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Western Historic Radio Museum
 

NATIONAL COMPANY, INC.

"MOVING COIL" COMMUNICATION RECEIVERS


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

by: Henry Rogers WA7YBS-WHRM

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

   

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

 

     PART 4 - Restoration Write-ups: RCE Airport Receiver, NC-101X, NC-80X

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

NATIONAL COMPANY, INC.

"MOVING COIL" COMMUNICATION RECEIVERS
 

PART 1

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.   >>>

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

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

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

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

NC-100, NC-100X

National NC-100 "MOVING COIL" Receiver from 1936 (sn 334D, first production run) showing the original "art deco" design of the aluminum overlay front panel. The art deco panel was only used on production runs D, E, F and G. The delicate artwork was easily damaged and the silk-screening wasn't very durable. National replaced the art deco panel with a black wrinkle finished panel that was identical to the panels being used on the NC-101X. The early NC-100 receivers have a PW-D micrometer dial that has a bluish-gray Index dial and an inner Number dial that is red with white numerals. The NC-100X used a black Number Dial with white numerals. Since 334D is the NC-100 version, note the relocated pilot lamp (to Phasing control hole) and the hole plugs covering the unused crystal filter Selectivity control hole and former pilot lamp location when compared to the NC-100X shown in the header B&W artwork. The matching loudspeaker is the early style cabinet with a 10" Rola K-10 electrodynamic speaker (only available for the 1936 production.)

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

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

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.) Some NC-81X receivers will be found equipped with a gray wrinkle finish cabinet and brown knobs.

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

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

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

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

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

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

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

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

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

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

photo above: A late-version NC-100 receiver with S-meter, Noise Limiter and black PW-D. Note the bar knob on the Limiter control. Serial number is 309-Y and receiver is owned by G1ZIM

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

The tuning dial cover found on most versions of the NC-200 receiver is a large, slightly convex, plastic cover that is actually more durable than it looks (other than with age the plastic will yellow and loose its transparency.) The very early NC-200 receivers had a flat glass dial cover. Note NC-200 artwork shown above, and in the photo to the right, that the exterior side dial cover metal mounting clamps aren't present. These early dial covers mounted with similar clamps that were located on the inside of the panel. Some early receivers used a plexiglass dial cover rather than a glass pane and other early receivers will have the later, slightly convex, vacuum-molded dial cover but mounted with the inside clamps. Evolution of the dial covers seems to start will glass, then plexiglass, then molded plastic mounted with interior clamps and, finally, molded plastic mounted with exterior clamps.

No logging scale was provided on the NC-200 probably because the dial accuracy was specified at 1%. The tuning drive system was a departure from the NC-100 gearbox with the NC-200 using a large diameter fiber disc that was rim-driven by a pinch-wheel on the tuning knob shaft. Though there appears to be a gearbox in front of the tuning condenser it doesn't have any gears inside and is only used as a bearing for the main tuning shaft from the rim-driven disc. This tuning drive system resulted in a very smooth feel to the tuning action and a good reduction in the tuning knob rotation to dial-tuning movement. The tuning knob itself was weighted via the large metal skirt which imparts a "flywheel effect" to the tuning action.   >>>

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

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

Silver Anniversary NC-200

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

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

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

Additionally, each Silver Anniversary NC-200 was to be fitted with special brown bakelite bar knobs, a brown bakelite tuning knob and a brown bakelite S-meter case. To complete the special finishes, the tuning knob skirt, all of the control nomenclature plates and the special NC-diamonds were finished in a gold tone.  >>>

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

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

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

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

>>>   Just how long the Silver Anniversary versions were produced is unknown. Of the "Silver Anniversary" SNs reported, the span is from C-33 up to D-639. It's unknown if the Silver Anniversary models were the only type of NC-200 that could be purchased for awhile or whether the buyer had a choice of the "standard finish" NC-200 or the Silver Anniversary. There are several other serial numbers logged between C-33 and D-639 and they aren't listed as Silver Anniversary models but that can't positively indicate that those receivers are standard finish models either (it just means that the specific information wasn't included with the serial number reporting.) Receiver D-700 is the earliest standard finish NC-200 reported and that is only 61 serial numbers higher than D-639 but whether that indicates that no more Silver Anniversary versions were being built by D-700 is also unknown. D-716 has also been reported as a standard NC-200 receiver. So far, the Silver Anniversary serial numbers that have been reported seem to indicate that all production between very early-C-run and late-D-run are Silver Anniversary models. If not all, then a significant portion of NC-200s were Silver Anniversary models.

In an effort to determine the quantity and other production information of the Silver Anniversary NC-200 receivers, we'll need some serial numbers from surviving receivers that are in collections. Our assumption is that the Silver Anniversary NC-200 was serialized within numbers used in early-Run C and into mid-Run D. What we don't know is if all serial numbers between C-33 and D-639 are exclusively used for Silver Anniversary models or whether the serial numbers were intermixed with the standard NC-200. A good quantity of serial numbers should reveal whether the Standard NC-200 and the Silver Anniversary NC-200 were produced during the same time period. Please report your Silver Anniversary NC-200's serial number (or any NC-200) using this e-mail link:

 SILVER ANNIVERSARY NC-200 SERIAL NUMBER

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

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

NC-240D, NC-240CS, NC-240C

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

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

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

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

Airport Communication Receivers and Airway Communication Receivers

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

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

photo left:  The Air-Ground Radio Station at Kansas City AP around 1936 showing the use of the stock NC-100 receiver at the time before the availability of the RCD Communication Receiver, the first of the National "Airway Receivers."  -  from Sterling's Radio Manual 3rd Ed.

The continuing improvement of airport to airplane radio communications along with improved radio-based airway navigation equipment had started in 1926 and was on-going through the 1930s. At the time, the Department of Commerce and the Bureau of Air Commerce were in charge of airports, airport communication and air navigation. The first National receiver specifically for airport communications was the RHM, a superheterodyne (National's first) supplied in 1932. The RHM evolved into the AGS receiver that was also used at some airport installations. Additionally, the AGS evolved in the RHP, RHQ and the AGU - all based on the RHM circuit but using ganged, plug-in coils rather than individual plug-in coils (a set of three coils were required for each frequency tuning range.)

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

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.

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

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

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

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

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

Details on Some National "Moving Coil" Airport and Airway Receivers

RCE   SN: 302

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

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

RCF-2   SN: 13

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

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

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."

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

The reason for the "N" suffix is that the U.S. Navy already had a VHF receiver that was designated as "RCK." Since the primary user for National's RCK was intended to be the Navy, National added the "N" suffix to avoid any designation confusion,... well ,... anymore than was normal.

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. 

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

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

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

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

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

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

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

WWII Versions

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

WWII had started in Europe and the U.S. military knew that much of their radio equipment was obsolete and needed to be replaced. The U. S. Navy ordered the NC-100XA which soon became the RAO series (along with several similar versions, e.g. RBH and others.) The second version of the RAO (RAO-2) added a second RF amplifier for reduction of Local Oscillator "leakage radiation" on the antenna with the added benefit of reducing images and maintaining decent sensitivity. By mid-war, the U. S. Army Signal Corps had ordered NC-100ASD receivers with 200kc to 400kc coverage instead of the AMBC band. By the end of WWII, thousands of RAO receivers had been produced by National and contactor Wells-Gardner & Co.

U. S. Navy

RAO 6, 7 & 9 - National continued on with the Navy contracts building the RAO-6, 7 & 9 (the designation RAO-8 ended up not being used.) RAO-2 and RAO-6 receivers will have an ID from National of NC-120 located on the crystal filter control nomenclature plate. Supposedly, early RAO-6 receivers had S-meters and were very similar to the RAO-2. But, the majority of RAO-6 production receivers were set-up for panadaptor and didn't have the S-meter but did provide a rear connection using a SO-239 UHF receptacle for the panoramic adaptor hook-up. The USN identification of the panadaptor version RAO-6 was CNA-46187-D.

The RAO-7/9 receivers were even more robust in construction and had increased shielding to further allow their use with other shipboard equipment without interference. The audio output Z remained at 600 ohms. The RAO-7/9 eliminated the S-meter in favor of panoramic adaptor connections as the late versions of the RAO-6 did.  The RAO 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. The early RAOs had used a separate shock mount cradle system but 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 designed for rack mounting since the copper-plated steel (painted black wrinkle) cabinet was part of the shielding necessary. The earlier RAO versions are 17.5" wide, with an integral panel-cabinet-chassis construction that requires major disassembly to service or repair.

photo above: This is the initial version of the RBH receivers. Its serial number is 840 and it was accepted by the Navy in May of 1942. The early versions of the RBH were single pre-selection and had many of the NC-100XA features like the silver background dial, the articulated-dial pointer band indicator and the BSW remote standby terminals. The RBH used a 1500kc IF to allow for complete tuning of the 400kc to 500kc part of the spectrum for Navy, marine and navigation uses. The initial RBH is identified on the Crystal Filter panel as "NC-156." The audio output is provided by a single 6V6 with an output transformer impedance of 5000Z ohms although the PHONES output is 600Z ohms. 

RBH CNA-46114
 National Co., Inc, - NC-156

The first RBH receivers were part of a Navy Department contract dated June 28, 1941 for a "Radio Frequency Tuner" and "Loudspeaker" with the designation CNA-46114 for the receiver and CNA-49166 for the loudspeaker. The first RBH receivers were part of a contract with General Electric Supply Corporation (Washington D.C.) who then sub-contracted with National Company to produce the receivers. This version of the RBH has no suffix number and is 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.) RBH receivers will have "NC-156" on the National Co. nameplate (part of the crystal filter panel.) The Crystal Filter has to operate at 1500kc and an internally mounted 1500kc crystal is utilized with the filter itself operating like the NC-100XA (the RAO receivers were supplied with Crystal Filters that operated like the NC-200, that is, with a stepped Selectivity switch and variable Phasing.) The first RBH receivers date from late-1941 and will have the typical modified audio outputs, that is, single-ended audio output stage driving a 5000Z ohm output transformer. The loudspeaker had a 5000Z to 8Z matching transformer mounted on the speaker frame. The PHONE output is 600Z ohms. This early version RBH wasn't suited for shipboard use (LO to antenna leakage) and the later "numbered" versions of the RBH receivers were modified for shipboard operation. The RBH-1 and all later numbered suffix RBH receivers were given the Navy designation of CNA-46188. All of the RBH series with number suffixes starting with RBH-1 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 dial background white rather than silver. It's possible that Wells-Gardner & Co. was a contractor for some of the RBH versions. The last of the RBH receivers incorporated the same improved cabinet like that used on the RAO-7/9 receivers. Numbered suffix RBH receivers had 600Z ohm audio output terminals that were connected in parallel with the PHONE jack. No matching speaker was specified or supplied with these later RBH receivers. The numbered RBH receivers will have NC-156-1 as the National Co. designation shown on the Crystal Filter panel. 

photo right: The chassis of RBH CNA-46114 SN:840

 U.S. Army Signal Corps

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

U.S. Coast Guard

 R-116
 

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

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

The front panel layout is significantly different with circular nomenclature plates (similar to the NC-200) used for all controls. The dial bezel is dramatically simplified when compared to the RAO. The R-116 doesn't have an S-meter. The Crystal Filter has just three positions, OFF-1-2. Additionally, the bar knob-operated dual toggle switches found on the NC-100XA/RAO were changed to single toggles on the R-116, each with its own nomenclature plate. The locations for the controls are quite different from the RAO. The band-in-use indicator is the same as the RAO receivers except for the six positions (Bands A thru F) rather than five positions. The dial is very similar to the RAO except for the bands' frequency scaling. Flanking the tuning knob are two logging charts. 

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

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

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

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

Rework - R-116 SN:46 overall was in good, complete condition. Most of the damage was to the rear cover. The cover was just jammed into place with no mounting screws (they wouldn't have lined up with the threaded holes anyway.) This cover was bent in several places and it appeared that someone had tried to pry it off without removing all of the screws. The lid was also bent and looked like someone had placed the power cable on top of the chassis and then tried to close the lid. The opposite side of the lid was bowed in. That was the bad stuff. The good stuff was that the front of the receiver was in good condition. Inside, the chassis is painted gray (original.)   >>>

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

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

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

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


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

CONTINUE TO PART 2