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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

by: Henry Rogers WA7YBS-WHRM

     PART 1 - History of the Moving Coil Receivers, Details on production models 1936 to 1948, Airport Receivers, US Navy models, US Army models, USCG models.


     PART 2 - Serial Numbers & Log, Production Engineering Changes, Catacomb details, PW Gearbox, PW-D, Tubes, Power Transformer info, Speakers
 
 

     PART 3 - Restoration Write-ups: NC-200 Silver Anniversary, NC-100XA, NC-100ASD
 

     PART 4 - Restoration Write-ups: RCE Airport Receiver, NC-101X, NC-80X, USN RAO-3, USN RAO-7




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
 

PART 1

 

NC-100, NC-100X


National NC-100 "MOVING COIL" Receiver from 1936 (sn 334-D, first production run) showing the original "art deco" design of the aluminum overlay front panel. These early receivers have a PW-D micrometer dial that has a bluish-gray Index dial and an inner Number dial that is red with white numerals. Note the relocated pilot lamp (to Phasing control hole) and the hole plugs covering the unused crystal filter Selectivity control hole and former pilot lamp location when compared to the NC-100X shown in the header B&W artwork. The matching loudspeaker is the early style cabinet with a 10" Rola K-10 electrodynamic speaker (only available for the 1936 production.)

History of the Design and Production

Many radio engineers of the thirties firmly 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.

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. $


photo above: The chassis of the NC-100

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. 

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.   >>>


photo above: Under the chassis showing the Coil 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 Type K-10 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 bluish-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 Type K-10 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 with black number wheel. The earliest versions of the receiver may have had this PW dial but most NC-101X receivers will be found with the black-tone lacquer, 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 (during production 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 during production run-N (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.) Some NC-81X receivers will be found equipped with a gray wrinkle finish cabinet and brown knobs.

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. Some versions had a gray wrinkle finish cabinet with brown bakelite knobs. 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 Later 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. On the earliest versions, the white scale S-meter was installed but these were soon replaced with the yellow scale meters. 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 and it might have been eliminated as early as run-G and replaced with a panel similar to the NC-101X. Certainly, when the NC-101X eye-tube was eliminated and replaced with an S-meter, then the NC-100 Art Deco panel overlay was gone for good. All of the changes were probably instituted along with the NC-101X upgrade from eye-tube to 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 above: 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 cover found on most versions of the NC-200 receiver is a large, slightly convex, plastic cover that is actually more durable than it looks (other than with age the plastic will yellow and loose its transparency.) The very early NC-200 receivers had a flat glass dial cover. Note NC-200 artwork shown above, and in the photo to the right, that the exterior side dial cover metal mounting clamps aren't present. These early dial covers mounted with similar clamps that were located on the inside of the panel. Some early receivers used a plexiglass dial cover rather than a glass pane and other early receivers will have the later, slightly convex, vacuum-molded dial cover but mounted with the inside clamps. Evolution of the dial covers seems to start will glass, then plexiglass, then molded plastic mounted with interior clamps and, finally, molded plastic mounted with exterior clamps.

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-78 showing the flat glass or plexiglass dial cover. Note the lack of exterior side metal clamps used to mount the plastic cover.  C-78 owned by PA3CTC - photo PA3CTC.

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. Lack of dial cover side clamps indicate this is an early version "Silver Anniversary" receiver.      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 "Silver Anniversary" SNs reported, the span is from C-33 up to D-639. 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 several other serial numbers logged between C-33 and D-639 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 the earliest standard finish NC-200 reported and that is only 61 serial numbers higher than D-639 but whether that indicates that no more Silver Anniversary versions were being built by D-700 is also unknown. D-716 has also been reported as a standard NC-200 receiver. So far, the Silver Anniversary serial numbers that have been reported seem to indicate that all production between very early-C-run and late-D-run are Silver Anniversary models. If not all, then a significant portion of NC-200s were Silver Anniversary models.

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 early-Run C and into mid-Run D. What we don't know is if all serial numbers between C-33 and D-639 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


UPDATE
: So far, all recently reported NC-200 receivers are Silver Anniversary versions within the C-33 to D-639 window. (5-2018)


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 "C" and "CS" 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. 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

Airports are municipal or government facilities providing runways, towers, navigation, communications for the landing and take-off of aircraft.
Airway is a defined route and altitude that an aircraft maintains while flying to a specific airport.

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 airway navigation equipment had started in 1926 and 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 airport 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 Airway 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" Airport and 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.

The RCF-2 is the only version that is specifically identified as an AIRWAY receiver implying that it was for use at Airway Range Beacon stations. All airports usually had a Range Beacon station for pilot radio navigation. In the thirties, there were areas within the USA where the great distances between some airports prevented the aircraft's reliable reception of the Range Beacon from the destination airport. For this navigation necessity there were other Airway Range Beacon stations located in remote areas that provided the navigation beam signals and weather reports. These stations could also provide pilot to ground communication if required.  

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 Airport receivers that utilized the NC-100 basic design. Note that the U.S. Civil Aeronautics Authority has now become the Civil Aeronautics Administration.

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 Airport Receivers did.

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 Airport and Airway Receivers

Due to the continued use of the National Airport 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 Airport 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 Airport 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 Airport 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 Airport 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 soon 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


photo above:  National RAO-7 sn:10 from the first contract, NXsr-38306  Sept.22, 1943. Received by the Navy in September 30, 1944 but never put into service.  Chassis sn: is H720 which is National's actual production run serial number. The data plate serial number and the chassis serial number will never match on these receivers - they aren't even the same format. The chassis serial number was assigned before assembly was complete and is a letter-prefix and number suffix format. The data plate serial number was assigned when the receiver was used to fulfill the Navy contract and is strictly numerical.

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 any of the shipboard superheterodyne receivers' Local Oscillator appearing on the antenna. This was primarily to allow the receivers to be used in the presence of other shipboard radio equipment without causing interference. Also, the receivers had to operate without interference from the other shipboard electronic equipment. Most Navy ships had multiple radio receivers and transmitters. Most of the large Navy ships had multiple radio rooms and many were equipped with radar. It was imperative that all radio equipment could operate simultaneously without mutual interference.

There was also the highly publicized reason for "low radiation receivers." There was 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! Most of these types of regenerative receivers had been removed from ships prior to 1940. The Navy believed that if the Germans put their minds to it, they could probably receive and DF inadvertent superheterodyne LO leakage radiation on the ship's radio antenna from up to 100 miles away. >>>

>>>  Consequently, the Navy came up with a specification of <400pW as the maximum of LO energy that could be measured at the antenna input terminals of the receiver. This assured that the receiver would not cause interference and it would be impossible to receive the LO leakage as radiation from a ship antenna at any significant 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 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 of the RAO-3 below shows the shock mount.



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 Company. 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. 

RAO-6 - National Company continued on with the Navy contracts building the RAO-6, a receiver that was similar to the earlier RAO-2 using the same multi-piece cabinet and cradle-type shock mount. Like the RAO-2, the RAO-6 had "NC-120" shown on the Crystal Filter nomenclature plate as the receiver identification. The data plate was mounted on the top lid and the RAO-6 designation was there. Apparently, during the production run for RAO-6 receivers, the USN decided to optimize the RAO receivers for surveillance and specifically to function with a panoramic adapter. The panoramic adaptor worked from the Mixer plate and allowed instant visual monitoring of wide segments of the spectrum which increased the ability of the radio operator to intercept enemy signals. The S-meter became superfluous and was eliminated. It's likely that the USN decision required another contract to build these modified RAO-6 receivers. The end result is that early versions of the RAO-6 will be equipped with an S-meter and are very similar to the RAO-2 in features. Later RAO-6 receivers would be equipped for use with a panoramic adaptor and will not have an S-meter installed. A different B+ nomenclature plate is used on these later RAO-6 receivers. The early RAO-6 had 500Z ohm audio output but it's possible that the later RAO-6 version had 600Z ohm audio output (like its successor, the RAO-7.)
 

Shown in the photo to the right is the RAO-6 version equipped for panoramic adapter use. This receiver belongs to N6MKC who also supplied the photograph.


photo above: RAO-7 SN: J444 - this receiver has been MFP treated giving the tags a yellowish color

RAO-7 and RAO-9 -  The RAO receiver underwent significant changes with the RAO-7 and RAO-9 (the designation RAO-8 was apparently not used.) The RAO-7 used CNA-46233 for its Navy designation and the RAO-9 used CNA-46263. 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 the standard 600 ohms and the design eliminated the S-meter in favor of a panadaptor connection. The RAO-7 and RAO-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. While the early RAOs used a separate shock mount system, the new RAO-7 and RAO-9 cabinets mounted the shocks directly to the bottom of the cabinet further easing construction and maintenance. The RAO-7 and RAO-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 (since the cabinet provided necessary shielding.)

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. 

Original receivers will have the Navy-style coaxial connector. Since these connectors are relatively difficult to find, many RAO receivers will have SO-239 coaxial connectors installed. If you still have the original Navy coaxial connector on your RAO, use a double female UHF coax adapter. The adapter will fit into the Navy connector and contact the center pin. Then just connect the PL-259 up for the antenna connection. Audio output is available either at the rear terminals or the front panel PHONES jack. Both are direct connections to the 600Z output transformer.

RAO-7 Serial Numbers and Contracts - There were two contracts for the RAO-7 (NXsr-38306-RAO-7 and NXsr-55614) and two contracts for the RAO-9 (NXsr-38306-RAO-9 and NXsr-85045.) A total of 1838 RAO-7 receivers were built and a total of 202 RAO-9 receivers were built. All contracts were issued between Sept.1943 and Dec.1944. Serial numbers on the data plates were assigned when the fully assembled receiver was used to fulfill a USN contract. The serial number on the chassis was assigned and stamped prior to production assembly of the receiver. These two serial numbers never match and shouldn't. They are different formats with the data plate SN being just numerical and the chassis SN being a letter prefix followed by numbers.

Data plate serial numbers appear to be non-sequential on these receivers. Serial numbers within the sequence may have been assigned to different accessories within the contract (e.g., the spares kit.) In fact, some data plate serial numbers are higher than the total quantity of receivers built for the contact. From this, one can infer that the numbers with the sequence must have been assigned to other items. To further the confusion, the two "J" production run RAO-7 receivers examined appear to have had their chassis serial number restamped (with the original stamping obliterated.) The build quantity of each production run is unknown but NAVSHIPS 900,356 indicates that 765 RAO-7s were on the first contract and 1073 RAO-7s were on the second contact. However, it appears that production runs aren't directly related to the contract quantities since it appears that National built somewhat ahead and then used completed receivers to fulfill the contracts.

More confusion can result in the possibility that the data plates found on a particular RAO receiver may not be original to that receiver. Of four RAO-7 receivers that have been examined closely, two positively have data plates that came from other receivers, another receiver's data plate originality is suspect and only one of the four (SN:10/H720) is positively the original data plate. The upshot of all of this is,...consider yourself lucky if your RAO-7 even has a data plate since many were swapped to different receivers during WWII for various reasons and many more were entirely removed post-WWII. 


photo above: RAO-7 SN: H720. 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. Other improvements are to the antenna input shielding and the addition of a panadaptor output - the SO-239 connector.

RAO 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. Use the logging scale for accurate resetability. 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 (requested by the Navy) 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. The audio peak response is at 800hz to 1000hz which favored CW copy. Overall audio response at -3db down at 290hz and at 3000hz (standard communications audio.) The bass response is -10db down at 125hz with the Tone control set to "N." Using the matching National speaker with the correct 500Z ohm or 600Z ohm matching transformer will help the audio response in the AM mode somewhat. Stability is very good with negligible drift after a 15 minute warm-up. The early RAO receivers provide a remote standby so it's easy to set one up for a vintage military radio station. RAO-7 and RAO-9 don't have remote standby and will require using the B+ OFF position of the POWER SUPPLY switch for standby. As a station receiver the RAO will have decent sensitivity, excellent selectivity (use the crystal filter,) good stability and good communications audio if a properly matched speaker is utilized. Best performance will be attained using a matched, resonant antenna. Antenna Input Z is approximately 70 ohms.

The Panoramic Adaptor - Using a Panoramic Adaptor with the RAO-7 (455kc IF) makes for an interesting receiver set-up. Panoramic Adaptor coupling is thru a series 50K resistor inside the coaxial cable from the Mixer plate. There is a SO-239 connector on the rear of the RAO-7 and also on the rear of the panoramic adaptor. All that's required is to connect the two together with a properly equipped RG-58 cable.

WWII panoramic adaptors aided surveillance by allowing the radio operator to see instantly if any unknown signals appeared within the slice of the spectrum he was monitoring. This eliminated the constant tuning that was necessary for receiver-only surveillance or for "guarding frequencies" using multiple receiver set-ups. The panoramic adaptor was primarily used for intercept of unknown signals. It wasn't used for signal analysis other than it was obvious to the radio op if the signal was CW or if it was somehow modulated. Surveillance and intercept was the panoramic adaptor's main role in WWII.

Shown to the right is the RAO-7 SN:10/H720 operating with a BC-1031-C Panoramic Adaptor (the Navy version was designated RCX.) The RAO-7 is tuned to the 25M Shortwave Band and is displaying several SWBC stations over about a 100kc bandwidth (50kc each side of center.) Only the signal at the center of the display is heard in the receiver. Note that there are two peaks close together. These are two SWBC stations separated by about 10kc. Down frequency (to the left) are two more stations that are about 10kc apart. I have the AVC on so the "tuned" signal level is reduced. MVC would show the comparative signal levels better. Up frequency are two more stations. The panoramic adaptor allows one to actually "see" signals within a portion of the RF spectrum centered around where the receiver is tuned. A large untuned wire antenna will tend to show more signals further away from "tuned center" since it will respond to an incoming signal frequency more-or-less equally. And, on the opposite side, a hi-Q resonant antenna may limit spectrum bandwidth.

You shouldn't expect to see too much detail in any individual signal with a vintage panoramic adaptor. You can expand the tuned signal using the Sweep Width control. You can see the modulation on an AM signal. You can see major malfunctions in a transmitted signal. However, seeing nuances of modulation is difficult. You can see in a broad sense the signal amplitude relationship to multiple signals within the tuned spectrum but you have to remember that the receiver front end (antenna, RF and Mixer) selectivity will somewhat attenuate signals that are out of bandwidth but the panoramic adaptor does have a spectrum amplifier circuit to compensate for this. However, the amplitude levels seen are merely for detecting that a signal is present and not for measuring their comparative amplitude. 

 

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 - RAO-2 and RAO-6 series are also marked as NC-120 on the Crystal Filter panel. 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.

 U.S. Army Signal Corps


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

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, were stored carefully and weren't victims of "hamster" rework. 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 (especially teenage hams who were "budget challenged.") 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 Radio Receiver Type R-116 was built on contract TCG-33675 with the Order Number of CG-80265. This contract was issued before WWII began for the USA with a date of May 15, 1941. Interestingly, the R-116 chassis is similar to the later RAO chassis in that an extra RF amplifier section is added to the rear of the receiver. This contract dates the double-preselection upgrade for military NC-100XA variants to pre-WWII and certainly must have affected the RAO evolution also. The R-116 uses eleven tubes as the RAO does however the circuit has several differences, uses different tubes and has several parts that aren't found in the USN RAO receivers.

The frequency coverage is different from the RAO with the R-116 covering 1.5mc to 27mc in six bands. This frequency coverage per band is spread out more than the NC-100XA or RAO receivers. The six bands required a different catacomb that was like the NC-200 receiver that also used a six band catacomb. The coil forms used in the catacomb are ceramic rather than National's normal coil form material (R-39.)

The catacomb band switch gear drive is very different in that the band switch knob shaft is coupled into a gearbox that rotates a shaft that goes back to the middle of the catacomb where the gear-rack is mounted to engage the pinion gear that's on the shaft. The RAO/NC-100XA had the rack mounted in the front of the catacomb driven directly by a short shaft and pinion gear from the band switch knob.

The front panel layout 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. The Crystal Filter has just three positions, OFF-1-2. Additionally, the bar knob-operated dual toggle switches found on the NC-100XA/RAO were changed to single toggles on the R-116, each with its own nomenclature plate. The locations for the controls are quite different from the RAO. The band-in-use indicator is the same as the RAO receivers except for the six positions (Bands A thru F) rather than five. The dial is very similar to the RAO except for the bands' frequency scaling. Flanking the tuning knob are two logging charts. 

Note the four threaded holes in each corner of the panel. These were for an optional rack mount adapter that looks something like a picture frame with grab handles on the front. Rear support was provided with threaded holes in the bottom rear corners. It was also possible to use a cradle-type shock mount on these receivers (the cradle-type shock mount was also used on the RAO-2 thru 6 receivers.) The cabinet is very similar to the RAO-2 thru 6 in that it is made up of several pieces rather than the one-piece cabinets that were used on the RAO-7 and 9 receivers.

The R-116 lacks an internal power supply requiring that the receiver operates from a separate AC power supply (rack mount dual supply) or from a DC source such as batteries, motor-generator/battery combo or other types of shipboard power sources. Voltages required are 6 volts for the tube heaters and +230vdc for the B+. A four wire power cable exits the rear of the receiver. Audio outputs are 300Z and 600Z. Serial number on the R-116 shown is 46. Also reported is R-116 SN: 103.

Tubes Used - RF1-6K7, RF2-6K7, Mix-6K8, LO-6J5, IF1-6K7, IF2-6K7, Detector-6J5, AVC Amp/Rectifer-6SF7, BFO-6SJ7, 1stAF-6J5, AFOUT-6V6,

Components - Photo to the right shows the chassis of the R-116. The circuit below the tuning condenser is very similar to the NC-100XA or RAO with the exception of the oil filled bypass capacitors that are mounted to the chassis. Above the tuning condenser, where the AC power supply normally is on the RAO or NC-100XA, are several unusual components. The large box is an audio filter that can be switched in from the front panel. The left choke is also part of the filter. The transformer to the right of the choke is the audio output transformer. To the left of the two tubes in front of the filter is the Relay. It appears that this might be a remote speaker on/off relay.


photo above: The rear "add-on" section of the cabinet that houses the 1st RF amp section of the receiver. Note the brown power cable. The black cable is for the antenna connections.

Rework - R-116 SN:46 overall was in good, complete condition. Most of the damage was to the rear cover. The cover was just jammed into place with no mounting screws (they wouldn't have lined up with the threaded holes anyway.) This cover was bent in several places and it appeared that someone had tried to pry it off without removing all of the screws. The lid was also bent and looked like someone had placed the power cable on top of the chassis and then tried to close the lid. The opposite side of the lid was bowed in. That was the bad stuff. The good stuff was that the front of the receiver was in good condition. Inside, the chassis is painted gray (original.) The chassis was dirty but looked good in that all of the nomenclature for the components was still present. Minor corrosion was on top of the Audio Filter unit but all other components looked in good condition. The cover for the First RF Amplifier tube compartment was missing (common on RAOs too.) All other chassis components were present. All components under the chassis were original and present. So, although the cabinet had been "manhandled" in the past, the chassis and front panel were in very good condition.

Operation - Since the power supply is external there are no filter capacitors in the R-116. All bypass condensers are oil-filled cans. There are three paper-wax caps associated with the Audio Filter. With the quality of components used, I decided to power-up the R-116. I tested the tubes and all were good. I connected a 600Z speaker, antenna and used a National 697 Dog House power supply for a power source. The R-116 came on after about 25 seconds with loads of audio, although that was just ambient QRN. Tuning around found 40M hams sounding great. Even 20M hams were received with no problems. SW-BC around 12mc came in fine. I used the 270' CF dipole and tuner as the antenna.

More details as this project proceeds along.  April 27, 2018

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

CONTINUE TO PART 2

 

NC-100 PART 3                    NC-100 PART 4                    Return to Home Index

 

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