Radio Boulevard
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-240 versions, Military Versions - RAO family, RBH, NC-100ASD,
Dept of Commerce & CAA Airway Communication Receivers

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

by: Henry Rogers WA7YBS-WHRM

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

NATIONAL CO., INC.

"MOVING COIL" COMMUNICATION RECEIVERS
 

NC-100 Series

History of the Design and Production - Many radio engineers of the thirties believed that the absolute, best sensitivity and stability of a communications 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.

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

photo above: The chassis of the NC-100

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

 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. When the NC-100A version was introduced, the elliptic hub had been replaced (along with the PW-D.)

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. 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. 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.

As with many of 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. 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

The NC-101X tuned 160M, 80M, 40M, 20M, 10M amateur bands using 400 divisions of 500 divisions of the PW-D. This was exactly as the HRO did with the exception of the 160M band which the HRO didn't band spread. 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 feet long.

The Crystal Filter was included on all versions. Note the artwork from the National Co. advertisement from the April 1937 issue of QST shown to the right. This shows the NC-101X with the light grey Index dial, similar to the NC-100X. The earliest versions of the receiver have this PW dial but most NC-101X receivers have the black PW-D, like the HRO Senior. To solidify the image of a ham receiver, the "magic eye" was soon replaced with a true S-meter. The NC-101X was priced at $129 from Allied Radio Corp. in 1939.

Internally, all PW-D dials are the same and work by rotating the Number Dial on an elliptic hub. The Number Dial is driven by the Index Dial that is set-screw coupled to the main tuning shaft of the N-PW gear drive. There are two moving parts and seven stationary parts that make up the micrometer dial. Besides the Index Dial (outer dial) and the Number Dial (inner dial) there are two springs that hold the two dials together, three flat-head screws to mount the HRK-style knob to the Index Dial and a single set screw on early versions. During WWII the set screws were increased to two. Elegant in its simplicity, about the only thing that ever goes wrong with a PW-D is caused by technicians who carelessly disassemble the dial and then reassemble it incorrectly. More details on PW-D maintenance will be found further down this article in the "Restoration Hints and Suggestions" section.

photo above: The early NC-100 PW-D

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-101X (late,)101XA:  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 = 26L6, Rect = 25Z5

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

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. A band indicator was provided on the right side of the dial that pointed to the band in use. Selling price for either the NC-80X or the NC-81X was $88 and the receivers were available from 1937 up to 1939.

photo right: NC-80X from QST ad October 1937

Airway Communication Receiver Versions

Starting in 1937, National began supplying somewhat modified NC-100 receivers for use at airports for tower communications and for other purposes in aviation communications and monitoring. The continuing improvement of airport to airplane radio communications along with improved radio-based navigation equipment had actually started much earlier when 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 first NC-100-based receiver was designated the RCD and it 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. These initial 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. 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 additional circuitry added to further adapt them to airport communication requirements.

Most of the CAA Airport versions used 12 tubes, had no Crystal Filter and no carrier level indicator. The audio output was changed from Push Pull tubes operating an output transformer mounted on the electro-dynamic speaker of the "civilian" models to a single audio tube with an output transformer internal to the receiver which allowed PM speakers to be used. The power supply was slightly modified to include an extra filter choke since the field coil of the ED speaker wasn't available for that function. The typical CAA receiver used a 3/16" thick aluminum front panel painted black wrinkle finish with engraved nomenclature.   

A black PW-D was usually fitted to these receivers but gray PW-D dials were also commonly used. IF was usually 457kc. Some later versions may be found that were professionally modified to include variable coupled IF transformers or a later version Crystal Filter. Many of these upgrades were incorporated by professional companies in the post-WWII period.    >>>

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

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

The early versions of the Airway Communication Receivers were designated as RC followed by a letter identifying the receiver type. Most designation letters run from D up to Q (though not inclusively.)

After WWII, many of the earlier versions were modified into the RCP and the RCQ versions. Most RCP modification sets date from around 1948 and the rework was done by companies other than National Company. Schuttig & Co. was one of the contract rework companies commonly used for the RCP rework.

photo right: Airway Receiver 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 professional installed within the original wiring harness  -  owned by Mike Everett W4DSE

Due to the continued use of the National Airway Communication Receivers by airports over a long period of time it's common to find the receivers in poor condition and with several modifications that probably aren't documented. Modifications were usually to upgrade the earlier receivers to later versions and these modifications are professional in quality and normally the work was performed by well-known companies. But the mods still may not be well documented. It also appears that the data plates were sometimes removed on the receivers for various purposes. Removal of a data plate might have been necessary to incorporate the upgrade or sometimes data plates were removed by later "ham owners" just to make the receiver look "newer" and not a military or commercial surplus unit. It's likely that many of the Airway receivers found today will be somewhat incomplete. Certainly the "slide on" dust cover is often times missing as is the frequency chart. Luckily, the frequency chart frame is identical to the HRO chart frames found on the HRO coil sets.

NC-100A Versions

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. 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. 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. The NL circuit added another tube and brought the total tube count up to twelve again. 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 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.

photo right: The 1941 NC-100ASD without the Crystal Filter

Along with the cosmetic changes, the NC-200 presents 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 six tuning ranges. Additionally, what would be better than to also offer the same sort of band spread that was available on the NC-101X receiver. With the NC-200, the operator had general coverage from the AM-BC band up to 30MC and separate band spread coverage of the 80, 40 20 and 10 meter ham bands. Also, why not make the tuning knob double for the band changing. By pulling out in 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. Since there are six main tuning ranges between each of these ranges four band spread ranges can be selected. Turning the knob one revolution selects the general coverage coils while rotating the knob about a quarter of a turn within the one revolution selects the band spread set of coils. The NC-200 replaced the articulated dial pointer and went with a separate dual flag-type indicator that pointed to the scale-in-use on the tuning dial.

National released the NC-200 just before WWII began and though it sold well, it wasn't in production all that long before changes had to be incorporated to make it acceptable for the military. First to be eliminated was the amateur band spread function. This was initially referred to as the NC-200FG. Soon, the NC-200 became the NC-240CS which was essentially the NC-200 without band spread. A few minor changes involved the S-meter style and the return of the type 80 rectifier tube.

The NC-200 and the NC-240 were built through WWII and it's possible that just after WWII a few NC-240CS receivers were sold to the post-war civilian market. National quickly returned to the band spread option and the receiver then offered was designated the NC-240D. The NC-240D was sold from 1946 up to 1949. It was the last of the moving coil catacomb receivers produced by National.

photo right: The 1948 NC-240D, in this case, a rack mount version. The post-war models returned to the amateur band spread feature that was lacking in the NC-200FG and the 1946 NC-240CS. The 240D was produced up to around 1949 or so.      Photo from eBay.

WWII Versions

WWII had broken out 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. This addition 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.)

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

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 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 specifically designed for rack mounting. Electronically, there is no difference between the RAO-7 and the RAO-9 with the exception that the RAO-9 has two RF amplifier inputs have an RC filter between the grid and the coil selected. This RC was a low-pass filter 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 stability. 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. 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 1500kc wavetrap on the antenna lead-in will usually cure any serious problem.

photo above: USN RBH aka NC-156, first version with single preselection

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

Performance - The NC-100ASD is just about as close to the civilian NC-100A as the military got. Like all of the military NC-100s, the ASD has oil-filled paper dielectric filter capacitors, so many will work fairly well on all original parts. Calibration is usually very good on the receivers that are in excellent condition and were stored carefully. Many ASD versions were available post-WWII as surplus and were used in many 1950s ham stations quite successfully.

Serial Numbering on the NC-100 Series

After enough numbers have been collected it may become possible to deduce from the serial number what model receiver it belongs to. Certainly, dating the year of manufacture will become possible. Please send your NC-100 Series receiver serial numbers to this e-mail:

  WHRM: NC-100 Series Serial Number LOG info
 

photo above: SN location on the NC-100XA

During WWII serial number locations vary and sometimes appear only on the data plate of the specific receiver. This also applies to the Airport receivers, the serial numbers are usually only on the data plate mounted on the front panel. With the NC-240 series, the serial number is stamped on the top of the chassis on the right side edge midway back. Note that with the NC-240D, the serial number is a seven digit sequence that incorporates the production run identification into the first three digits and the receiver identification into the last four digits.

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

NC-100X - 227-D, 435-D (has original rack mounting brackets), 216-E (early TC),  253-E, 99-J,

NC-101X - 62-E, 217-F, 365-G (S-meter),184-N (S-meter, thin dividers TC, brass hubs TC),

NC-80X, NC-81X -
 

NC-100A -

NC-100XA - 58-R (thin dividers TC, brass hubs),

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

*TC = Tuning Condenser - early ones have thin metal dividers between sections

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

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

RBH -

NC-100ASD - A-49, 948,

Airport Receivers - 15 (RCD), 40 (RCD-40H), 42 (RCD-42H), 101(RCD), 234 (RCF),
13 (RCF-2), 221 (RCK-N), 531-G (NC-100 mod'd .2-.4mc on highest range, separate PS)

Production and Engineering Changes

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.

1937

NC-101X is introduced with ham band only coverage, black PW-D on most of production, eye tube on early versions

Contracts for Airport "Communication Receiver" from Dept. of Commerce, Bureau of Air Commerce Air Navigation Division

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

S-meter replaced eye-tube in NC-101X

Contacts for Airway Communication Receivers begin - the U.S. Civil Aeronautics Authority, the CAA, was formed in 1938

1939

NC-101XA introduced. Both versions are available up into the early 1940s

Noise Limiter circuit added to NC-100A versions

1940

LO tube changed to a 6J5 triode

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.

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 - features both general coverage and band spread coils within the catacomb thus eliminating the need for a separate "ham bands only" receiver like the NC-101X

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.

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: 1941 - replaced AM BC coverage with 200kc to 400kc, single-ended audio output with 600 Z ohm output transformer

NC-200FG and NC-240CS produced during WWII. These receivers do not have band spread function

1946-49

NC-240CS sold in post-war 1945 as a 1946 model.

NC-240D returned the band spread function and was sold from early-1946 up to 1949

Servicing the NC100 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.

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.)

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. 

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

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.

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 communications 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.

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

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.

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

After the pieces are stripped, the holes are filled with Bondo. After the Bondo has set-up it can be sanded flat. At this time prime the Bondo patches with a small paint brush using lacquer paint.

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

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

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

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

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.

- completion coming soon -

Restoration of the Wells-Gardner RAO-3

How I Found the W-G 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 up on 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 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 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 above. 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. 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.

Expected Performance Today

 - completion coming soon -

Hardcopy Material

1. Original Manuals for NC-100, NC-100ASD, RAO-7/9, RCF-2 - 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.

Online Material

4. BAMA, Boatanchor Manual Archive - aka BAMA Mirror site 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.