find that going to MVC and reducing the RF Gain will help with QRM or heavy static. The Limiter normally works best in the AM mode with the receiver in AVC with the RF Gain at maximum.

The ASD is relatively small and the weight is around 60 lbs. It's easy to find a place for it and it's not too heavy to move it there. It's a nice looking receiver that's easy to use and has pleasant, natural sounding audio on AM.

UPDATE: After using the NC-100ASD as the station receiver with the ART-13A for several months I have to say that I'm impressed with the receiver's performance on 75M. The audio is very natural sounding and selectivity is more than adequate for the normal adjacent QRM. In fact, our Vintage Military Radio Net has been plagued for the last couple of months with a pair of SSB stations "parked" 2kc up from our net frequency. I have to say that the NC-100ASD does a good job handling the adjacent frequency QRM but I do have to tune down slightly (~ 2kc) to cope with the interference. The fact that a WWII-era receiver with no crystal filter can effectively null QRM is impressive. However, I was really impressed a few weeks ago when KA7NGT from Mineral, Washington (using an ART-13 xmtr) checked into our Vintage Military Net and his signal was Q-5 on the NC-100ASD. So, I'd have to rate the NC-100ASD sensitivity on 75M as excellent and it ability to cope with QRM, impressive. Naturally, as the operating frequency is increased the difficulties encountered will also increase but I'm sure any operations on 160M, 80M or 40M would present no problems.

 

RCE  SN: 302  -  Airport Communication Receiver - Mechanical Rebuild

This restoration was more mechanical and cosmetic than it was electronic. It involved finding and installing correct vintage parts along with correcting other mechanical problems. Additionally, front panel paint "touch-up" and the mechanical approach to "body work" is covered. Further, the methods for operating a vintage receiver on "original parts" is presented.
I received an e-mail from fellow Vintage Military Radio Net member WA6OPE asking if I would be interested in a National RCE Airport Receiver. He included some photos that showed the RCE to be complete but in rather "rough" condition. A deal was made and Tom brought the RCE up to the N7RCA Ham Swap Meet in Minden, Nevada (first weekend in June 2014) for delivery and payment. Tom had found the receiver in the SF Bay Area, apparently from a "non-ham" as the RCE had never been worked on or modified since it had been retired from commercial use. Storage in the SF Bay Area, with its salt-air environment, is problematic and long-term storage in a garage or shed can result in significant corrosion. The photo of the chassis below shows the typical surface rust that is found on metal that is stored in unheated buildings. Not terrible but certainly significant. Fortunately, the underneath of the chassis was nearly perfect which implies that condensation was probably responsible for most of the rust.

The dust cover being present is somewhat rare. It seems like many of the Airport receivers now turn up with the dust cover long-gone. This dust cover is made out of aluminum - the later Airport receiver dust covers were made out of steel. The condition of the dust cover is very good but somewhat bent in the back because the rear screws weren't installed. >>>

photo right: The RCE exterior as received before any cleaning by me. The exterior had been wiped down.

>>>  The knobs are correct but it's odd that the RF Gain, Audio Gain and CW Osc knobs are long-boss versions and the Tone is a short-boss type. The knobs appear to have the same patina and probably are original. This implies that National inter-mixed the knob styles on production. This seems normal for National and the fact the the difference is hardly noticeable further convinces me that the knobs are all original.

UPDATE - July 21, 2014 -  Someone in the past "forced" the PW-D to turn when the tuning condenser or part of the gear box was "frozen." This stripped the main condenser drive gear and the right side reduction gear of several teeth and deformed both gears. The end result was although the PW-D appeared to rotate it only did so for about one turn and then "locked up" due to the deformed gear teeth. Fortunately, I had a few extra gear boxes and I thought that a gear box replacement would fix the problem. However, the original gear box shaft is designed to fit the tuning condenser shaft that is about .375" in diameter. The "after-market" gear boxes were the type National sold for projects and were designed to fit a .250" shaft. Additionally, the original gear box output shaft was "pinned" to the tuning condenser shaft. Needless to say, the two types of gear boxes weren't directly interchangeable.

As I thought about this project, it seemed like the way to go was to find a complete NC-100 gearbox and tuning condenser assembly. That would solve the severe rust problem on this tuning condenser and would solve the broken gearbox problem too. About a nine months later (April 2015,) I was able to purchase a complete NC-100 N-PW gearbox with the three gang tuning condenser assembly attached. Also, included was a good condition PW-D. Thanks to George W7HDL.
     
photo left: The RCE interior before any cleaning. Includes lots of spider webs, rust, grease and dust.

Tuning Capacitor Transplant  - May 31, 2015 - This operation required removal of the front panel which is easily accomplished. With as much corrosion present, it was quite a surprise that the PW-D wasn't a problem to remove. The BFO knob required some light oil and heat to break its set screw loose but all other knobs came off easily. I gave the chassis a quick cleaning and although there are quite a lot of small rust pits, it's not so bad that the chassis needs repainting. Cleaning has really helped the overall appearance (see photo below.)

The first thing to do in the removal of the old tuning capacitor is to desolder the seven TC (tinned copper wire) connections at the solder lugs on the tuning capacitor. The lugs aren't difficult to access if you have a small soldering iron ( I use a 25W Weller.) National didn't wrap the TC wire connections probably because they knew if removal became necessary it would be difficult to "unwrap" the connections. Luckily, the TC wires just slide out when the solder is molten. The LO grid leak is on a tie point that is soldered to the stator lug. Just desolder the entire tie point from the stator lug. Once the seven connections are off, the capacitor can be dismounted by removing the mounting screws.

The tuning capacitor has four screws under the chassis that mount the gear box. These need to be removed. You have to move the catacomb from one end to the other to access all of the screws. There is also a bracket at the rear of the tuning capacitor. Remove the chassis screw and leave the bracket mounted to the capacitor (for now.) At this point you should be able to lift the capacitor/gear box assembly off of the chassis. This reveals the tray under the capacitor that has the three insulator blocks with the terminal connections exposed. This area will probably be very dirty and filled with all sorts of solder bits, wire ends, dust and grease - mine was (see photo right.) Clean the tray area using Q-tips to clean the insulator blocks and hard to access areas of the tray. I use WD-40 as a degreaser followed with Glass Plus to remove the WD-40 residue.

Be sure the seven lugs on the replacement tuning capacitor are tinned and the holes are fully open. On the terminal blocks, you'll note that there are three short wires and three longer wires plus the wire that has the LO grid leak. The short wires connect to the stators and the long wires connect to the rotors. You have to route the longer wires with a slight bend under the stators and then up to the rotor lugs. Originally, these wires were not sleeved but, if you are worried, you can sleeve them since sleeving was used on some of the later Moving Coil receivers. Guide the TC wires into place as you lower the replacement capacitor into place. You will need a slotted soldering aid tool and long thin needle-nose pliers to guide the TC wire through the holes in the soldering lugs. 

Once you've soldered the seven wires you can then mount the gear box and the bracket. If your replacement capacitor doesn't have the bracket (mine didn't) use the bracket from the original capacitor. After the capacitor is mounted, disengage the catacomb (place between bands) and check with an ohm meter if you have any shorts from the stators to chassis. Check rotor to stator which should also not have any shorts. Mount the PW-D and check operation of the entire assembly. No problems should be encountered.

photo above: This is a shot of the tray under the tuning capacitor of the RCE with the tuning cap removed. Note the dirt and grease that collects in this area. If you don't need to remove the tuning cap, this area can be cleaned with Q-tips and small long paint brushes. With the cap out though, the area is easy to clean.

Note the TC wire. On each block, the terminal to the left is the stator connection and uses a short length wire. The remaining longer TC wires connect to the rotor lugs. When mounting the replacement cap, you'll have to guide these wires into place as you lower the cap into place. It sounds difficult but it just takes a little patience and good illumination. Be sure to solder the TC connections first and then install the screws to mount the gear box and bracket.

Note the corrosion on the chassis. While a perfect condition chassis would be great, sometimes we have to accept that some receivers were located in humid areas and stored in sheds or unsealed garages. This corrosion is not bad enough for a repaint and most will be hidden once all of the parts are reinstalled. Some minor "touch-up" painting will also be required.

Front Panel Restoration - The front panel on the RCE was bent and had several nicks to the black wrinkle paint. The panel needed to be straightened out first.  I "eyeball" down the edges to see where the bends are. The panel is then placed on a large pine board that is perfectly flat (well, as flat as wood is going to be.) I then use two inch square oak wooden blocks about four inches long and a weighted hammer to gently bend the panel into proper alignment. If carefully done, the result will be a straightened panel with no damage to the paint. More delicate types of paint will require paper or cloth be placed on the pine board for additional protection but old wrinkle finish is tough and quite resistant to scratching or chipping and can be placed directly on the pine board.

After the panel is straight I clean it with Glass Plus. I then use nitrocellulose black lacquer that has been thinned down (~ 3 to 1) with lacquer thinner. I use this paint to touch up any significant nicks or scratches. Of course, when the "touching up" is finished, the panel paint looks uneven and spotted. The next step is to use several cotton pads - the round 2" diameter flat pads work best. Wearing nitrile gloves, dab the cotton pad with thinned black lacquer and rub the entire panel to even out the finish. Don't saturate the engraving but don't be too worried if some of the engraving is darkened by the thinned paint. >>>

>>>  The lacquer will dry in a few minutes and then the engraving has to be "reconditioned." I don't want to scratch the metal so I use a long round wooden skewer (like you use for barbequing) that I've sharpened the end of. I use this tool to go over the engraving to remove any paint, dirt or corrosion that's in the engraving. I use a head-mounted magnifier to enhance my view of the engraved nomenclature as I'm working. The result will be nice looking engraving that doesn't look "scratched out" but just looks clean and somewhat bright.

Dust Cover Body Work - The rear panel of the dust cover was really bent up. I used the same body work method of a flat pine board and oak wooden blocks with a weighted hammer to straighten the panel. The rest of the dust cover just needed a little "tweaking" to get all sides and corners square. The back of the dust cover was pretty scratched and the top wasn't much better. Sides were okay. I decided to give the dust cover a light coat of "Satin Black" paint to even out it's overall appearance.

Chassis Cleaning - Since there was quite a bit of surface rust I first cleaned the chassis with WD-40. This removes all of the grease and most of the dirt. I tried some different methods on the rust spots but found that the best results were to use WD-40 and lightly rub with 0000 steel wool. This reduces the rust spots to a minimum and doesn't hurt the chassis paint. Don't be too aggressive though. Lightly use the steel wool. Avoid the ink-stamped tube identifications as the steel wool will remove these. Next, wash the chassis with Glass Plus and a paper towel. You can somewhat "polish" the paint with the paper towels. This gets the chassis paint looking about as good as it can.  The aluminum cans were slightly pitted under the dirt. I lightly removed the dirt with WD-40 applied with a cotton cloth. Use a circular motion to avoid streaking of the cleaning action. Don't rub too hard or you'll polish the aluminum which then doesn't look original. Lightly clean the WD-40 with Glass Plus. The end result should look flat matte finish that is clean. The audio output transformer was the worst condition and really didn't respond to much cleaning but the IF cans turned out quite nice.
Bar Knob Skirts - The procedure to clean these solid German Silver skirts is in the section "Detailing the NC-101X" further down this page. Essentially, NaOH (Easy-Off Oven Cleaner) is used to remove corrosion and grunge. Then the knob is washed in water to remove the NaOH and cleaned with Glass Plus. Then the skirt is polished with Wenol's. The skirts are the same material all the way through so this method works great.

photo left
:  The RCE chassis after cleaning and installation of the replacement tuning condenser and gear box. Although the chassis paint isn't perfect, it's not too bad. The aluminum cans turned out matte finish after cleaning.
PW-D Repaint - The original PW-D was "lumpy" with corrosion under the paint. This is difficult to remove since the base metal is damaged. I decided to just use another PW-D that was in better condition but still needed a repaint. The Airport receivers used a light-gray paint on the Index dial and then black with white numbers on the Number dial. I had an excellent condition gray PW-D that was on my RCF-2 receiver. This was used to "match" some automotive lacquer paint from All Auto Paints in Carson City. They will do the match and then fill a spray can with the paint for $18.

The old paint has to be removed from the Index dial. This can be accomplished with Jasco Epoxy Stripper (methol chloride - so do it outside using Platex and nitrile gloves.) Clean with steel wool and alcohol and then clean the Index dial with lacquer finish afterwards. This prep is enough to then apply the lacquer without any primer (that's the way the originals were painted.) Don't spray more than two coats since modern auto paint has some fillers in it which might "fill" the index lines too much. Two coats should be enough. Let the paint set overnight. Then use a mixture Artist's Acrylic paint of white and raw sienna to create a manila color (beige color) to fill the index lines. Don't use pure white - it will look way too bright. Apply the manila paint a few index lines at a time (five to ten works well.) Let the paint set for one minute and then wipe off with a damp paper towel piece (dampened with Glass Plus works best.) You might have to do two applications of paint to get a really good looking fill. Let the index lines dry for a day. Wipe the Index dial with a cotton cloth. Reassemble the PW-D if the Number dial is is good shape.

Initial Testing - Since the commercial and military versions of the NC-100 receivers use oil-filled paper dielectric filter capacitors, frequently they can be operated with all original parts. I really wasn't planning on using the RCE as a station receiver and the set would only be powered up for testing and maybe a casual listen around the bands. When long-term use is planned (like the NC-100ASD or the NC-200 profiled above) it is necessary that all paper-wax capacitors be replaced since their leakage current is only going to get worse as the receiver is operated. But, I felt that the RCE could be safely operated with all of the original components if the filter capacitors tested okay. I just checked these parts for shorts and for value and found them to be okay. Before power-up, it's a good idea to just check the DCR of the B+ to chassis. It should be very high resistance which would indicate that there are no "power-off" shorts to chassis.

Upon power-up, I watch the rectifier tube carefully. If there is a short or other type of excessive load on the B+, the rectifier tube will glow bluish-purple inside the filament to plate area. If the rectifier appears normal, then B+ should come up to voltage in about 20 seconds or so. I measure it to be sure it's in spec and not loaded down. I watch for any indications of sudden heating which would indicate a heavy current flow through a resistor due to a shorted or nearly shorted bypass capacitor. Luckily, none of these things happened and the RCE powered up fine and did work somewhat. Of course, since I had replaced the tuning capacitor, it was pretty much out-of-alignment but still a few AM BC stations could be tuned in which let me know the receiver basically was operational. I performed an IF alignment and found that was pretty close. The RF tracking was way out, as expected. I kept having trouble with the contacts of the catacomb with the receiver on its side (necessary for RF tracking alignment.) The catacomb had to come out for further checking of the pins and receiving contacts. Mechanically, the catacomb seemed to be "flexing" the chassis when bands were changed which also warranted a thorough inspection.

Catacomb Movement Issues - The catacomb problem was due to some corrosion spots here and there on the shaft. These were removed with 600 grit Alu-Oxide paper and oil. The bearings in the catacomb were also oiled. The flexing of the chassis was due to the shaft mounting screws not being fully tightened. The front bearing for the pinion gear needed another spacer to be added to have the rack and pinion gears fully mesh and to have the shaft bearing fully within the catacomb slot. A light coating of grease was applied to both the shaft and to the rack.

On the pins and split-contacts, they were of course dry which increased the force necessary to have all fifteen pins to engage the contacts. Originally clean grease was applied to the pins although nowadays this has dried up or has turned into a "dirt-trap." I cleaned all of the pins and split-contacts with DeOxit and Q-tips, reapplied new clean grease (very light coating) and then reassembled. The bandswitching was now a fairly light action and there was no more flexing of the chassis.

Back to Aligning the RF Tracking - The LO kept dropping out at the low end of the bands on bands A, B and C. Bands D and H seemed okay. The entire operation was somewhat erratic. I checked the connections to the tuning condenser. I found that during soldering of the TC connections I had not soldered the lower TC wire to the LO stator. The wire was thru the hole in the lug but just not soldered. This was enough for the LO to not operate correctly on the higher frequencies. Soldering the TC wire corrected the problem and the alignment was completed on all bands. 

Final Testing - Since the RCE was operating on all original paper capacitors, I was particularly careful about checking the heat build-up in the power transformer. This is a crude method of determining how much of a load there is on the B+ due to leaking capacitors (one can also measure the current draw of the B+ - it should be around 70mA nominal.) During an alignment, the receiver is going to be in operation for at least a half-hour. Checking the power transformer and the chokes several times for heat, I was surprised that nothing got hot. Again, I would stress though,...I'm not planning on operating the RCE for extended periods of time. If I was going to do that then it would be almost mandatory that the paper caps be replaced. For short test periods, original parts are okay if they not defective (other than the expected minor leakage current.)

Performance - I'm quite surprised that this RCE actually works better than my much nicer condition RCF-2. I guess that goes to show that appearances can be deceiving. Anyway, the RCE now looks pretty good but how does it perform?

I used a 7000Z ohm to 8Z ohm transformer on a 10" speaker that is mounted in the larger style National speaker housing for the RCE. When I tested the National RCF-2, I used a military LS-3 speaker that had a 600Z ohm transformer installed. However, the DCR of the output transformer on the RCE measures 280 ohms which seems too high for a 600Z ohm output which would normally be around 50 to 100 ohms DCR. The 7K set-up seemed to provide a normal reproduction level. AVC has a rather slow attack, probably a capacitor problem. Sensitivity is respectable with 40M ham signals coming in strong. I tried 20M and also heard a few hams there. The surprise was 11M CB with many strong AM signals in that region of the spectrum. No ham 10M sigs were heard. The AM BC signals (above 1300kc) were strong but the audio not very good. The DGPS "screamers" around 300kc were quite strong on band H. I didn't hear any NDBs because I was testing performance during the daytime.

I'm sure that if I went ahead an did a full "re-cap" and replaced any "drifting" resistors followed by another complete alignment, the RCE would be a really great performer,... when compared to other similar pre-WWII receivers.

 

Detailing the NC-101X Receiver

Many times the receivers we find don't really require a total restoration or rebuild. Sometimes only a little cleaning and repair of a few minor problems will result in a usable receiver that is performing quite well. I call this "detailing" a receiver. Mainly a cosmetic job with only a little electronic work necessary. Here's how we did this type of task on a NC-101X receiver recently.

Initial Inspection - I bought this NC-101X from an ad on QTH.com with only the photos and description to go by. Dave N7RK was very accurate in both departments and his packing job was first rate. The NC-101X arrived with no damage. It is shown in "as acquired" condition in the photo to the left. An inspection found that the receiver was as described,...very nice external condition with just a few blemishes and dirty but mostly complete on the inside. The dirt was a dusty type with some grease and maybe some tobacco deposits mixed in. This "dirt" made a very good preservative for the gray paint used on the chassis. Dave had said the receiver worked so I wasn't shy about just "going for it" and applying power. Although the NC-101X did function it had some hum problems. Removing the bottom cover revealed that the dual electrolytic was disconnected. I figured this was probably for a reason and decided to replace the filter caps before going any further.

I mounted the three 10uf filter caps under the chassis. Since all of the original paper caps had been replaced several decades ago with Sprague molded capacitors (not Black Beauties but the later versions with a black plastic body with red nomenclature) I felt these were probably okay. Upon power up the NC-101X now seemed to work fairly well.

I hadn't tested any of the tubes and really hadn't done much of anything other than confirm that the receiver did function and all of the major parts of the set worked fine. All that was now necessary was detailing the cosmetics and a complete servicing of the electronics.

Chassis Inspection: The chassis "as received" is shown in the photo to the right. The dirt is obvious and as mentioned above actually acted as a preservative for the paint. The first thing noticed is that only one can-type electrolytic capacitor is present. Although it is very similar to the originals, it actually isn't an original type. The original filters were all 10uf 450wvdc individual capacitors. The one installed is a dual capacitor which is not correct.

The next thing noticeable is that the power transformer appears rather tall. A quick look under the chassis showed that the power transformer was indeed marked "NC-100" but the complete ink stamp was "NC-100-25" which indicated that this transformer was for 25 cycle AC. All power transformers for 25 cycle operation will have extra laminations installed to increase the iron and prevent over-heating on such a low frequency AC. Operating a 25 cycle transformer on 60 cycles will cause no problems and many so-called 25 cycle transformers are actually rated "25 to 60 cycles."

The tuning condenser appeared to be slightly rusty however this wasn't the case. What appeared to be rust was actually greasy dirt probably mixed with tobacco smoke that turned the metal pieces a rusty color. The rotor hubs are brass but it's difficult to tell through the dirt.

The tuning seemed to work fine so that indicated that the gear box probably only needed to be cleaned and lubricated.

I was lucky that nobody had tried to clean the aluminum IF transformer cans and BFO can. Improper cleaning of this soft aluminum will end up polishing the matte finish.

Under the Chassis -  This area had been recapped several years ago using Sprague black plastic molded capacitors (with red nomenclature - not Black Beauties.) The installation appeared to be a professional job with all of the capacitors matching in manufacturer and type used. Soldering appeared professional. Many hams WERE professionals in the electronics business besides being hams. All of the resistors were Centralab types that National had changed over to by 1937. These Centralab resistors are excellent in quality with carbon drifting almost never being encountered. Cosmetic Inspection - The NC-101X only had some minor blemishes to the original black wrinkle finish. The skirted knobs were very dirty with tobacco deposits and some tarnish. The PW-D dial, while in pretty nice original condition, did show some wear especially at the point where the Index dial angles off. Here the paint was worn through to the metal. There were no gouges or scratches as is normally found on poor condition PW-D dials. There were no extra holes drilled anywhere on the chassis or the cabinet.
Getting the NC-101X Operational - This isn't restoring, it's just getting the receiver to a point where I could use it and do some evaluation of its performance. The new filters got the NC-101X running with no hum. The receiver came with 6V6 tubes installed in the audio output. Original tubes were 6F6 and while you can substitute the 6V6 and it will draw only 2 amps of heater current versus the 3 amps required for the 6F6, there are internal structural differences between the two tube types. The 6V6 is a beam-power pentode while the 6F6 is a regular pentode. To some ears, the 6V6 will sound a bit harsh compared to the 6F6 if the circuit and transformers were designed for 6F6 tubes. This was the case with the NC-101X, the 6F6 tubes sound a little bit mellower and slightly more bassy - at least with the original 10" Rola electrodynamic speaker.

At this point the NC-101X was performing well enough to try using it as a station receiver. I used it on both 80M and on 40M on the AM nets available on those bands. The NC-101X was surprisingly effective as a station receiver. Sensitivity was ample although successful 80M or 40M reception doesn't require too much of a receiver anyway. The important thing was how the receiver sounded and how immune to adjacent SSB activity it would be. Performance was good enough that I decided to proceed and do the complete "detailing" followed by a complete IF/RF alignment.

Disassembly - Without removing the cabinet it's next to impossible to access all of the areas of the chassis for complete cleaning. The cabinet is easy to remove. First, remove all of the tubes. Then take off all of the knobs. Then note that the RF Gain control is mounted to the front panel as is the Tone Control. Remove the mounting nuts on these two controls. Remove the mounting nut for the phone jack assembly. Dismount the pilot lamp assembly. Remove the back cover and lid assembly by removing eight mounting screws. Now remove the four large screws, nuts and lock washers that mount the cabinet and front panel assembly to the chassis. The cabinet and front panel assembly should now slide forward to clear the control shafts and then can be lifted off. Cabinet removal exposes all of the chassis and chassis mounted components for thorough cleaning. Cleaning the Chassis - As mentioned, the dirt and grunge was a dust combined with grease and tobacco deposits. Since grease was involved I used WD-40 as a cleaner. WD-40 generally will not react with older types of paint, which in this case was lacquer. The dirt and grunge came off very easily and left the chassis paint clean and having somewhat of a shine. The application of the WD-40 was with a medium size paint brush and sometimes Q-tips when the area was too small for the brush. Paper towel strips were used for WD-40 removal. This was followed by another cleaning using Glass Plus to remove the WD-40 residue. Glass Plus doesn't contain ammonia and won't react with painted surfaces (don't use Windex.) The ceramic tube sockets were cleaned with De-Oxit and a clean tooth brush and wiped down with a paper towel strip. As expected, the grunge had acted as a paint preservative and the chassis turned out to be in excellent condition.
Cleaning the Tuning Condenser - Again, WD-40 was used as a cleaning agent due to the greasy nature of the grunge. Small paint brushes and Q-tips were used for the most part. The nickel plated spacers were cleaned with a brass tooth brush (brass won't scratch the Nickel plating) and WD-40 which removed the discoloration completely. The ceramic spacers had to be cleaned with Q-tips. After all of the grunge was removed, the tuning condenser was further cleaned using Glass Plus to remove the WD-40 residue. Paper towel strips or Q-tips were used to remove excess Glass Plus. After the tuning condenser had time to dry (overnight) it was lubricated with DeOxit on the rotor contacts and 10W oil on the bearings.

Cleaning Under the Tuning Condenser - This area seems to really attract dirt. I didn't want to remove the tuning condenser so I used long handled small paint brushes and bent Q-tips to clean this area with WD-40 followed by Glass Plus. You have to fit the brushes and Q-tips under and sometimes through the tuning condenser. 

Photo Right: This shows the chassis after half of it has been cleaned with WD-40 and Glass Plus. The tuning condenser has also been cleaned in this shot. Note that the three electrolytics were removed for cleaning. Actually, two were missing and the third wasn't original and wasn't connected up anyway. With the electrolytics out of the way, cleaning is easy around the tuning condenser and reinstallation of these capacitors is not difficult. I didn't do any polishing of the chassis paint. This is how certain kinds of dirt and grunge end up protecting the original paint finish. The WD-40 cuts through the grease and removes it and the associated dirt. Then the Glass Plus is used to remove the WD-40 residue.

The PW-D - The original PW-D had a badly worn Number Dial that "chattered" if the dial was turned too fast. Even lubricating the elliptic hub didn't stop the "chattering." I swapped another Number Dial to correct the problem temporarily. The Index Dial was in pretty good shape with no scratches but the paint was worn through around the tuning knob. Eventually, I wanted to find a good condition replacement. It ended up I didn't have to wait all that long. A couple of weeks after acquiring the NC-101X, I was at the annual Minden, Nevada swap meet put on by N7RCA. What should turn up but an excellent condition PW-D in a protective plastic bag - nice. The paint was very close to the original NC-101X's PW-D in that it was the dark bronze-black shading even though the new PW-D was actually a WWII vintage piece. All the new PW-D needed was to be disassembled, cleaned, lubricated and reassembled to end up functioning perfectly - and looking incredibly nice. Cleaning the Skirted Knobs - The knobs on this NC-101X were the early style knobs with the short boss on the bakelite bar knob section. These knobs were advertised by National as having skirts that were "solid German Silver." German Silver is an alloy of Nickel, Copper and Zinc that generally is used in plating but also can be used as a solid metal alloy. In this case, the metal is the same all the way through so it can be polished heavily without wearing through any plating. The skirts on these knobs were pretty tarnished and I initially cleaned off the oxidation, tobacco deposits and grunge by carefully using NaOH (Sodium Hydroxide) commonly found in Easy Off Oven Cleaner. Using a small paint brush, I carefully painted some EOOC (that I had sprayed into a paper cup) on the skirt. Be sure to avoid getting EOOC on the bakelite bar knob. Work the EOOC around on the skirt and then rinse under cold water. This will remove all crud and oxidation. Follow this by polishing the skirt and the bar knob with Wenol's Polish (similar to Semichrome.) Since the skirt on these early knobs is solid German Silver, the polishing works great and if the EOOC has removed any of the nomenclature fill, the polishing residue is "jet black" and ends up filling the nomenclature perfectly.
The Can Electrolytic Capacitors - This wasn't going to be one of my "museum quality" restorations where I restuffed caps and made the chassis look totally original both on top and underneath. Since it had been professionally recapped in the past with excellent components and workmanship, I decided to leave the underneath "as-is." I had already connected three new 10uf 450wvdc electrolytic capacitors under the chassis so the receiver was functional. I did want the top of the chassis to look correct however. So, I dug through the "junk boxes" until I found three close matching can electrolytics. These had to not only match each other but had to be similar to what was originally in the NC-101X. Luckily, I was able to come up with three good candidates. Since these were dummy-cans, I removed the wires from the bottom of the can. I also repainted to top silver (although some NC-100s are found with red or brown tops.) When mounted, everything looked convincing correct. Now, anytime in the future, if I want to do a "museum quality" restoration, these cans can be rebuilt into functional components.

Photo Right: This shows the NC-101X chassis top after detailing. This is after the painted surfaces have been given the final cleaning with Glass Plus and polished with a soft paper towel. Note that tuning condenser hubs now look like they should - brass. Also, the nickel plated spacers look like nickel. Don't get too aggressive when cleaning the tops of the IF transformers, the top of the BFO can or the interstage transformer can. These aluminum cans are pretty soft and will "polish" quickly if cleaned or rubbed aggressively. Use WD-40 first with a soft brush to gently remove any grunge then follow with Glass Plus. Dab with a soft cloth to dry - don't rub. This should leave the cans appearing as a matte aluminum finish - as original.

Cabinet Detailing - The cabinet pieces were in very good condition and only required a little touch up painting. I use Artist's Acrylic Mars Black for most touch ups but sometimes I also will use nitrocellulose jet black lacquer thinned and applied with a small brush. It depends on the blemish. In this case, the inner surface of the cabinet where it contacted the chassis had some surface rust. Cleaning this up removed the original black paint (which was lacquer.) This was touched up with the lacquer. The wrinkle finish blemishes were touched up with the acrylic paint applied with a small brush or Q-tip. Front Panel Detailing - The front panel was in very good condition and only required cleaning and touch up. I used Glass Plus applied with a 2" paint brush and then the Glass Plus was worked into the panel using the paint brush. After a couple of minutes cleaning the dirty residue was wiped off with a paper towel. Then a second spray down with Glass Plus and wipe down with a clean paper towel. This gentle cleaning will usually get most of the grunge off. I then touched up the panel and let the acrylic dry overnight. The next day I used Armor-All to polish the panel and also gave the cabinet the same treatment. If I was doing this today (2016) I'd use 10W machine oil instead of Armor-All. I find the results less harsh-looking.
Reassembly and Test - About the only thing to mention on reassembly is to be sure to set the PW-D correctly as described in the section "PW-D Micrometer Dial Servicing." Basically, you have to set the tuning condenser to "half mesh" and then install the PW-D which will be set to "250." When installed correctly you should be able to cover 0 to 500 with an overlap on each end that is around 10 divisions.

Alignment - The IF is aligned first and you have to determine the frequency of the crystal that's installed in the crystal filter. Using this exact frequency in your IF alignment will assure that the crystal filter is a useful tool for combating QRM. Switch on the crystal filter and adjust the Selectivity and Phasing controls for a narrow bandwidth. With the RF signal generator connected to the grid of the mixer tube, sweep the generator frequency through 456kc and listen for a peaking noise as you pass the crystal's frequency. With a digital frequency counter connected to the RF signal generator note the exact frequency that the crystal responds to. This will be the IF used for alignment. In my case, the crystal was 456kc. Adjust the IF capacitors for a peak indication using the "eye-tube." You can also connect a VTVM up to the AVC line and monitor that voltage for alignment purposes. Be sure to use a 400 hz modulated signal on your RF generator and have the receiver in AVC.

The RF tracking alignment requires the receiver to be on its side with access to the coil catacomb. Generally, the inductance which adjusts the lower end of the band was set at the factory and shouldn't change unless there's severe environmental problems with the storage that allowed moisture to ingress into the coils. Most of the time the catacombs are sealed pretty well and are protected by their location under the chassis. Usually, only the trimmer capacitors needs to be adjusted for each band. Connect the RF signal generator to the Antenna Input and use a 300 ohm load (a 300 ohm carbon resistor in series with the input.) You'll have to reference your NC-101X calibration charts for each band since these provide the frequencies that should be used at 450 on the PW-D for the high end and 50 for the low end of each band. Set the PW-D at 450 and the proper frequency input on the RF signal generator. You'll probably find that the LO is pretty close but tweak it in at the exact frequency. >>>

>>>  Now check the tracking by tuning down to the low end (PW-D at 50 and the correct input frequency from the signal generator) and you'll probably find it is very close. Return to 450 and the correct input frequency and set the Mixer and the RF stages for peak. You can measure the AVC voltage or you can use the "eye-tube" for a peak indicator. I just used the "eye-tube." Perform this same procedure on each band. Be careful on the 10M band as it is very easy to align to an image. With only one RF amplifier stage the image ratio isn't all that great on 10M so keep the RF generator amplitude as low as possible and check your LO tracking carefully. If you align to an image the tracking will not be accurate. You can also align the Mixer stage to an image and this would show up as uneven gain across the 10M band. Usually if you keep the RF input as low as possible and adjust for the strongest peak that will be the correct setting.

photo above: The NC-101X SN: 181-G after detailing

Performance - I'm impressed by the NC-101X. It's certainly sensitive enough on 80M and 40M. I've listened on 20M and it seems to do very well on that band too. I haven't tried 10M but I think it would also be adequate there. Certainly, if 10M performance was an issue, an external RF Preselector could be added. IF bandwidth is probably about 8kc without the Crystal Filter. QRM hasn't been a problem but I'm sure the Crystal Filter would be able to eliminate all but on-frequency interference. The audio is really nice. I'm using the matching 10" Rola electrodynamic speaker that came installed in the larger National speaker box. This seems to reproduce AM signals quite well and there is quite a bit of bass in some signals. The Tone control is not very responsive and does most of its action in the last 10% of rotation. On the station antenna, which is a 135 foot center-fed tuned inverted vee, the signals on 80M are so strong that I have to reduce the RF gain to about 7, even with the AVC on. This is to get the "eye-tube" to show some reaction since with the RF gain at full the eye-tube will always remain closed. Since my use of the NC-101X has been on 80M and 40M AM, I haven't noticed all that much drift. The bandspread action of the NC-101X will make drift show up more since a slight drift of a few kc would be indicated with as many as 8 or 10 divisions on the PW-D. The most drift I've noticed is about three divisions over an entire net time (about an hour.) Remember though, this is in the AM mode. Of course if you're monitoring a SSB net or doing CW, the drift in the BFO will also be a factor and more drift will probably be noticeable. Overall, I'm impressed enough with the NC-101X that I've set it up with the Collins 32V-2 transmitter and have been using it as the station receiver (off and on) for several weeks now. Using the Eye-Tube - I've been using the NC-101X as a station receiver for a while now. I've found that for AM on either 80M or 40M, I can't really run the RF gain "wide open" (receiving on the station antenna) and have the eye-tube show anything other than fully closed. Backing the RF gain down is necessary and provides some interesting possibilities. I've been listening to the AM "round-table" stations and adjusting the RF gain to have the eye-tube fully closed on the strongest station. This usually has the RF gain set at about 7 or 8. Now all of the other stations are normally going to show something less than fully closed on the eye-tube when they transmit. After a couple of sessions you get to where you can sort of estimate signal strength relative to the strongest station. It might have been something that the original owners of early NC-101X receivers did - at least if they operated AM.
 

Detailing the NC-80X Receiver

This isn't a restoration write-up. It's detailing a receiver that had already been recapped but was not completed as far as mechanics and assembly. Also, this write-up provides a way to present more detailed information on this "seldom-encountered" receiver, including photos of the chain drive assembly for moving the catacomb.

I had been seeing this NC-80X being offered for sale for awhile. I was interested but, as usual, I was distracted away from an actual purchase many times. Eventually, I had to contact N7RK and work out  how to purchase this NC-80X. As always, Dave provided excellent photos and I knew in advance what I was getting into as a project. Packing and shipping from Arizona was first class all the way. As described, the receiver was already recapped but had bad filter caps and some of the dial bezel and dial lamp assembly was still apart. The first step though was complete disassembly for a thorough clean up.
Disassembly - The cabinet is held together with sheet metal screws. The front and sides are one sheet metal piece. The back is one sheet metal piece. The top and bottom are sheet metal pieces. The only unusual feature is the eleven screws and shoulder-nuts that isolate the chassis from the cabinet.

Serial Number - I looked and looked for the serial number on this NC-80X. Most National receivers have the serial number stamped into the chassis but not the NC-80X. Complete cleaning and some disassembly proved to me that there wasn't a serial number anywhere on the chassis. Sometime into the project, I was thinking, "I wonder if National stamped the cabinet instead." Looking at the back panel in the proper light and there it was,...stamped just below the opening for the headphone jack,...C 475. Difficult to see since the black wrinkle paint was applied after the stamping of the serial number.

The Bezel and the Station Markers - The bezel was brown plastic and there were six curved metal clips that were inserted into a lengthwise slot at the bottom of the bezel. What did these clips do? At first I thought it was some sort of mechanical method to mount the bezel - it wasn't. Looking at the advertising artwork, I could see that the points of the clips were next to dial scale. By reverse-mounting the clips, they then revealed their purpose - they were station indicators. The clips have a portion that protrudes out the bottom slot and one can then grab this "tab" and slide the clip in its slot until the "pointer" is aligned with where your favorite station was located on the dial.

The bezel was missing its clear plastic dial cover. I assume it was plastic because whatever was original was held in place with drive pins. I think glass would have broken when the drive pins were installed but plastic would work fine. I used .030" thick clear plastic cut to fit and mounted with Duco cement. Only one drive pin was present so the Duco cement was an option that worked fine.

Catacomb Chain Drive System - The small coil catacomb of the NC-80X only requires a chain drive to move it into position for the four bands. The catacomb seemed to be locked in place so it was disassembled. There wasn't anything wrong but, as with the NC-80X's big brother the NC-100, the pins and the receiving contacts must be clean and the rod bearing must be lubricated for smooth, easy operation of the band switching system. There are only 10 pins to engage (rather than the 15 pins on the NC-100) so the chain drive is adequate for moving the catacomb into position. Thorough cleaning got everything working smoothly.

Coil Assemblies - These are identified as W, X, Y and Z for frequency coverage and 1 for Mixer function and 2 for Local Oscillator function. They mount in the same manner as the NC-100 coil assemblies. Since the IF is 1560kc, these coils are different than the NC-100 series coils.

Chassis Cleaning - The chassis was cleaned as described in the NC-101X write-up above. I use WD-40 to cut the grease and Glass Plus to clean up the residue. This chassis had mostly soot deposits - weird. Anyway, they cleaned up with just the Glass Plus. Complete disassembly allows for a thorough cleaning job.


photo right: The smaller-than-usual coil catacomb showing the chain drive system.

Cabinet - Four pieces comprise the cabinet. The rear panel, the front/sides piece, the top and the bottom. All pieces were cleaned and then touched-up with black nitrocellulose lacquer. Afterward the pieces were rubbed down with 10W machine oil (this does work great - better than ArmorAll.) The two nomenclature panels are held in place with the control mounting hardware so their removal and reinstallation is very easy. The "ground wire" lug for the wire for the antenna input coil return is riveted to the cabinet. This was missing and a new lug, wire and spade lug needed to be installed.

Bezel Mounting - The bezel can only be mounted or dismounted after the cabinet has been taken off of the chassis. This allows access to the back side of the front panel. Four 6-32 screws hold the bezel in place. They screw in from the inside of the bezel behind the clear dial cover. A gasket is needed so the bezel will mount "tight" against the cabinet. I used a thin piece of friction tape to make a gasket that accomplished the proper spacing.

Loud Speaker - The NC-80X doesn't have an internal audio output transformer. The pin jacks connect to B+ and to the 25L6 plate. The speaker that was intended for use with the NC-80X was the standard National 8" table speaker that used a PM speaker with an audio output transformer mounted on the speaker. This is the same speaker used with the HRO Senior. Primary impedance is about 7000Z and the secondary is 8.0Z.

photo left: This shows the favorite station clips and how they can slide along the slot in the bezel

Filter Capacitors - Originally the NC-80X used two 40uf 150wvdc electrolytic capacitors that mounted to the chassis via large threaded bases. The negative of each capacitor is connected to chassis (B-.) I left the original caps in place and removed their connecting wires. I mounted two Sprague 50uf 150wvdc axial mount electrolytics (yes, they were old ones from 1983.) I tested and reformed the caps before installing.

AC-DC and Chassis Isolation - Everyone knows that in a receiver with no power transformer, the circuit is directly connected to the line. B- is usually chassis but in some cases, with improper power plug orientation, the chassis can have AC voltage present. If this were also on the metal cabinet, then the operator would likely receive a line voltage shock if he was touching the cabinet and another part of his body was at ground. Realizing this possibility, all designers of AC-DC radios isolate the chassis from the metal cabinet. National did this using eleven rubber grommets and special shouldered nuts and screws to provide complete isolation of the chassis. Additionally, realizing that someone could lift the lid on the receiver and touch the chassis parts while the receiver was powered up, National didn't provide a lift-lid. Instead the lid is screw-mounted.

All of the rubber grommets were natural rubber and had disintegrated or were in that process. All grommets had to be replaced. Modern black rubber grommets were used since the brown natural rubber ones are difficult to find these days.

Pre-power-up - I tested all of the tubes and found that the 6B8 was weak but all other tubes tested fine. I installed a vintage power plug on the AC line cord and was ready to test the NC-80X. But wait a minute,...I didn't want to just plug this into the AC line direct. So, I had to drag out the hefty isolation transformer that I have (good for 20 amps) and actually have something between the house AC line and the NC-80X. Anyway, I was going to need the isolation transformer for the alignment since if you try to hook up test gear to an AC-DC radio there will usually be a conflict between grounds and how the radio is connected to the AC line. Best to use an isolation transformer to eliminate any problems.

Power-up - With power applied the NC-80X lamps didn't come on. This was just a loose lamp. The receiver came to life tuned to the broadcast band. Very good audio from the 8" National speaker. Just a 25 foot wire for an antenna. Bands W and X functioned fine and calibration was fairly close. I tuned into some 75 meter AM signals which sounded quite nice. Bands Y and Z only tuned in signals on a portion of their coverage which indicated a LO problem.

Minor Problem - There was no B+ going to the plate of the LO tube. Originally there had been a jumper between pins 3 and 4 which allowed the plate to connect to the screen for the B+ connection. For some reason the jumper was missing and installing a new jumper got the LO working on all bands correctly.

Alignment - Since the NC-80X doesn't have a TRF stage it might be possible to align the LO or the Mixer to an image, especially on the two highest frequency bands. However, this would require an extremely high level input from the signal generator. Since the IF is at 1560kc, an image would appear 3120kc lower than its actual frequency. Images are caused when an "untuned" signal gets pass a non-selective RF or Antenna stage and mixes with the LO to produce the signal at the IF. Normally the LO would be operating above the tuned signal but mixing can also take place with the LO below the received signal and that's what happens with an image. It's usually the non-tuned signal mixing with the LO below its frequency. That's why images are normally two times the IF below the actual frequency of the signal that is "leaking thru." Since the NC-80X's images are 3120kc down, the normal selectivity of the antenna coils are sufficient to practically eliminate image response.

IF alignment is straight forward except that the frequency is 1560kc. Be sure to use the actual crystal frequency for IF alignment and then your crystal filter will function correctly. You can use either an audio output level meter or use a VTVM monitoring the AVC line as a "peak" indicator.

Performance - Surprising! I was not expecting the NC-80X to be a very good performer but it has plenty of sensitivity and pulls in signals even up to 10 meters. The selectivity is via the crystal filter and it works like the HRO crystal filter in that there is a "peak" adjustment on the selectivity control that should be set, even when the crystal filter is off. I used an eight inch HRO speaker box and the audio on AM BC is very good. SW BC also sounds good on strong signals. Dial accuracy is fair and the resolution is very limited. The BFO provides a fairly stable oscillation and allows decent CW or even SSB however drift would be an issue if you wanted to monitor a SSB net over a long period of time. Drift is certainly normal for the vintage of the NC-80X and not objectionable.

The down side of the NC-80X is mechanical. The chain driven band change feels like the chassis flexes whenever it is operated. The band in use indicator is very crude and although it does its function it's basically a string and a wire spring operated device. The clips for indicating favorite stations are crude and always seems to be leaning one way or the other. It's obvious that National put most of the cost cutting into the mechanical side of the NC-80X.

The NC-80X is a sensitive receiver that is certainly capable of functioning as a station receiver. However, the AC-DC power and some of the mechanical issues might limit its compatibility with other equipment.

 

Cosmetic Restoration of the Wells-Gardner RAO-3

This 2006 restoration was more cosmetic than electronic. While more involved than a "detailing" job, it still can't be considered a "museum-quality" restoration. What is interesting is that I've restored this RAO-3 more than once. "Rios Radio Revisited" is a re-telling of how I found this receiver in 1969.

How I Found the Wells-Gardner RAO-3
A Brief Re-telling of "Rios Radio Revisited * "
 

When I was a 19 year old kid, I did a short stint at the local Post Office that involved substitute deliveries. I generally was at a different office and different route each day. On one of those "new to me" routes I came upon a house that had an old National receiver setting in the drive-way. A lot of weeds and grease around but the receiver looked pretty good. I lifted up the lid on it and saw the inside was full of small rocks and pebbles. Not too many tubes but lots of rocks. I knocked on the door, gave the resident his mail and pointed over at the receiver in the drive-way but I didn't get a chance to say anything before the guy blurted out, "Oh, you want that radio? Go ahead and take it. It's your's." I thanked him and loaded it into the back of the mail truck. I finished that route in record time so I could get off work and go down to see my old friend, Phil Rios. >>>

>>>  Phil had been in the Radio-TV repair business since 1946 and I had worked for him during the summers when I was in high school. I knew he would have the tubes that were missing and some suggestions on restoration. Of course, in 1969, restoring a WWII relic was more of "get it running" than anything else. With the rocks removed, I saw that about half of the tubes were still present and we needed the easy ones, the 6K7, 6J7, etc. After "tubing up" the RAO, we found a power cord that fit and a speaker (that was the wrong impedance - but who knew that then.) The RAO came to life and played pretty well with just a connection to the test TV antenna at the shop. Phil said, "Sounds pretty good. Run it for a week or so and then do an alignment and it should be a good set."

It wasn't too long before the audio output transformer opened up. I had taken the receiver down to Rios' shop and had the bottom off. After poking around for a while with no progress, Phil must have felt sorry for my inexperience and started issuing orders. "Measure the 6K6 pin 3. Okay, now measure pin 4. Okay, you have an open audio output transformer." Phil knew all the connections in his head from years of experience. I went over to Schirmer's Electronics, the local parts house, and bought a small universal audio output transformer. Who knew that the RAO was supposed to have a 500 ohm audio output impedance? Well, I installed the new transformer inside the old housing and everything looked original but of course the output impedance was now 8 ohms. The RAO came back to life and was ready for use as my new ham receiver. How I made any contacts on 15M CW I'll never know, but I did. As far away as VE3BAW. Exciting stuff then.

I later replaced the RAO with a Collins 75A-4 which was a vast difference in performance to a fairly new ham. I didn't get rid of the RAO though. I kept it in storage through several moves. In 2006, after more than thirty years of storage, I thought that the old RAO needed to be "revisited" again. Poor ol' Phil was long gone having been hit by a car while crossing the street in front of his shop. That was in 1989. I was a different person now, with 25 years of Industrial Electronics experience in engineering prototyping, test fixture design and field service repairing behind me. I also now had about 12 years (at that time) experience with owning and operating the Western Historic Radio Museum. The things I found that I had done to the RAO over thirty years ago were amazing - maybe even embarrassing! The goal now was to bring the RAO back to working condition and original appearance. This was going to require a total repaint and locating several correct type parts for the restoration.

 

 

*"Rios Radio Revisited" was an article that I wrote for "Antique Radio Classified" that was published in that magazine around 1990. It combined a "found radio story" about the RAO-3 with a bio of Phil Rios.

Plexiglass Dial Cover - Who knew this cover was supposed to be plexiglass? Not me, obviously, since I had installed a pane of glass. I guess that assumption was based on seeing an old NC-100ASD at the neighboring ham's shack when I was a kid (that was Dennis Antilla, WA6LHL, another mentor of mine mentioned in the NC-100ASD restoration section above.) I remember that receiver's dial cover was cracked, so it had to be glass (it's supposed to be glass in the ASD.) Like most RAOs, the plexiglass on this one was warped and discolored, so glass went in. Thirty years later, after realizing for some time that the glass was incorrect for an RAO, during this rebuild I replaced the glass with a correct plexiglass dial cover.

Interesting Painting Problems - At first I thought I was going to be able to just spray over the original wrinkle finish with new wrinkle finish paint and the cabinet would look okay. Luckily, I tested the idea on the dial escutcheon first. Wells-Gardner used some sort of dark olive drab primer on the sheet metal that reacted with the new paint by "lifting" - almost like paint stripper was being used. So, with that idea eliminated, I had to strip each of the cabinet sheet metal pieces first. This was probably better anyway, since the paint job will be higher quality.

The RAO-3 was cabinet pieces were prepped by light sanding with 400 grit Al-Ox paper and washing with lacquer thinner. You have to paint one side at a time because the wrinkle paint has to be hot to "wrinkle." I use heat lamps along with a heat gun to assure that the wrinkle is complete. Since the lamps are in a fixed position, only one side at a time can be painted and then put under the lamps. Apply at least three heavy coats of wrinkle paint with a time spacing of about 1 or 2 minutes between each coat. Then place that painted side under the lamps and standby with the heat gun. Wrinkling will take place in about 5 to 10 minutes. Use the heat gun on the corners and edges where the lights don't provide enough heat. Don't use too much heat from the heat gun or you'll "gloss" the wrinkle and it won't match. You don't need much heat, just a little to assure the you get full wrinkling on the entire panel-surface. If you're careful painting each side to not "over-shoot" the edge you'll find that each side will blend and when finished the cabinet looks like it was all painted at one time. Be sure to allow a little time for curing before assembly. Overnight is okay but a few days is better.


Shown above is the main cabinet piece after wrinkle finish was applied. NOTE - this application is using Krylon Black Wrinkle Finish paint, not the VHT paint I now use. At the time I painted the RAO in 2006, only Krylon had wrinkle finish paint and when the Krylon was compared to true "black" paint, one will see that the Krylon is sort of a charcoal black, not deep, jet black.
Minor Stuff - I had to make a cover for the small housing for the 1st RF Amplifier tube. This box is mounted to the rear panel of the cabinet and due to the necessity for complete shielding, the box does have a small cover. Additionally, the dial lamp harness was in terrible shape so I rebuilt it using correct period lamp sockets since the originals were missing their mounting brackets and the dial lamps were just shoved into the lamp holes. The four small control panels were straightened and then repainted. Mounting these panels, which were originally riveted, required making "fake rivets." These are 4-40 slotted head screws that have the heads turned down to look like rivets. The panels are then mounted with "fake rivets" and nuts. I usually remove any excess length of screw that extends past the nut to prevent any interference problems during reassembly.

Observing the photo above in the RAO section of this article which shows the RAO-3 after detailing of the chassis, it can be noted that there are some minor problems remaining. Due to the cleaning most of the silk screened nomenclature is missing on the IF cans and from the chassis. Remember, this receiver was filled with rocks when I found it!  Note that W-G used a different power transformer cover than National did. I'm not sure that this cover is correct since it was merely one that fit and the original cover was missing. The filter capacitors are not electrolytics. They are oil-filled paper dielectric capacitors which is why most of the RAO receivers will usually work somewhat with mostly original parts. Replacement of the paper-wax capacitors is recommended for best and safest performance, however.

On the Air, Again - After many years of storage and then a second restoration, I finally did use my old RAO-3 "on the air." It was on the Saturday morning West Coast AM Net on 3870kc at 8AM. The transmitter was a Collins 32V-2. All I can say is that the RAO-3 performed quite well with its typical great sensitivity. Of note though, the RAO receivers do not have any significant bass reproduction in the stock configuration. The audio reproduction was pretty thin sounding - but, that's normal for the RAO. My initial use in 1970 was as a CW receiver, which really was it's most common mode of operation with the Navy. The Net operations went fine and the RAO-3 finished the entire Net without having to resort to using a back-up receiver. Although I wasn't configured for remote stand-by, that option is available on the RAO-3. I used the B+ OFF switch as the remote stand-by. Although I haven't used the RAO-3 with any military transmitters on the Vintage Military Radio Net, I have used my RAO-7 on several occasions and it also is a dependable receiver, providing plenty of sensitivity and stability, although not much bass response.

The header photo in this section shows the finished RAO-3 mounted on an original RAO-3 shock-mount.

 
Conclusion - With over sixty different versions of National's Moving Coil receiver design having been produced over a period of thirteen years, including increased production for WWII, one has to conclude that the NC-100 Series were rugged performers that were very popular with all of the different types of end-users. From hams to the military and to the users at many airports around the country. From the late-thirties up to the early fifties, the Moving Coil receivers provided successful communications to a diverse group of radio users and listeners. Although the NC-100 "catacomb" may have started out as a mechanical compromise for plug-in coils, its reliability soon proved that the movable coil catacomb was not only virtually indestructible but also gave the user top performance. With a few "tweaks" from National, the NC-100X went on to become the NC-101X, a popular ham bands-only receiver. With other "tweaks," the NC-100 went on to become a popular, well-used Airways Receiver with versions produced for over a decade, even being rebuilt and used after WWII. With further additions, the NC-100XA became the famous WWII USN RAO receiver. Today, any of the Moving Coil receivers can be rebuilt, aligned and then used for solid reception on the ham bands all while providing great audio. Truly, the Moving Coil receivers were National's "second-greatest" creation and certainly must be considered another "masterpiece."

photo above: NC-101X sn: 40-X with it's matching loudspeaker - ready to use "on the air!"

 
References 

Hardcopy Material

1. Original Manuals for NC-100, NC-100ASD, NC-200, NC-2-40D, RAO-7/9, RCF, RCQ, RCR - Original manuals or copies of original manuals will provide circuit descriptions and operational procedures that reflect the manufacturers' design intent.

2. QST Magazine 1936 to 1948 - Original advertisements are good for providing a time line in the evolution of the design and which of the features the manufacturer believed were important at the time.

3. Communications Receivers, 4th Ed. - Raymond Moore - The best reference book for production details on early receivers.

4. The Coil Catacomb Radios and Variations on a Theme   by: Lawrence R. Ware - AWA Old Timers Bulletin - 1998 - This article covers all of the receivers that used the moving coil system with lots of photos of seldom encountered versions.
 

Online Material

4. BAMA, Boatanchor Manual Archive - aka BAMA Mirror site (or BAMA edebris) is a great online source for many old manuals. Great for reference on rigs you don't own but maybe want to.

5. Thanks to all of the National Co., Inc. radio enthusiasts  - who have provided photos, serial numbers, circuit and performance data and other information via the National Radio Reflector and via e-mails.

Henry Rogers/Western Historic Radio Museum © 2012-2017 - info on NC-101X, updates on NC-100XA project, new photo additions - June-July 2014. Catacomb Coil Assembly info added, NC-100A dial lamp assembly info added, NC-100ASD restoration added, RCE restoration added - June 2015. Added NC-80X restoration, photos,...Added RCR photo and info, Feb 2016,...Added additional information and photo NC-200, May 2016,...Added RCQ photo and info, Jun 2016,...re-edit and expand Airport Receiver section, June 2016,...added NC-101X SN:141-N photo, July 2016,...added info on "Silver Anniversary" NC-200, Dec. 2016,..., added power transformer replacement info, Jan 2017,...

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Donations to Radio Boulevard - Western Historic Radio Museum's Website

If you enjoy using Radio Boulevard - Western Historic Radio Museum's website as an information resource and have found our photos, our hard to find information or our restoration articles helpful, then please consider a donation to the WHRM website. A small donation will help with the expenses of website operation, which includes research, photographing and composition. WHRM was a real museum that was "Open-to-the-Public" from 1994 to 2012 - eighteen years of operation. WHRM will continue to provide its on-line information source with this website, which has been in operation since 1997.

Please use PayPal for sending a donation by clicking on the "Donate" Button below


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Website Navigation Index

-  WHRM History  ~  Nevada Radio History  ~  The KOWL XMTR  ~  Full Length Articles with Photos -

Home-Index

Western Historic Radio Museum Information
 Contact Info, Museum History 1994-2012, Museum Photo Tour, Using Photos and Info from this Website & Radio Value Info

Nevada Radio History - 1906 to 1930
Arthur Raycraft, Nevada's "Father of Wireless," America's First Radio Tour, Early Nevada BC Stations & More

KOWL's Gates BC-250L BC Transmitter
2007 Move from Lake Tahoe - Restoration - PLUS -  2013 Move to Dayton, Nevada & Getting on 160M

Parish House History
1876 to Present
Virginia City, Nevada

Lots of Photos

 

- Wireless Apparatus, 1920s Radio and Communications Equipment  ~  Full Length Articles with Photos -

M.H. Dodd's 1912 Wireless Station
100th Anniversary  Edition 
Includes New Photos, Reassembly Info and Lots of Original Vintage 1912 B&W Photos + Reassembly in Dayton

 THE COLIN B. KENNEDY COMPANY
"RADIO APPARATUS OF QUALITY"
Universal, Intermediate Wave and Short Wave Models History, Restoration and Operation - Lots of Photos

A.H. GREBE & CO., INC.
"A Guide to the Synchrophase MU-1"
Comprehensive Manufacturing History, Restoration, Neutralizing, Performance Information - Lots of Photos

 

 SE-1420, IP-501 & IP-501A
"The Classic Shipboard Wireless Receivers"
Comprehensive History, Restoration and Operation Info - Tuning in NDBs with IP-501-A

Vintage Long Wave Receivers
Long Wave Receiver Profiles, Loop Antenna Info, NDB Info and Log,
Fallon NV "Master - M" Loran Station Tour

 

 

- Vintage Communications & Amateur Radio Equipment  ~  Full Length Articles with Photos -

National Co. - HRO Receiver
"The Cream of the Crop" 
Comprehensive History, Serial Numbers, Restoration, Lots of Photos & More

National Co. - NC-100 Series
"Moving Coil"  Receivers 
Comprehensive History, Serial Numbers, Restoration & More - Includes Civilian Versions, Military Versions & Airport Versions

Hallicrafters SX-28
"A Pre-war Masterpiece"

Comprehensive History, Serial Number Analysis, Restoration Details & More

Patterson Radio Company
         
   PR-10 Receiver & Pre-selector              

Comprehensive History, Los Angeles Radio Mfgs History, Circuit Details, Serial Numbers, Restoration Details & More

 RCA's Amazing AR-88 Receivers
Comprehensive History, Restoration Info, How to do IF Sweep Alignments, Serial Numbers & More

RCA's Legendary AR-60 Receiver
Comprehensive History, Serial Number Analysis, Restoration Details & More - including the AR-60 connection to Amelia Earhart's Disappearance

Hammarlund Mfg.Co.,Inc
The Incredible Pre-War 'Super-Pro'
C
omprehensive History, Serial Number Analysis, Restoration Details. Includes info on the Hammarlund Comet Pro

Hallicrafters DD-1 "Skyrider Diversity"
Comprehensive History, Serial Numbers & Restoration Details

Hallicrafters' "Super-Pro" R-274 Receiver
Comparison of the SP-600 to the R-274(SX-73) in detail, best features of each. VOTE for your favorite Super Pro

 

-  Rebuilding Communications Equipment  ~  Full Length Articles with Photos -

Rebuilding the R-390A Receiver
Detailed Restoration Information for each module with Lots of Photos

Rebuilding the ART-13 Transmitter
Detailed Restoration info - includes details on building AC power supplies (with schematics) Lots of Photos

Rebuilding the Hammarlund SP-600
Detailed Restoration Information with Lots of Photos

NEW!       T-368 Military Transmitter                    
Detailed Information on Reworking, Testing and
Operation with Lots of Photos

Rebuilding and Operating the AN/GRC-19
T-195 XMTR & R-392 RCVR

 Detailed Information with Lots of Photos

Successfully Operating the BC-375 on the Ham Bands Today
Detailed Information on Power Set-ups that Work, Dynamic Neutralization, BC-191 Info & More

Rebuilding the Collins 51J Series Receivers
Detailed Restoration Information with Lots of Photos - Includes R-388 Receiver

Rebuilding the BC-348 Receiver
Detailed Information on all BC-348 Types, Dynamotor Retrofit Information, AC Power Supply Enhancement - Lots of Photos

Building an Authentic 1937 Ham Station
Utah Radio Products - UAT-1 Transmitter

 

- WHRM Radio Photo Galleries with Text -

Entertainment Radios from 1922 to 1950

Roaring 20s Radios
1922 to 1929

Vintage Table Radios
1930 to 1950

Floor Model Radios (Consoles)
1929 to 1939

Only Zenith Radios
1930 to 1940

Communications Equipment from 1909 to 1959 - Commercial, Military & Amateur

 Early Ham & Commercial Wireless Gear
1909 to 1927

Classic Pre-WWII Ham Gear
1928 to 1941

WWII Communications Equipment
 U.S. Navy & U.S. Army Signal Corps  1941 to 1945

Commercial & Military
Communications Gear
1932-1941 & 1946-1967

Post-WWII Ham Gear
1946 to 1959

Vintage Broadcast Equipment, RTTY, Telegraph Keys & Vintage Test Equipment

Vintage Microphones
 & Vintage Broadcast Gear
1930 to 1950s

Radio Teletype - RTTY - with Real Machines
includes TTY Machines, Military TUs and Amateur TUs

Telegraph Keys - 1900 to 1955
"From Straight Keys to Bugs"
Hand Keys and Semi-Automatic Telegraph Keys

Vintage Test Equipment
1900 to 1970

Includes Tube Testers, Freq Meters, Wobulators and More

 

Radio Boulevard
Western Historic Radio Museum

 Vintage Radio Communication Equipment Rebuilding & Restoration Articles,

 Vintage Radio History and WHRM Radio Photo Galleries

1909 - 1959

 

 

This website created and maintained by: Henry Rogers - Radio Boulevard, Western Historic Radio Museum 1997/2017