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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,

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

by: Henry Rogers WA7YBS-WHRM

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

NATIONAL COMPANY, INC.

"MOVING COIL" COMMUNICATION RECEIVERS
 

NC-100, NC-100X

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

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

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

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

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

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

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

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

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

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

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

NC-101X

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

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

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

NC-80X, NC-81X

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

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

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

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

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

NC-100A and NC-100XA

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 indicator. Small wonder that during WWII this feature was eliminated and replaced with an indicator dial mounted to the band change shaft. Initially, the dial cover was a pane of glass. Most later military versions had the glass replaced with plexiglass.
 

photo left: National ad for the new NC-100XA from June 1938 QST

A Crystal Filter was added to the NC-100A with the designation changed to NC-100XA. Some of the controls 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.

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 versions featured a "weighted" tuning knob that added a fly-wheel affect to tuning the receiver. The weighted knobs are not found on military versions.

Production of the NC-100A and NC-100XA probably lasted until the NC-200 was fully in production (Sept.- 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 continued on through WWII with these 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.

NC-200

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

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 (see photo below.) 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 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 five 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. >>>

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 and the receiver shown to the right is serial number C-536. Run C probably lasted until the "slow-down" that usually happened in the Spring. Run D probably started in September 1941 and also produced a high quantity of receivers. D-700 has been reported. Since 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.

Silver Anniversary NC-200

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 that had "SILVER ANNIVERSARY" embossed around the perimeter of the diamond. Although not mentioned in the ad, each matching NC-200 speaker also had the "Silver Anniversary" diamond. Additionally, each Silver Anniversary NC-200 was fitted with special brown bakelite bar knobs, a brown bakelite tuning knob and a brown bakelite S-meter case. The dial scale was finished in a gold-beige color. 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.

Since the NC-200 was introduced only a couple of months earlier, in October 1940, only some of the later "C" production run NC-200 receivers, those built after December 1940, had the "Silver Anniversary" finishes.    >>>

NC-200FG/CS and Other WWII NC-200 Variants

Shown to the left is an odd variant of the NC-200. 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. More than likely, this receiver was built for the U.S. Coast Guard. Unfortunately, the data plates are missing and therefore a positive identification is difficult. The bar knobs are not correct and should be National-type bar knobs.   Receiver photo from: Art Lebermann

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

NC-240D - The NC-240C and CS were built through WWII. The band spread function was eliminated in the WWII versions. The NC-240CS had a 200kc to 400kc band in place of the lower section of the AM BC band. Just after WWII, a few NC-240CS receivers were sold to the post-war civilian market but National quickly designated the "CS" as a "commercial" receiver. 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.) By 1946, National had quickly returned to the band spread option to the NC-240 and added the suffix "D" to distinguish that the receiver was the latest version. National also added pedestal-type feet to the receiver cabinet and 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 receivers will use a letter prefix serial number while later versions went to the seven digit numeral serial number.  >>>

Airport Communication Receivers and Airway Communication Receivers

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

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

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

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

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

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

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.    

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 RCF-2 receiver is shown in the photo below.

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. 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 was changed from 457kc on this model to a higher frequency, probably around 1560kc (same as the RBH receiver.) The designation RCK was also used for a piece of VHF multi-channel equipment 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.

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 National "Moving Coil" Airway Receivers

RCE   SN: 302

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.

RCF-2   SN: 13

This RCF-2 is almost complete. When acquired, all of the skirted National knobs were missing. What is shown on the receiver in the photo are close but not entirely correct knobs. Since the photo was taken I have acquired the correct knobs. The proper dust cover still needs to be located. These are always missing but, luckily, this RCF-2 does have the correct side panels.

RCK-N  SN: 182

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."

The designation RCK was also used for a VHF piece of gear used by the USN, so the suffix "-N" was added to the RCK designation to specifically identify the National receiver.

This RCK-N is partially restored but it needs a thorough cleaning and assessment of the rework that was done. Functionability hasn't been tested yet. As usual, the dust cover and side panels are missing.

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.

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. 

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

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

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."

RCR   SN: 17

photo left: The data plate from the RCR receiver SN: 17. Note that the tag refers to the RCR as a "Communication Receiver" as the earlier Airways Receivers.

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

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

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

Restoration and Repair of National Airway Receivers

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

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

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

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

WWII Versions

RAO Series, RBH Series, NC-100ASD

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. Army Signal Corps ordered NC-100ASD receivers with 200kc to 400kc coverage instead of the AMBC band. The U. S. Navy ordered the NC-100XA which became the RAO series (along with several similar versions, e.g. RBH and others.) The second version of the RAO added a second RF amplifier for reduction of Local Oscillator "leakage radiation" on the antenna with the added benefit of reducing images and increasing sensitivity. By the end of WWII, thousands of RAO receivers had been produced by National and contactor Wells-Gardner.

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

photo left: 1944 RAO-7 sn:10 built by National Co., Inc.

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

Beginning with the second of the numbered suffixes, the RAO-2, National added an extra RF Amplifier with an additional coil catacomb and tuning condenser housed in a bolt-on rear chassis and bolt-on cover. The extra RF Amp provided the isolation necessary to keep the LO radiation on the antenna below the designated level of <400pW. The second RF amplifier gain was set at unity-gain, so the overall receiver sensitivity is similar to the single RF stage NC-100A although with the extra coil set (TRF stage) the image ratio is improved and LO isolation from the antenna is increased. This additional RF amplifier section increased the depth dimension by about four inches and increased the weight significantly, running the scale up to about 75 lbs for most of the later RAO receivers (see photo of the underneath of the RAO-3 chassis below.) Tube total in RAO-2 (and later) is eleven.

The 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 or alignment is needed. These early versions of the RAO used a cradle-type shock mount that is seldom found with the receiver today. Note in the photo to the right that the RAO receiver and the RBL receiver both are mounted in their proper cradle-type shock mounts.  

photo above: This snapshot of a couple of Navy shipboard radiomen at work shows the typical receivers used during WWII. Far left is an early RAO receiver on it's shock mount. Next to the RAO is the RBL longwave receiver. The larger receivers are the RAK and RAL regenerative receivers.

photo above: Wells-Gardner RAO-3 on its original shock mount

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.

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

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

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

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

The RBH Series - 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.

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

Serial Numbering on the NC-100 Series

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

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

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

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

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

     WHRM: NC-100 Series Serial Number LOG info
 

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

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

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

NC-80X, NC-81X -  C-745 (80X),
 

NC-100A - 145-T, A-49 (reported as ASD)

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

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

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

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

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

RBH -

NC-100ASD - 194, 402, 426, 931, 948,

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

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

SA = Silver Anniversary NC-200

Production and Engineering Changes

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

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

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

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

6E5 cathode ray tuning eye tube is used

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

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

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

1937

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

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

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

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

1938

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

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

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

1939

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

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

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

1940

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

Antenna/Ground thumb-screw terminals with Polystyrene insulator replaces the old "push terminals" with fiber board insulator. HRO went through same change at the time.

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

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

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

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

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

1941-45

RAO USN versions - RAO, RAO-1,2,6,7 & 9 built by National, RAO-3,4 & 5 built by Wells-Gardner. RAO-2 through 9 have double preselection. All versions have .54mc to 30mc coverage. Early versions have 500 Z ohm audio outputs, RAO 7 & 9 have 600 Z ohm audio output. RAO-2 thru 6 are 17.5" wide, RAO-7 & 9 are 19" wide. RAO-7 & 9 don't have S-meters installed but have an output for panadapter use instead. Gears used in the gearbox are changed from brass gears to cast pot metal gears for wartime material conservation.

RBH USN version of NC-100XA with special frequency coverage of 300kc to 1200kc and 1700kc to 17.0mc, IF changed to 1500kc, single ended audio, 500 Z ohm output

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

NC-200FG, NC-240C and NC-240CS produced during WWII. These receivers do not have band spread function. The CS version had 200kc to 400kc band to replace the AM BC band.

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

In 1945, Schuttig & Co. modified RCL and RCK Airway receivers into RCP Airway receivers.

1946-49

NC-240CS sold to the civilian market in post-war 1945 as a 1946 model.

NC-240D returned the band spread function and was sold from early-1946 up to 1949. Note in SN Log that F-104 is a CS model w/o BS and that F-478 is a D model with BS. Both receivers are from the same production run "F" which indicates that this change was rapidly incorporated. In 1946, National designated the CS version as a "commercial receiver"

In 1948, National Electrical Machine Shops (NEMS) modified RCL and RCK Airway receivers into RCQ Airway receivers

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

Coil Catacomb Details

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The RBH also has its bands identified as A through E but bands E and D cover different frequencies than E and D on the RAO. The RBH coil assemblies are also identified as A through E even though they are different from the RAO coil assemblies - they are identified just like the RAO receiver's coil assemblies. RAO IF is 455kc and the RBH IF is 1500kc which means the LO and Mixer coil assemblies are different. In fact, band D and E LO assemblies both have padder capacitors on the RBH coil assemblies. As far as any markings other than the letter-number ID, there aren't any.

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

On the NC-200 series, with it's very different coil assemblies that included four sets that had a bandspread function, still the letter-number ID remains the same. That is, the highest frequency coverage band is ID'd as Band A and the associated coil assemblies are also ID'd as A1, A2 and A3. NC-200 Series coil assemblies have six stubby pins to provide for the bandspread function on 80M, 40M, 20M and 10M.

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

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

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

PW-D Details

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

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

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

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

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

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

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

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

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

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

6. Push the inner dial so that the "250" appears closer to the bottom of the window. You should hear a slight "click" as the inner dial gear teeth mesh with the outer dial gear teeth. You may have to slightly "rock" the inner dial as you push.

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

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

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

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

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

Servicing the NC-100 Series Gear Boxes

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Overall Length =  8.375"                Wall Thickness =  0.030"

Outer Diameter =  0.50"                 Slot Opening Width =  0.375"

Material is fiberboard that is painted white. The material has to be non-conductive, somewhat flexible and has to be able to withstand the heat from the two dial lamps. That probably would eliminate some types of plastics, especially the types of plastics available around 1938 to 1945. Fiberboard tubes are available from various vendors. Check eBay or McMaster-Carr for suitable material. The original lamp sockets were for screw-base lamps. I recommend using the #46 lamps that are listed in the manual as #40 lamps don't provide adequate dial illumination. Be sure to use the low current #40 lamp for the S-meter illumination since a #46 is "hot" enough to really discolor the plastic meter scale.  >>>

Power Transformers for the "Moving Coil" Receivers

Replacement Power Transformers - National used the same basic power transformer in all of the different models of Moving Coil receivers. Today, most restorers "harvest" a usable power transformer out of a "parts set" that's the same model as the receiver they are rebuilding. Sometimes, one has to use power transformer from a similar Moving Coil receiver. All Moving Coil receiver power transformer versions have wires for the primary connections and wires for the ends of the HV winding (but not the CT.) All other connections are solder terminals. Although there are some versions that have a two-wire primary winding and others with a three-wire primary winding, nearly all transformers were originally three-wire primary windings. Close inspection of a two-wire transformer will usually reveal that the "third wire" has been cut close to the core by National assemblers. This means that a three-wire primary transformer can be used to replace a two-wire but a three-wire primary will be needed if the receiver originally had that type installed. All transformers versions are the same physical size. You may have to adapt the core shield connection as this was used in some receivers and not in others. Some later transformers have a separate terminal for the core shield. Some schematics show the shield connected to the 6.3vac winding CT but this CT isn't connected to chassis in some receivers. However in these receivers, one side of the 6.3vac winding is connected to chassis to provide both 6.3vac to the tube heaters return and a low resistance connection to chassis for the shield.

Photo to the right shows a typical "Moving Coil" receiver power transformer installation. At the top of the photo the two large wires exiting the transformer are the primary connections. The three terminals are 6.3vac-CT-6.3vac. Note that the CT is not connected to chassis in this receiver (NC-200.) On the bottom, the two large wires are the HV connections to the rectifier plates. The left-most terminal is 5vac and the terminal with the red wire connected to it is the other 5vac (rectifier filaments.) The right-most terminal is HV-CT.

photo above: NC-200 power transformer. This transformer had a shorted primary. It was replaced with a transformer from a junk NC-100ASD receiver. Only the 3-wire primary had to be changed to a 2-wire primary to use the "ASD" transformer in the NC-200.

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

Some Observations - Check your receiver's original power transformer connections. Some production wiring is different from the published schematics. Most NC-100A and NC-200 receivers don't use the 6.3vac CT even though it's present. Some transformers have an internal shield that doesn't have a terminal and relies on the mechanical mounting for chassis ground. Most 3-wire primary windings are for "HI" and "LO" line voltages, assume 120vac for "HI" and 110vac for "LO." If 3-wire is used in a 2-wire receiver, use the "HI" 120vac connection. The unused ("LO") wire can be insulated and tucked into the apron area of the transformer mounting or cut short next to core (as originally done.) RAO-7 and 9 use a dual voltage primary (115vac & 230vac) so replacement transformer must come from same model receiver. RAO 3, 4 and 5 were built by Wells-Gardner and use an entirely different (not National) power transformer. Reuse the original power transformer top cover and bottom skirt to preserve original appearance unless original pieces are damaged. Some Airport receiver transformers will have a vented top cover consisting of either four large holes on top of the cover or several small holes on two sides of the cover. When looking at the receiver chassis from the front, the NC diamond on the transformer top cover should be rightside-up. There was a 25 cycle version that can be identified by its height (it's much taller) and also it's ink-stamped "NC-100-25" on the bottom side of the core.

NC-100A, NC-100XA, NC-101XA - 2-wire primary, CT HV sec., CT 6.3vac, 2-term 5.0vac

Airway Receivers - Tapped 3-wire primary, CT HV sec., CT 6.3vac, 2-term 5.0vac, shield terminal

NC-100ASD - Tapped 3-wire primary, CT HV sec, CT 6.3vac, 2-term 5.0vac, shield terminal

RAO-2, 6 - Tapped 3-wire primary, CT HV sec, CT 6.3vac, 2-term 5.0vac, shield terminal

RAO-7, 9 - Dual primary 115vac, 230vac, CT HV sec, CT 6.3vac, 2-term 5.0vac, shield terminal

NC-200 - 2-wire primary, CT HV sec, CT 6.3vac, 2 term 5.0vac

NC-240, NC-240D - 2-wire primary, CT HV sec, CT 6.3vac, 2 term 5.0vac

Most 2-wire primary windings are actually 3-wire with the "LO" wire cut short at the core by National assemblers.

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

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

HV = 270vac - CT - 270vac (540vac across ends)       FIL = 6.8vac (across ends)       RECT = 5.5vac

Restoration Hints and Suggestions

Military Manuals - These are generally higher quality and contain a lot of detailed information not found in the civilian versions. However, ALL of the military NC-100A versions are significantly different from earlier civilian models and these manuals should only be used either for specific military models or as a casual reference.

Importance of "Parts Sets" - It's possible to do any restoration without having a "parts set" as an immediate source of replacement components. However, the restoration is much easier and takes far less time when there's a "parts set" to supply needed replacement parts and to use as a reference when a question comes up regarding component placement or originality. Unfortunately, the expense of shipping heavy communication receivers has made it almost "cost prohibitive" to obtain a receiver just as a "parts set" anymore. Despite the cost, it will probably be worth the expense though since your restoration will be able to use the best parts from either receiver.

Restoring the NC-100ASD

 This restoration is a combination of "museum quality" electronic restoration and some cosmetic rejuvenation. It involved using two NC-100ASD receivers to end-up with one receiver that functioned correctly with a nice, original appearance.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Detailing the NC-101X Receiver

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

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

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

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

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

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

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

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

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

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

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

RCE  SN: 302  -  Airport Communication Receiver

This restoration was more mechanical and cosmetic than it was electronic. It involved finding and installing correct vintage parts along with correcting other mechanical problems. Additionally, front panel paint "touch-up" and the mechanical approach to "body work" is covered. Further, the methods for operating a vintage receiver on "original parts" is presented.

I received an e-mail from fellow Vintage Military Radio Net member WA6OPE asking if I would be interested in a National RCE Airport Receiver. He included some photos that showed the RCE to be complete but in rather "rough" condition. A deal was made and Tom brought the RCE up to the N7RCA Ham Swap Meet in Minden, Nevada (first weekend in June 2014) for delivery and payment. Tom had found the receiver in the SF Bay Area, apparently from a "non-ham" as the RCE had never been worked on or modified since it had been retired from commercial use. Storage in the SF Bay Area, with its salt-air environment, is problematic and long-term storage in a garage or shed can result in significant corrosion. The photo of the chassis below shows the typical surface rust that is found on metal that is stored in unheated buildings. Not terrible but certainly significant. Fortunately, the underneath of the chassis was nearly perfect which implies that condensation was probably responsible for most of the rust. The dust cover being present is somewhat rare. It seems like many of the Airport receivers now turn up with the dust cover long-gone. This dust cover is made out of aluminum - the later Airport receiver dust covers were made out of steel. The condition of the dust cover is very good but somewhat bent in the back because the rear screws weren't installed. >>> photo right: The RCE exterior as received before any cleaning by me. The exterior had been wiped down.

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

As I thought about this project, it seemed like the way to go was to find a complete NC-100 gearbox and tuning condenser assembly. That would solve the severe rust problem on this tuning condenser and would solve the broken gearbox problem too. About a year later (April 2015,) I was able to purchase a complete NC-100 N-PW gearbox with the three gang tuning condenser assembly attached. Also, included was a good condition PW-D. Thanks to George W7HDL.     

photo left: The RCE interior before any cleaning. Includes lots of spider webs, rust, grease and dust.

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

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

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

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

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

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

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

Upon power-up, I watch the rectifier tube carefully. If there is a short or other type of excessive load on the B+, the rectifier tube will glow purple inside the filament to plate area. If the rectifier appears normal, then B+ should come up to voltage in about 20 seconds or so. I measure it to be sure it's in spec and not loaded down. I watch for any indications of sudden heating which would indicate a heavy current flow through a resistor due to a shorted or nearly shorted bypass capacitor. Luckily, none of these things happened and the RCE powered up fine and did work somewhat. Of course, since I had replaced the tuning capacitor, it was pretty much out-of-alignment but still a few AM BC stations could be tuned in which let me know the receiver basically was operational. I performed an IF alignment and found that was pretty close. The RF tracking was way out, as expected. I kept having trouble with the contacts of the catacomb with the receiver on its side (necessary for RF tracking alignment.) The catacomb had to come out for further checking of the pins and receiving contacts. Mechanically, the catacomb seemed to be "flexing" the chassis when bands were changed which also warranted a thorough inspection. Catacomb removal is covered in the NC-100AX restoration article below.

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

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

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

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

Detailing the NC-80X Receiver

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Restoration of the Wells-Gardner RAO-3

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

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

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

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

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

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

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

The RAO-3 was painted using the same procedure that is described for painting the NC-100XA profiled in the next section below. Heat lamps, heat gun to assure that the wrinkle is complete and a little time for curing before assembly.

Shown in the photo to the right is the main cabinet piece after wrinkle finish was applied. NOTE - this application is using Krylon Black Wrinkle Finish paint, not the VHT paint I now use. At the time I painted the RAO in 2006, only Krylon had wrinkle finish paint and when the Krylon was compared to true "black" paint, one will see that the Krylon is sort of a charcoal black, not deep, jet black.

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

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

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

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

I'm wondering if this restoration project is ever going to be completed. It started out as a "museum-quality" restoration that stalled with our move to Dayton, Nevada. Also, the difficulty of the re-stuffing capacitors is a daunting job. Still, there are lots of details on catacomb removal, tuning condenser removal, wrinkle finish painting techniques.

Under the chassis was pretty stock and, although the phono addition constituted most of the modifications, there were several wires replaced with the red-striped white vinyl wire and some of the original wires had black electrical tape used as insulation. The entire dial light system had been replaced with bayonet base sockets and new bracket mounts that were riveted to the front panel. The replacement electrolytic capacitors appeared to have been from 10 to 20 years ago. Unfortunately, the original can electrolytics had been removed. These would have to be replaced with originals from "parts sets." A quick check of the transformers revealed that they were in good condition so the restoration of this early version NC-100XA could begin. 

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

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

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

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

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

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

photo above: This is the tray that is under the tuning condenser with the tuning condenser removed. Note the dirt, foreign objects, condition problems on the chassis and poor rework that was performed by a former owner. The only way have access to thoroughly clean this area and repair the other problems is to remove the tuning condenser.

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

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

More Delays - Here it is mid-2015 and I still haven't finished this NC-100XA. As soon as I can get to this project I'll complete this section. Restuffing caps, restringing the dial and reconditioning the small nomenclature panel plates remain unfinished at this time.

Hardcopy Material

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

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

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

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

Online Material

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

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