NATIONAL COMPANY, INC.
by: Henry Rogers WA7YBS-WHRM
|Besides National's masterpiece, the HRO, another series of receivers produced by National Company, Inc. were very popular and accounted for a lengthy production history. The NC-100 Series "MOVING COIL" Receiver production spanned from 1936 up to 1949. Another masterpiece? Certainly the design of the "MOVING COIL" method for band change used in the NC-100 receivers impressed the commercial operators of the late thirties with modified versions being ordered by the Bureau of Air Commerce and the CAA for use in airports around the country (even as late as post-WWII.) The NC-100XA version impressed the U. S. Navy who ordered special versions with low Local Oscillator radiation that became the famous RAO family of WWII receivers. Even post-WWII buyers could still purchase the descendants of the NC-100 in the modernized NC-240D receivers. This web-article details most of the various members of the NC-100 family and it became an extensive family (over sixty different variations) that grew as National up-graded and improved the receiver over its 13 years of production history. - Henry Rogers - March 19, 2012|
NATIONAL COMPANY, INC.
COIL" COMMUNICATION RECEIVERS
Certainly, National was having fabulous success with the HRO receiver, which had been in production since early 1935. It was well-known that the HRO's legendary performance was in-part due to its plug-in coil sets. The HRO wasn't hassle-free though and Millen realized that for the SWL (Short Wave Listener) and intermediate-level hams, in other words, those who didn't have the experience or couldn't afford the $200+ HRO receiver, there had to be a design that would provide the excellent performance of plug-in coils without all of the hassles and expense.
The NC-100 Features - National's mechanical engineers offered a solution that solved most of the negatives of plug-in coils and retained most of the advantages. With the use of a movable cast aluminum coil box called a "catacomb," all of the coils would be mounted in individual shielded compartments with short contact pins mounted in molded insulators on top of the catacomb. A large band selector knob on the front panel of the receiver would turn a rack and pinion gear mechanism that would move the coil catacomb into place, thus engaging the proper coil set pins into short, fixed position, spring-contacts mounted under the tuning condenser in insulator blocks.
The mechanical action simulated plugging in a three coil set for each band with the ease of turning a knob while keeping all of the unused coils isolated and shielded. "Switching noise" was eliminated by routing the RF and IF amplifier screen voltage through the foremost pin split-contacts of the LO coil section. The split-contacts were not soldered together but rather had the screen voltage wires connected to each of the two halves of the contact and when the coil pin, which wasn't connected to anything inside the coil catacomb, engaged in the two halves the circuit is completed and the screen voltage was then routed to the RF and IF amplifiers. >>>
|>>> To keep the costs down by keeping
the physical size of the catacomb relatively small only three sets of
coils were used per tuning range. A single RF amplifier provided
pre-selection and good reduction of images up to about 15mc. A separate
Local Oscillator reduced the noise associated with the typical
"inexpensive" Converter stage and a separated Mixer stage accounted for
the three tuned circuits that were necessary in each coil set. Due to
the nature of physically moving a metal box underneath the receiver's
chassis the catacomb width dimension was limited by the chassis width
and five tuning ranges were what could be fit into a metal box half the
width of the chassis.
A finely finished round metal rod that ran the full length of the chassis was mounted at the rear of the chassis to act as a rear bearing for the catacomb. The front of the coil catacomb had a "track" machined into it and the band change shaft, which also carried a pinion gear, protruded beyond the pinion gear and rode in this "track," thus supporting the front of the catacomb. The rack gear was mounted to the front of the catacomb at the proper level to engage the pinion gear to allow moving the catacomb via the band changing shaft. The "detent" action was actually caused by the 15 coil pins engaging the 15 contacts which provided the positive feel of the catacomb "locking" into position. Five marked holes in the front panel allowed viewing a white "flag" that was mounted to the catacomb and indicated which tuning range was selected. When shipped, the coil catacomb was screwed to one side where a guide pin was located to prevent any damage due to rough handling. When the receiver was installed, this screw had to be removed to "unlock" the coil catacomb.
Another portion of the design involved the PW gear drive used on the NC-100 series. The famous HRO PW gear drive used a "precision worm" (PW) gear to drive a large split-gear with two ganged variable condensers flanking each side of the gearbox. The NC-100 series was only going to require a three-gang tuning condenser and this was going to be centrally located on the chassis running front to rear with the power supply of the receiver on the left and the receiver circuitry to the right. This required the gear box to drive the condenser from the rear of the box. As with the HRO, an elliptic hub was required to make the PW-D work but inside the NC-100 gear box were several changes. Gone was the large split-gear, replaced with dual driven gears, one of which was spring-loaded for anti-backlash, driving via reduction gearing a large condenser drive gear. This approach eliminated the spring loaded drive of the HRO gear box and replaced it with a much easier to operate gear box.
Since the NC-100 was intended for the SWL or intermediate-level ham, a different type of signal strength or tuning indicator was utilized. RCA had just released their "Magic Eye" in 1936 (everything RCA offered in 1936 had "magic" tied to it, "Magic Brain was their radio front-end, "Magic Voice" was a special sound chamber for their console radios, etc.) The "Magic Eye" was a cathode ray tuning indicator tube that glowed a mysterious green color and, as signals were tuned, the "eye" would open and close a "pie-shaped" shadow. Since the "Magic Eye" required a license from RCA to use in a design, not too many manufacturers incorporated it into their designs in 1936 (Zenith waited until 1938 and Philco never did use the tuning eye tube in any of their designs.) Since a cathode ray tuning indicator was part of the NC-100 design it was going to require strong and consistent AVC voltage. This required the incorporation of an Amplified AVC stage in the receiver. Since nearly all hams were running on CW in the thirties and their receivers were always operated with the BFO on and the AVC off, the use of an amplified AVC stage implies that National was designing the NC-100 more for voice reception of AM signals from Shortwave Broadcasters. IF is National's standard 456kc.
Finally, there was the Push-Pull Audio output stage using a pair of 6F6 tubes driving a 10" Rola electro-dynamic speaker. Even an optional 12" electro-dynamic speaker was available. Again this great audio section implies the reception of voice and music, not the "dots and dashes" of International Morse. Since the intended market for the NC-100 was not necessarily hams, the receiver was introduced with a fabulous "art deco" front panel that featured geometric-linear black panels and black octagon control nomenclature layout combined with the natural aluminum finish along with a red highlighted central panel for band in use indication. The PW-D combined a gray index dial with a red number dial along with a green pilot lamp and the green "eye tube" (when in operation) finished off the striking "art deco" design of the receiver. The NC-100 and the NC-100X were introduced in August 1936 and were available from dealers in September 1936, with the initial pricing at $105 and $127 respectively.
Like several of the National receivers, the
NC-100 had the potential for commercial use. When supplied as a
Commercial Receiver the NC-100 would be a rack mounted receiver. This
could be accomplished by added brackets to the sides of a standard table
model, by building special racks that adapted to the standard table
model (as in the Highway Patrol photo below) or by supplying the
receiver with a special rack mount front panel that usually didn't have
the "art deco" aluminum overlay but was rather painted black wrinkle. Commercial versions
usually don't have extensive circuit modifications that the later Airway
Communication Receivers employed. The Airway Receivers are detailed in a
section below, "Airway Communication Receiver
|National was aware
that the NC-100 didn't really meet the needs of the average
intermediate-level ham who operated mostly CW so they simultaneously released a version of the
receiver with a crystal filter, the NC-100X (shown in the header photo.) However, it was a
"stop-gap" until National could release the "ham bands
only" version of the NC-100X receiver, the NC-101X. NC-101X
production starts with the second production run of the NC-100 Series
(run- E, probably October-November 1936.) The NC-101X version seemed to be more of a ham's
receiver, appealing to both CW operators and the few AM phone operators
that were around in the late thirties. The art deco tri-color panel of
the NC-100 was replaced with a black wrinkle finish panel with engraved
nomenclature. The tuning-eye tune was retained with a fancy "eyelid"
bezel that was also finished in black wrinkle. The B&W artwork to the left shows the NC-101X
as advertised in QST.
The NC-101X tuned 160M, 80M, 40M, 20M, 10M amateur bands using 400 of the 500 divisions of the PW-D. This was exactly as the HRO band spread with the exception of the 160M band which the HRO didn't band spread at all. This band spread action gives the operator the impression that the particular ham band being tuned will just go "on and on." The 400 divisions equates to a linear dial over 9.5 feet long (see PW-D section below for details on the micrometer dial mechanics.) >>>
photo left: B&W artwork for the NC-101X as it appeared in QST ad April 1937
|>>> The Crystal Filter was included on
all versions working with the National standard IF of 456kc. Push-pull
audio (6F6s) was used to drive the 10" Rola electrodynamic
speaker that was usually included (there was a 12" optional
speaker also available.) Note in the B&W artwork (above) from the National Co. advertisement from
the April 1937 issue of QST showing the NC-101X with the light grey
Index dial, similar to the NC-100X. The earliest versions of the
receiver may have had this PW dial but most NC-101X
receivers will be found with the black (actually olive-black,) HRO-type,
To solidify the image that the NC-101X was a ham receiver, the "magic eye" was soon replaced with a true S-meter (sometime around run-J, mid-1937.) The earliest S-meters used a white scale with 0-9 S-units printed in black. These meters were illuminated since the green pilot lamp had been eliminated to allow for the installation of the S-meter switch. By mid-1938, a light yellow meter face with 0-9 S-units in black and "db over" in red scales was being installed. The change appears to have happened around run N or P (see SN log.) Both meter types were built by Marion Electric. The later yellow meter scales darken considerably with exposure to UV light and the red numbers will tend to fade to the point of invisibility.
With the introduction of the NC-100A direct-read dial in June 1938, National began to also produce the NC-101XA - a ham bands only version with the direct-read dial. Interestingly, the ham purchaser could select either the NC-101X with its PW-D type dial or the NC-101XA with the "A" type direct-read dial for the same price of $129. In November 1939, a Noise Limiter circuit was added to all of the NC-100 series receivers. The Noise Limiter circuit required changing the 6C5 detector tube to a 6C8 tube duplex-triode to allow both the second detector and noise limiter functions. These late-version NC-101X receivers will have the Noise Limiter control installed between the RF Gain control and the Band Change knob. >>>
|>>> In October 1940, National introduced the NC-200 and
this model offered the user
both general coverage and bandspread coils in one receiver thus
effectively eliminating the need for either the NC-101X or the
NC-101XA receivers. Demand for the NC-101X had been dropping
significantly since the introduction of the "A" version. In fact, the
last NC-101X receivers are built on the NC-100XA chassis with all of the
holes pre-punched for the "A" direct-read dial present but not needed or
used. National ads for the NC-101X last appear in QST in April 1940 and the last discount dealer ads in QST are in October,
1940. It's very likely that the last few NC-101X receivers were built
long before the
summer months of 1940 and the discount dealer ads were actually for selling
receivers they had in stock (since they probably weren't selling that
fast anyway.) However, the production of the NC-101XA direct-read
dial receiver continued on for a short time, probably until the NC-200
was in full production.
Need the calibration curves for the NC-101X? NC-101X CALIBRATION CURVES
>>> A band indicator was provided on the right side of the dial that pointed to the band in use. Four tuning ranges were used for coverage of .55mc to 30mc and the receiver used 10 tubes. There was a gap in the tuning coverage between 1.5mc and 1.7mc to allow for the IF of 1560kc. The gap is placed between the top of the first band and the bottom of the second band.
National also introduced a "ham bands only" version of the receiver which was designated as the NC-81X. This receiver increased the number of tuning ranges to five so that 160M, 80M, 40M, 20M and 10M could be covered. This also required a different catacomb than the NC-80X used (but still used only two coils per band.)
A "B" version was available for either the NC-80X or the '81X that was battery operated and eliminated the rectifier tube from the circuit. There was also a power transformer available to convert the operation to AC (essentially a chassis-mounted isolation transformer.) Mounting holes and "metal-grommeted" holes were provided on the chassis for mounting the optional power transformer.
The advertising artwork shown bottom-right possibly represents the earliest versions of the NC-80X. Later versions have vertical nomenclature panels at each side of the front panel as seen in the photo left. The initial selling price for either the NC-80X or the NC-81X was $88 but this was soon increased to $99. The receivers were available from October 1937 up into late-1939.
NC-80X production evolved rapidly and most of the receivers sold were somewhat different than the advertising artwork. The dual-speed tuning was the first to be eliminated and replaced with single speed tuning system. The bezel was dark brown plastic and the dial cover is also transparent plastic. The bezel had a cutout slot that ran along the bottom and contained six "clips." These clips could be slid along the slot and the "points" lined up on the dial for "marking" the location of favorite stations. The tuning knob was weighted to give the tuning a flywheel effect. Since the NC-80X was an AC-DC circuit, it was necessary to have the chassis isolated from the cabinet. This was accomplished by mounting the cabinet to the chassis using several natural rubber grommets. The corners were further isolated using heavy cardboard. Since it was possible to have a voltage potential difference between the chassis and the cabinet, the top lid doesn't lift up - it's held down with screws.
NC-100A and NC-100XA
By mid-1938, the use of an indirect-readout device such as
the PW-D probably accounted for more than a few complaints from casual users who had to constantly refer to the
manual for a graph that provided frequency versus dial readout
correlations. Perhaps the expense of the micrometer dial itself was also
a consideration. The competition's direct-readout dials also could have
factor that resulted in National
revamping the entire NC-100 line to replace the PW-D micrometer dial with a
direct readout illuminated tuning dial. The suffix "A" was added to all
NC-100 receivers that had the new dial installed. For sometime, the
NC-101X was available with either the micrometer dial or the "A" version
dial as the NC-101XA. After all, it was a ham receiver and many hams enjoyed the
correlation exercises involved with using the PW-D.
One of the interesting features of
the new "A" version dial was its articulated pointer. When the band was
changed, the dial pointer would automatically increase or reduce its
apparent length so that its red tip would line up with the selected
band's tuning scale, thus indicating the "band in use." This required
the pointer "lifter" mechanism to track the tuning while maintaining the
proper length of the pointer. A dial cord that was anchored to the
band changing shaft and then routed via a pulley system to pivot against
the tuning condenser drive shaft so that the action of tuning the
receiver didn't affect the apparent length of the pointer while it
tracked around the dial. The
pointer-lifter mechanism and the entire articulation system seemed
for the simple task of a band indicator. Small wonder that during WWII
this feature was eliminated and replaced with an indicator dial mounted
to the band change shaft. Initially, the dial cover was a pane of
glass. Most later military versions had the glass
replaced with plexiglass.
photo left: National ad for the new NC-100XA from June 1938 QST
|In addition to the new tuning dial,
the cabinet itself was increased in height from the nine inch height of
the NC-100 to ten and a half inches of the NC-100A. A
"behind the panel" S-meter was installed on
all versions now. A push-pull switch allowed disabling the S-meter when CW was being received, just like the HRO receiver.
When the S-meter was installed, the total tube count of the NC-100A
reduced to eleven tubes since the cathode ray tuning eye tube was
A Crystal Filter was added to the NC-100A with the designation changed to NC-100XA. Some of the controls were moved with the addition of the Crystal Filter (still at 456kc.) The S-meter switch was moved from adjacent to the meter to the lower part of the panel. Different nomenclature was required so the small panels were changed to reflect the additional controls needed on the "XA" version. The selling price for the NC-100XA was $147.00 from Allied Radio in 1939.
Throughout the NC-100A production, the circuit went through several minor changes. Later in production, an adjustable Noise Limiter circuit was added (Nov. 1939.) The NL circuit changed the 6C5 second detector to a 6C8 duplex triode to provide the second detector function and the Noise Limiter function (keeping the tube total at eleven tubes.) The 6J7 AVC Amp was replaced with a 6F8 duplex triode to provide AVC amplification and also to add an audio preamp (1st AF Amp.) Also, with the NL addition, the location of some of the controls was rearranged. The Audio Gain control was changed to a standard grid input on the new 1st AF amp and the Tone control was changed to an RC type control which eliminated the audio choke that was used in the earlier versions. Some of the NC-100A versions featured a "weighted" tuning knob that added a fly-wheel affect to tuning the receiver. The weighted knobs are not found on military versions.
Production of the NC-100A and NC-100XA probably lasted until the NC-200 was fully in production (Sept.- Oct. 1940.) However, National was supplying the USN with the RAO and RBH receivers and would soon be supplying the US Army Signal Corps with the NC-100ASD receivers so the NC-100A series production essentially continued on through WWII with these receivers. The NC-100A was advertised in the 1945 Radio Amateur's Handbook in the National advertising section in the back of the book. All items shown have a disclaimer that "priorities" were required until released by the War Production Board. Most likely National was just showing what might be available after WWII ended.
||In 1940, many of the communications receiver manufacturers decided
that their models needed some modernization. In most cases this was more
cosmetic than anything to do with the circuitry, which usually was in a
constant "up-dating" mode anyway. Gray seemed to be the "hot" color for
gear and many manufacturers were offering their products in either gray
or black. National decided to go for a color change in a
two-tone gray scheme with the dark gray in wrinkle finish and the
lighter gray in smooth finish. Two wrap-around chrome bars finish the trim.
some substantial circuit changes were incorporated into their new model, the NC-200
(introduced in October 1940.)
Along with the cosmetic changes, the NC-200 presented another change in the catacomb design (the first was the NC-80.) By reducing the size of the chambers for the coils and trimmers, another band could be squeezed in (see photo below.) The NC-200 featured ten tuning ranges - six general coverage and four amateur band spread ranges. Essentially, the NC-200 offered the combination coverage of the NC-100XA and the NC-101XA in one receiver. With the NC-200, the operator had general coverage from the 490kc up to 30MC and separate band spread coverage of the 80, 40 20 and 10 meter ham bands. The 160M band was "band spread" covered on the NC-101X but it was tuned as "general coverage" on the NC-200. National also combined the function of tuning and band changing into one control. By pulling outward with the tuning knob the tuning dial is disengaged and the pinion gear is the engaged into the rack of the coil catacomb allowing the band to be changed. Pulling and turning the tuning knob one revolution selects any of the general coverage coils. Rotating the knob about a quarter of a turn within the one revolution selects the band spread set of coils between the five upper general coverage bands. The NC-200 replaced the articulated dial pointer and went with a separate dual flag-type indicator that pointed to the scale-in-use thru slots on the tuning dial. >>>
|>>> Some tubes were changed with the most obvious being the push-pull 6F6 audio output tubes being replaced with 6V6 tubes in push-pull. A closer look shows that the interstage transformer coupling used in the NC-100 Series was replaced with a phase inverter circuit that repurposed part of the 6F8G tube. Single-ended tubes like the 6SK7 were used as the RF amplifier and one of the IF amplifiers. The 80 rectifier was replaced with an octal 5Y3 tube. The receiver used a Marion Electric S-meter that had a square front. The S-meter toggle switch from the NC-100XA series was eliminated and the new S-meter switch combined with the RF Gain control. Only with the RF Gain fully advanced did the switch actuate and allow the S-meter to function. AVC had to be on for the S-meter to indicate signal strength. The IF frequency on the initial NC-200 receivers was 456kc but this was quickly changed to the industry standard of 455kc. The tuning dial had a large plastic cover that was more durable than it looked. There are some reports of NC-200 receivers that have an original glass pane dial cover rather than the plastic type commonly seen. Note in the artwork shown above that the side dial cover metal mounting clamps aren't present implying the dial cover is mounted by some other method and could be a glass cover (like the proceeding NC-100A receivers were.) No logging scale was provided on the NC-200. 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 Noise Limiter was the same type used on the late-NC-100A receivers.
Later versions of the NC-200 will have an Audio Input jack on the lower
right front panel. Battery operation was possible using the seven pin
auxiliary socket on the rear of the chassis. The matching speaker had
the output transformer mounted on the 10" diameter PM speaker.
It appears that the NC-200 receivers were built within three production runs, C, D and E. Run C was probably started in September 1940 and appears to have produced around 1000 receivers. Serial numbers as high as C-951 have been reported and the receiver shown to the right is serial number C-536. Run C probably lasted until the "slow-down" that usually happened in the Spring. Run D probably started in September 1941 and also produced a high quantity of receivers. D-700 has been reported. Since WWII war production was going to start in April 1942, it's likely that Run D ended before that. Run E seems to be WWII production of the NC-200 variants. This puts the production of pre-war NC-200 receivers at around 1800, more or less. The pre-war NC-200 receivers aren't encountered as often as other models and should be considered relatively scarce.
Silver Anniversary NC-200
|If any of National's "Moving Coil" receivers
has a confused origin, it's the Silver Anniversary NC-200. Whose
anniversary was it, anyway? Well, it wasn't National's as many collectors think.
The Silver Anniversary NC-200 was announced in the December 1940 issue of QST. This QST was a "special issue" with a silver cover and articles galore on the origins of ham radio, the ARRL and QST. It was all to celebrate QST's 25th "Silver" anniversary. The back-inside cover of the magazine had a full-page National advertisement introducing a special version of the NC-200, the Silver Anniversary NC-200.
The ad states that this special NC-200 was "Dedicated to amateurs on the twenty-fifth anniversary of their own QST. A toast to QST, the ARRL and the Amateur!"
National's ad stated that each Silver Anniversary NC-200 would have a special NC diamond that had "SILVER ANNIVERSARY" embossed around the perimeter of the diamond. Although not mentioned in the ad, each matching NC-200 speaker also had the "Silver Anniversary" diamond. Additionally, each Silver Anniversary NC-200 was fitted with special brown bakelite bar knobs, a brown bakelite tuning knob and a brown bakelite S-meter case. To complete the special finishes, the tuning knob skirt, all of the control nomenclature plates and the special NC-diamonds were finished in a gold tone.
Since the NC-200 was introduced only a couple of months earlier, in October 1940, only some of the later "C" production run NC-200 receivers, those built after December 1940, had the "Silver Anniversary" finishes. >>>
NC-200FG/CS and Other WWII NC-200 Variants
|National released the NC-200 in October 1940. WWII had started in Europe but was still a little
over a year away for the US. Though the NC-200 sold well before
late-1941, by the time the USA entered WWII, National had already
started planning for changes to be incorporated into the receiver to make
it acceptable for the military. First, the amateur
band spread function was eliminated. This receiver was initially referred to as the NC-200FG.
Soon, the NC-200FG became the NC-240CS which was essentially the NC-200
without band spread and with 200kc to 400kc installed to replace the AM
Broadcast band. The NC-240CS was later referred to as a "Commercial
Receiver." The IF was changed to the industry standard 455kc. A
logging scale was added to the tuning dial. The square S-meter style
was replaced with the round front type as used on the HRO receiver and
the replacement of the 5Y3G rectifier tube with the type 80 rectifier tube.
Shown to the left is an odd variant of the NC-200. It tunes from 200kc to 400kc and then from 500kc up to 18mc, thus including the AM BC band. The receiver controls appear to have been reduced to a minimum and there is no Crystal Filter, S-meter, Tone control, Logging Scale or Noise Limiter. More than likely, this receiver was built for the U.S. Coast Guard. Unfortunately, the data plates are missing and therefore a positive identification is difficult. The bar knobs are not correct and should be National-type bar knobs. Receiver photo from: Art Lebermann
NC-2-40D aka NC-240D, NC-240CS
NC-240D - The NC-240C and CS were built through WWII. The band spread function was eliminated in the WWII versions. The NC-240CS had a 200kc to 400kc band in place of the lower section of the AM BC band. Just after WWII, a few NC-240CS receivers were sold to the post-war civilian market but National quickly designated the "CS" as a "commercial" receiver. In 1947, there was a contract for the NC-240CS to be was used as an Airways receiver. It was identified as the RCR (see RCR in Airport Receiver section below.) By 1946, National had quickly returned to the band spread option to the NC-240 and added the suffix "D" to distinguish that the receiver was the latest version. National also added pedestal-type feet to the receiver cabinet and the speaker cabinet. The NC-240D sometimes will have an additional T or R suffix, depending if the receiver was a table model or a rack mount style. Many of National's advertisements and manuals show the receiver model as "NC-2-40D" however it seems to have also been shown as NC-240D in some National literature. Early receivers will use a letter prefix serial number while later versions went to the seven digit numeral serial number. >>>
|>>> As with the post-WWII HRO-5 and its many build
variations, the NC-240D receivers also vary depending on what surplus
parts National had on-hand. Many of the NC-240D receivers will have used
chassis that were punched for earlier versions of the receiver with the
chassis having several unused large holes. By 1947, most of the surplus
parts at National were used up and the receivers produced after that are
all standard parts and similar in appearance. The tube line up replaced
the two dual triode tubes (6C8G and 6F8G) with the 6SL7G and 6SN7G.
Also, the 80 rectifier used in some versions went back to the 5Y3G. Later versions of the
NC-240D will have the square S-meter (shown in photo of the rack mount
version below.) The NC-240D was sold from 1946 up to 1949 and was the
last of National's Moving Coil receivers.
Airport Communication Receivers and Airway Communication Receivers
Built for: Department of Commerce, Bureau of Air Commerce or Civil Aeronautics Authority
The continuing improvement of airport to airplane radio communications along with improved radio-based navigation equipment was on-going through the 1930s. At the time, the Department of Commerce and the Bureau of Air Commerce were in charge of airports, airport communication and air navigation. The first National receiver specifically for airways communications was the RHM, a superheterodyne (National's first) supplied in 1932. The RHM evolved into the AGS receiver that was also used at some airport installations. Additionally, the AGS evolved in the RHP, RHQ and the AGU - all based on the RHM circuit but using ganged, plug-in coils rather than individual plug-in coils (a set of three coils were required for each frequency tuning range.)
From 1935 up to around 1937, the HRO was favored by many airlines but the HRO had numerous accessories that required additional storage be provided. Each HRO came equipped with four coil sets, a separate power supply and a loud speaker. Custom installations usually were able to integrate the HRO and its accessories into the airport communications equipment racks.
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.The power supply was slightly modified to include an extra filter choke since the field coil of the electrodynamic speaker wasn't available for that function. The typical CAA receiver used a 3/16" thick aluminum front panel painted black wrinkle finish with engraved nomenclature. This description also applied to the earlier RCE receiver built for the DOC-BAC but not for the earlier RCD. >>>
>>> A gray painted PW-D was standard for all Airways
receivers. The IF was usually 457kc. Later
versions will have a two-position selectivity control and, later still, a crystal-controlled fixed-frequency
function. For fixed-frequency operation, the receiver had to be tuned
near the crystal frequency minus the IF for the RF amplifier stage and
the Mixer to be tuned correctly. If the fixed-frequency desired was, for
example, 4495kc, then the crystal required was 4495kc + 457kc = 4952kc
crystal frequency. This put the fixed-frequency LO operating higher than
the tuned frequency which is the normal configuration.
Most CAA early versions will have a squelch added that is referred to as the Interchannel Noise Suppressor, or I.N.S., which was activated by a front panel toggle switch. The I.N.S. circuit used a 6J7 tube that was operated from the 6J7 AVC tube and when the AVC bias voltage was being driven negative by lack of a signal, the I.N.S. tube would bias off the 1st AF Amplifier tube (6C5) which reduced the audio output to a very low level. Although the I.N.S. could be adjusted to "full squelch," National recommended that the I.N.S "suppression" be set to allow a very slight background noise to be just audible and then when a desired signal was received the I.N.S. would provide a "normal" audio level. The I.N.S. is adjusted with the two potentiometers that are mounted at the rear of the chassis directly behind the tuning condenser. Typically, the eye-tube of the standard NC-100 or other type of carrier level measuring device was not used on the CAA receivers but at least one RCF-2 example has turned up with a National S-meter that appears to be a factory installation.
The audio output was rolled off at 3000 Hz by using an in-circuit audio filter that is between the output of the first AF amp and the input of the 6V6 audio output tube. National felt that the necessary voice characteristics that affect intelligibility are all contained in the audio frequencies below 3000 Hz. As with military versions of National receivers, the P-P audio was replaced with a single-ended audio output tube and an internal output transformer provided 600 Z ohm output along with a Hi-Z audio output (20K Z ohm.) The phone jack on the front panel is a 600 Z ohm output. Some versions had an internal relay that operated on 6vdc (supplied externally) to disconnect the speaker but not affect the headset output. A single loud speaker was supplied and also a rack mounted dual speaker assembly was sometimes supplied. Some versions also had remote control available for RF and AF Gain functions. Some versions had a dual fused AC line input while others have a HI AC or LO AC primary on the power transformer which is selected by which fuse clips are used in a dual fuse holder. All versions used oil-filled paper dielectric filter capacitors and have two filter chokes.
With the RCK-N versions, the I.N.S. circuit was replaced with the C.O.N.S. circuit, or Carrier Operated Noise Suppression. This was an improvement that operated a relay that silenced the receiver if no carrier was present. Additionally, the pilot lamp would illuminate when a carrier was present. The RCK-N was built for the U.S. Navy during WWII. It covers 200kc to 800kc in two bands and 2.5mc to 23.5mc in the other three bands. The IF was changed from 457kc on this model to a higher frequency, probably around 1560kc (same as the RBH receiver.) The designation RCK was also used for a piece of VHF multi-channel equipment used by the USN, thus the suffix "-N" to specifically identify this Airway receiver.
The RCL added a switch that allowed selecting either a Broad or Sharp selectivity. Later, it was found that the selection process slightly changed the IF center frequency. When crystal-controlled, fixed-frequency operation was installed the Broad-Sharp switch was removed.
After WWII, many of the earlier version receivers (RCL and RCK versions) were modified into the RCP and the RCQ versions. The modifications were to add a selectable crystal-controlled fixed-frequency operation. These receivers also had modifications to the AVC and the addition of a series noise limiter (that was always on.) Also, the Broad-Sharp selectivity switch was removed due to IF instability problems. RCP modifications date from around 1945 and the rework was done by Schuttig & Company. The RCQ modifications date from 1948 and the rework performed by National Electrical Machine Shops, Inc. (NEMS.) The last of the National Airport Receivers was the RCR dating from 1948. It was essentially an unmodified NC-240CS from the late-forties.
Since Airport communications and electronics, in general, were evolving rapidly in the early-1950s, it's likely that the life of most of the National Airport and Airway receiver didn't last past the late-1950s. Many were repurposed into other functions, perhaps at airports and maybe other locations. Eventually, most were sold off to surplus vendors or scrap dealers.
Details on National "Moving Coil" Airway Receivers
RCE SN: 302
|The RCE was the second of the "Moving Coil" Airport receivers built
by National. The contract is from November 1937 and the data plate
indicates that the U.S. Department of Commerce and the Bureau of Air
Commerce were to be
the end users. The RCE eliminated the Crystal Filter and the Tuning
Eye-tube and added several other features that were to become standards
for the Airport (Airway) receivers. First was the I.N.S. circuit, the
Inter-channel Noise Suppression circuit that was essentially a Squelch
circuit. Additionally, two output impedances were supplied, a 600Z ohm
and a 20K Z ohm output. Other features were the ability to silence the
remote speaker via an internal relay that was powered externally.
The receiver uses a separate bottom cover and a full top dust cover. The upper dust cover is made out of aluminum. This type of dust cover is only used on the RCE. Later receivers had a slide-on full cover that interfaced with chassis-mounted side panels.
I mechanically restored this RCE because it had a serious breakage in the gearbox and a completely rusted tuning condenser. I was able to use a complete tuning condenser and gearbox assembly from a "parts set" NC-100 receiver. The restoration is described in the "Restoration" section further down the page in this web-article.
RCF-2 SN: 13
|Shown in the photo above 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. The photo to the above right is the RCF-2 sn 13 chassis showing some of the differences the "Airport Receivers" have
when compared to the standard NC-100 receiver.
The small cover with the handle on top (next to the power transformer)
relay that is used for remote silencing of the speaker. Note the extra tubes to the
left of the gear box. These are for the "I.N.S." (6J7) and First AF
Amplifier (6C5) functions. Also, note the single 6V6 audio output tube next to the 80
rectifier tube and the relocation of the audio output transformer to the
other side of the tuning condenser. The
antenna terminal insulator is the later polystyrene type and next to the
antenna terminals are the two potentiometers that adjust the "I.N.S."
threshold and suppression. Note that the power transformer cover appears
to be mounted opposite of normal. Usually, National always had the "NC"
logo right-side up when viewed from the front panel.
This RCF-2 is almost complete. When acquired, all of the skirted National knobs were missing. What is shown on the receiver in the photo are close but not entirely correct knobs. Since the photo was taken I have acquired the correct knobs. The proper dust cover still needs to be located. These are always missing but, luckily, this RCF-2 does have the correct side panels.
RCK-N SN: 182
|photo left: This is the RCK-N SN:182 from
the WWII-era. These receivers were built for the U.S. Navy and cover
different frequencies than the standard National Airway Receivers. Note
that the nomenclature indicates that the lowest frequency band is 200kc
to 400kc followed by 400kc to 800kc. The three highest frequency bands
cover 2.5mc up to 23.5mc. Obviously, the standard 457kc IF wasn't used
in the RCK-N.
The switch identified as "C.O.N.S." is an updated version of the older I.N.S. control on the RCE and RCF-2 receivers. The C.O.N.S. squelch control operates a relay that silences the receiver when no carrier is present. Additionally, the pilot lamp will only turn on when a carrier is present. It was also possible to manually silence the receiver with a remote switch. The acronym C.O.N.S. stands for "Carrier Operated Noise Suppression."
The designation RCK was also used for a VHF piece of gear used by the USN, so the suffix "-N" was added to the RCK designation to specifically identify the National receiver.
This RCK-N was partially restored when acquired. It needs a thorough cleaning and assessment of the rework that was done. Functionability hasn't been tested yet. As usual, the dust cover and side panels are missing.
RCQ SN: 288, SN: 242
Shown to the left is the RCQ receiver SN: 288. RCQ receivers were modified from earlier RCL or RCK version receivers in 1948 to add a crystal-controlled fixed-frequency function. Note the additional toggle switch that is identified as TUNE-ABLE and XTAL. The XTAL position selects the fixed-frequency mode.
After WWII, fixed-frequency became more and more necessary because it helped to eliminate tuning errors during operation and eliminated the possibility that the receiver might "drift" off-frequency. Since many of the receivers were operated with a squelch circuit, frequency drift would go unnoticed unless the receiver was in fairly constant use. The crystal-control fixed frequency kept the receiver "on frequency" over long periods of inactivity. Even though the LO is crystal-controlled, the tuning dial must be set to the correct receive frequency so the RF amp and Mixer stages are "in tune." A dial-lock was added to keep the RF and Mixer correctly tuned for fixed-frequency operation.
Note the "vented" top cover on the power transformer. This is commonly found on Airway receivers.
|Airport communications functions required fewer (if
any) frequency changes and most set-ups had dedicated receivers for each
frequency needed. The small paper label indicates the fixed- frequency
is 4494kc but the crystal installed is 3448kc which would then provide
crystal-control of 2993kc, so it's likely that the crystal was changed
and the label wasn't.
To calculate the crystal frequency necessary just add 455kc (IF) to the desired receive frequency. For example, if 2993kc was the desired receive f, then 2993 + 455 = 3448kc for the crystal frequency. Note that the IF was changed from 457kc to 455kc with the modifications of the RCQ.
Note that there is a "butch plate" attached to the front panel that has the nomenclature "TUNE-ABLE" and "XTAL" engraved on it. This butch plate is covering up the original engraving that was "BROAD" and "SHARP" for the original selectivity switch. This indicates that this RCQ started out as a RCL receiver. The hole at about "2 o'clock" by the PW-D was to mount the dial-lock assembly. It's missing on SN: 288 but is present on the RCQ shown in the photo to the right. The RCQ shown to the right is SN: 242. The panel has been repainted gray. The power transformer cover is missing and, of course, the side panels and dust cover are long-gone. I did get an original RCQ manual with SN: 242, however.
Another note is that the RCQ is referred to as a "Communications
Receiver" instead of a "Communication Receiver." The same reference is
used on the RCP tag shown further below.
Shown to the left is a close-up of the data plate from the 1948 contract RCQ receiver SN: 288. All of the RCQ and RCP receivers were originally earlier versions of National Airport receivers (RCL or RCK) that were modified to update the receivers to perform to the specifications required by post-WWII airports. The RCQ modification added a crystal-controlled fixed-frequency function to the receiver. The RCQ was the last of the Airways receivers that utilized the NC-100 basic design.
RCQ SN:288 is not restored. In fact, it's in "as found" condition. I have an entire "packet" of information that came with the receiver that indicates a former owner was thoroughly engrossed in the rebuilding of this receiver without any actual documentation or without using any National parts. Actually, it's not too bad, an after-market audio output transformer and several circuit mods. Dust cover and side panels missing, as usual.
|NOTE: On SN: 288,
the IF cans are stamped "RCK" implying that this RCQ was built by
modifying a RCK-N receiver. This would have required a different front
panel, a different coil catacomb and different IF transformers. It seems
likely that NEMS consolidated many of the parts necessary to rebuild either RCK
or RCL receivers into RCQ receivers. RCK-N front panels
couldn't be used because of the frequency coverage nomenclature would be
incorrect for the RCQ. The RCK-N IF transformers couldn't be used since
they are 1560kc. The coil catacomb would have required standard coils be
installed or it was probably easier just replace the RCK-N coil catacomb
with a catacomb from a RCL receiver.
So, even though the IF cans on SN: 288 are marked "RCK," inside the cans are standard 455kc transformers. The front panel and coil catacomb must have come from an RCL receiver.
The RCQ manual states "This receiver (RCQ) was originally a Type RCK or RCL receiver." Probably a more accurate statement would be that "parts from both RCL and RCK receivers were used to build RCQ receivers."
|NEMS - Note that the contractor for the RCQ was National Electrical Machine Shops, Inc., otherwise known as NEMS. NEMS formerly was known as NESCO, or National Electrical Supply Company but the name change dates from around 1938. The company NESCO dates from the late-nineteenth century. NESCO was supplying SE-1420 receivers to the U.S. Army in WWI. There was always some confusion between National Electrical Supply Company and another company called NESCO (National Electrical Signaling Company) that was associated with Reginald Fessenden but Fessenden's company was gone before 1920. In the early 1950s, as NEMS, National Electrical Machine Shops, Inc. formed a partnership with Alan Clarke and became "NEMS-CLARKE COMPANY" building VHF receivers for the military, government and commercial users. Nems-Clarke was bought by Vitro Electronics in 1957 but Vitro kept the Nems-Clarke name on the receivers.|
RCR SN: 17
|National built the RCR on contract number Cca26391, probably in 1948.
Note that the contract number on the RCQ Airport Receiver above is Cca
26227 and the date is 2-5-48. Compare that to the contract number on the
RCR Airport Receiver - Cca 26391. The contract numbers are 164 contracts
apart which certainly dates the RCR receiver contract to mid-to-late
1948 (there's no date on the data plate.) The receiver is essentially National's NC-240CS
Commercial Communications Receiver. The NC-240CS differs from the
NC-240D in frequency coverage, lack of amateur bandspread and an amber
Inside the RCR is a standard NC-240CS using a 12 tube circuit with single preselection, two IF amplifiers, crystal filter, noise limiter and tone control. Frequency coverage is 200kc to 400kc on Band F and then the remaining five bands, E thru A, cover 1.0mc to 30mc. Only a portion of the AM BC band is covered and no amateur bandspread coverage is provided. Audio output impedances provided are 8.0Z and 500Z. Additionally, 10,000Z can be used by paralleling the internal output transformer primary with a speaker-mounted audio output transformer with a hi-z primary. This allowed users to "plug-in" the standard National NC-240-type table speaker, if desired. >>>
RCP tag and a "One-off" Version of the RCF-2
photo right: RCF-2 Airway Receiver that was probably modified by National to have an S-meter. Although the first Airport receiver, the RCD, did have a tuning eye tube, most receivers didn't have any sort of carrier level indicating device. This receiver is an exception and it has all of the indications that the rework was carried out at National. The S-meter circuit wiring uses matching cloth-covered wire and it is professionally installed within the original wiring harness. The function of the toggle switch to the right of the band change knob is not known. - owned by Mike Everett W4DSE
Restoration and Repair of National Airway Receivers
|Due to the continued use of the National Airway Communication
Receivers by airports over a long period of time it's common to find
the receivers in poor condition with missing parts and with several
modifications that probably aren't documented. Modifications were
usually to upgrade the earlier receivers to later versions and these
modifications were professional in quality and normally the work was
performed by well-known companies. These mods are well-documented since
professional companies performed the rework and each receiver was redesignated as either a RCP or RCQ. But many receivers had mods
installed "on site" and these may not have been well-documented. Many
Airways receivers fell into the hands of hams that modified the
receivers to their needs without regard to providing any documentation.
Many repairs didn't used OEM parts (National parts.) Most ham mods were
poorly installed and seldom helped performance.
It also appears that the data plates were sometimes removed on the receivers for various reasons. Removal of an original data plate would have been necessary when the receiver was upgraded to a newer version by a professional company but always a new data plate was installed to correctly identify these receivers. Some "unofficial" mods might have removed the data plate because it was "in the way" of installing the upgrade. Sometimes data plates were removed by later "ham owners" just to make the receiver not look like it was military or commercial surplus.
Since the Airway receivers were in commercial use, normal maintenance-type repairs were generally performed by airport technicians. It's common to find non-OEM parts used for these types of repairs. Sometimes, when considering removing a vintage repair that used a non-National (OEM) part, one should consider that the repair was part of the receiver's history. Possibly the quality of workmanship would determine whether the non-OEM part should be replaced. "Hamster" mods or sloppy repairs should be corrected to a workmanship level that would be consistent with what professional technicians were capable of.
It's very common that most of the Airway receivers found today will be somewhat incomplete. Dust covers especially seem to be often times missing. Sometimes even the side panels were removed. It was probably done by airport technicians who felt that the dust cover and side panels retained heat and caused heat-related failures. Of course, one would think that proper ventilation would have been provided in the racks for all of the airport electronic equipment. Most of the time the original frequency chart is missing. Usually, the chart frame will still be mounted but, if it's missing, many of the frequency chart frames used are identical to the HRO chart frames found on the HRO coil sets. Luckily, nearly all of the mechanical parts and circuit components are interchangeable with the standard NC-100 series receivers. For example, I needed a complete tuning condenser and gear box to rebuild the RCE receiver shown above in this section. I was able to find a complete gear box and condenser removed from a standard NC-100 and it fit into the RCE perfectly. The RCF-2 shown above had a defective BFO coil and a replacement was taken from a "parts set" NC-200 receiver. It's lucky that most of the Airway receivers used standard National parts that are available on several different types of National receivers.
See the RCE restoration profile in the "Restoration Section" further down this page.
RAO Series, RBH Series, NC-100ASD
WWII had started in Europe and the U.S. military knew that much of their radio equipment was obsolete and needed to be replaced. The U. S. Army Signal Corps ordered NC-100ASD receivers with 200kc to 400kc coverage instead of the AMBC band. The U. S. Navy ordered the NC-100XA which became the RAO series (along with several similar versions, e.g. RBH and others.) The second version of the RAO added a second RF amplifier for reduction of Local Oscillator "leakage radiation" on the antenna with the added benefit of reducing images and increasing sensitivity. By the end of WWII, thousands of RAO receivers had been produced by National and contactor Wells-Gardner.
|U.S. Navy RAO & RAO-1
- National began
supplying the U.S. Navy with the NC-100A as early as 1940. Designated as RAO,
the first versions are somewhat similar to the standard NC-100XA
receivers. Initially, the RAO circuit used was the standard NC-100XA
although it's likely that a 500 Z ohm audio output transformer was
incorporated into the circuit along with power supply modifications. This was standard procedure for Navy
receivers. In fact, the civilian NC-100A receiver used a field coil
type speaker and push-pull audio output, neither of which the Navy would
have wanted on their receivers. Consequently, the RAO and the RAO-1 should have
single-ended audio output along with an extra filter choke to take the place
of the "speaker field coil used as a choke." Early versions
probably use National's 456kc IF but by the RAO-2 the IF was changed to
RAO-2 - When WWII began, the Navy wanted minimal radiation from the receiver's Local Oscillator appearing on the antenna. This was primarily to allow the receiver to be used in the presence of other shipboard radio equipment without interference. There was also the remote possibility that an enemy could "Direction Find" (DF) the radio's position from receiving the LO signal. It would be possible to also discern at what frequency the receiver was tuned. Prior to WWII, many of the Regenerative Medium Wave receivers used on commercial ships could easily be received at a distance of five miles or more - and this was from other commercial ships, not the enemy! The Navy believed that if the Germans put their minds to it, they could probably receive and DF inadvertent LO leakage radiation from some types of superheterodyne receivers up to 100 miles away. >>>
photo left: 1944 RAO-7 sn:10 built by National Co., Inc.
>>> Consequently, the Navy came up with a specification of <400pW as the maximum of LO energy could appear on the antenna. This assured that the receiver would not cause interference and it would be impossible to receive the LO leakage radiation at any distance away from the ship.
Beginning with the second of the numbered suffixes, the RAO-2, National added an extra RF Amplifier with an additional coil catacomb and tuning condenser housed in a bolt-on rear chassis and bolt-on cover. The extra RF Amp provided the isolation necessary to keep the LO radiation on the antenna below the designated level of <400pW. The second RF amplifier gain was set at unity-gain, so the overall receiver sensitivity is similar to the single RF stage NC-100A although with the extra coil set (TRF stage) the image ratio is improved and LO isolation from the antenna is increased. This additional RF amplifier section increased the depth dimension by about four inches and increased the weight significantly, running the scale up to about 75 lbs for most of the later RAO receivers (see photo of the underneath of the RAO-3 chassis below.) Tube total in RAO-2 (and later) is eleven.
The dial scale background was changed from silver to off-white paint and the articulated dial pointer was eliminated. Instead, a rotating dial mounted to the band change shaft indicated which tuning range is selected. RAO-2 and some RAO-6 receivers had an ID from National of NC-120 on the control panel for the Crystal Filter (along with the National Co. identification and the "NC" diamond insignia.)
The early RAO receivers (pre-RAO-7) are 17.5" wide, with an integral panel-cabinet-chassis construction that requires major disassembly if more than tube replacement or alignment is needed. These early versions of the RAO used a cradle-type shock mount that is seldom found with the receiver today. Note in the photo to the right that the RAO receiver and the RBL receiver both are mounted in their proper cradle-type shock mounts.
||RAO-3, 4 &
5 - Most WWII equipment was
built under contract and not all RAOs were built by National.
Wells-Gardner Company (Chicago, IL) was the second contractor for the RAO series,
building the RAO-3, 4 & 5. The Wells-Gardner versions were generally
intended for shore stations and were not considered "heavy-duty
military-type" construction by the Navy. All W-G versions have the extra RF
amplifier for double preselection and all versions have S-meters
installed. Though W-G used National Co. parts, where specified, many other
components were strictly W-G manufactured. The IF
transformers, power transformer, chokes and the chassis all appear to be
W-G components while the knobs, dial, tuning condenser and coil catacomb
were definitely from National and were even marked so. The Crystal
Filter panel was marked with the WELLS-GARDNER logo and name. The RAO-3
operated on 115vac only while the RAO-4 and 5 had a dual primary power
transformer to allow operation on either 115vac or 230vac.
Interestingly, the W-G versions of the receiver had embossed "feet" on
the bottom cover that allowed using the receiver on a table top without
the shock mount assembly. The "feet" were specifically intended for
shore use only and is also probably why so many W-G versions are found
without the shock mount assembly today. Also today, many W-G RAOs
show up without data plates. Oddly, it doesn't appear that they were removed -
they were just never installed (all mounting hardware is present.) It's possible that W-G did have
many of the RAO receivers unshipped when WWII ended and they merely "dumped"
them on the surplus market, which would account for the large number of
W-G RAOs found without data plates or shock mounts today.
The photos below show the chassis of the RAO-3 version of the receiver. The top of the chassis (left photo) shows that though many parts are from Wells-Gardner the main components are made by National. Note the small shielded box for the antenna input next to the First RF tuning condenser. This was one of the first attempts to reduce the LO leakage.
|The photo to the right shows underneath the chassis of the RAO-3. Note how the 1st RF Amplifier section consists of a "bolt on" chassis and utilizes a smaller catacomb that contains the 1st RF Amplifier coils for the five tuning ranges. The mechanical coupling from the main catacomb to the smaller catacomb is through a slot in the rear of the main chassis and a matching slot in the smaller chassis. On top of the bolt-on smaller chassis is a single tuning condenser that is mechanically coupled to the main tuning condenser which can be seen in the photo to the right. The 1st RF Amplifier tube is housed in a small metal box that is mounted to the inside rear wall of the main cabinet with a shielded harness to make the connections to the tube socket. This small box also has a top cover for shielding of the entire box. To complete the shielding, the bottom of the chassis is covered by two plates as can be seen by the mounting flanges on each of the boxes. The RAO-3 shown in the photos was in poor condition and its clean-up and restoration has removed much of the silk-screened nomenclature on the various components. See the photo below of the interior of the RAO-7 for the typical IDs found on chassis components in the RAO Series.|
photo above: The top of the RAO-7 chassis showing the improvements to shielding and the addition of a panadapter output - the SO-239 connector.
RAO 6, 7 & 9 - National continued on with the Navy contracts building the RAO-6, 7 & 9 (the designation RAO-8 was apparently not used.) These receivers were of robust construction and had increased shielding to further allow their use with other shipboard equipment without interference. The audio output Z was changed to 600 ohms with the later versions. The RAO-7 & 9 eliminated the S-meter in favor of panadapter connections. The early RAO-6 receivers had S-meters but later production receivers did not. The later RAOs (7 & 9) simplified the maintenance of the receiver by designing the chassis so that it was easily removable from the cabinet (handles were added to the receiver front panel to assist removal.) The cabinet itself was redesigned for better shielding and easier mechanical construction by making it a one piece unit and, though some later cabinets did have lids, most did not. While the early RAOs used a separate shock mount system, the new cabinets mounted the shocks directly to the bottom of the cabinet further easing construction and maintenance. The RAO-7 & 9 receivers are physically larger than the earlier versions with full 19" rack width panels although the receivers are not specifically designed for rack mounting. Electronically, there is no difference between the RAO-7 and the RAO-9 with the exception that, in the RAO-9, the two RF amplifier inputs each have a RC filter installed between the grid and the coil selected. These RC filters were low-pass types to keep VHF radar from interfering with the receiver's operation onboard ship.
The photo to the left shows the top of the chassis of the later RAO receivers, this one is an RAO-7. Note the large and completely shielded RF input box. The SO-239 connector to the left is for the panadaper input. Note that the separate chassis for the 1st RF catacomb is still in use. The side panels provide rigidity for the chassis since it is now mounted in a complete cabinet rather than the sheet metal "pieces" that make up the earlier type cabinet.
Performance - All of the versions of the RAO receivers have impressive sensitivity and freedom from images. Dial accuracy is usually quite good given that the resolution is limited. The tuning rate, which is standard for the National NC-100XA Series, seems fairly fast but it's still easy to tune in CW signals and SSB can be "fine tuned" using the BFO control. The audio output was modified for the RAO series to eliminate the P-P output in favor of a single audio tube stage that was primarily designed for CW reception using earphones. Audio output of the RAO, especially the later versions, will have the bass rolled off significantly since this enhanced the copy of CW signals. AM signals will be significantly lacking in bass response. Stability is very good with very little drift after a 15 minute warm-up. The RAO does provide a remote standby so it's easy to set one up for a vintage military radio station. As a station receiver the RAO will have great sensitivity, reasonable selectivity, good stability and fair audio. I've used both the RAO-3 and the RAO-7 as station receivers with good results.
The RBH Series - RBH was the Navy designation for the NC-156 receiver, a 10 tube superhet that covered 300kc to 1200kc and 1700kc to 17mc in five bands based on the NC-100XA. To allow continuous coverage of the 300kc to 500kc range the IF operates at 1500kc (which is why there is a gap in the tuning from 1200kc to 1700kc.) The initial RBH receivers will have "NC-156" on the National Co. nameplate (part of the crystal filter panel.) The first RBH receivers date from around 1940 and will have the typical modified audio outputs, this is, single-ended audio output stage driving a 500 Z ohm output transformer. The later versions of the RBH receivers were modified for use at sea. All of the RBH series with number suffixes starting with RBH-2 have an additional stage of preselection added with a bolt-in chassis and cabinet to house the additional catacomb section for the coils and an additional tuning condenser for tuning the stage. This addition was very similar to the RAO receiver change and was for the same purpose of reducing the LO radiation from the antenna (>400pW on the antenna.) Like the RAO, the dial system changed on the later RBH receivers, eliminating the articulated pointer and painting the background white rather than silver. It's likely that Wells-Gardner was a contractor for some of the RBH versions. The last of the RBH receivers incorporated the same improved cabinet of the later RAO receivers. The early RBH receiver shown in the photo to the right unfortunately has had all of its Navy tags removed. Sometimes tag removal was a requirement for sales of surplus equipment but more often the new owner wanted visitors to the shack to think his receiver was a new purchase of civilian equipment and not surplus military gear.
Performance - When operating an RBH today, strong AM-BC stations around 1500kc might resonate with the RBH's 1500kc IF amplifiers and can cause strong heterodynes when tuning in stations or, if the 1500kc AM-BC station is particularly strong, it may dominate the IF system of the RBH. A tuned antenna will help with most IF interference however a 1500kc wavetrap on the antenna lead-in may be required to cure any serious problem.
|Miscellaneous U.S.N. Versions - Some of the early RAO series are also marked as NC-120 on the Crystal Filter panel. This generally appears on RAO-2 and some RAO-6 receivers only. NC-156 appears on early versions of the RBH receivers with this designation appearing also on the Crystal Filter panel. The NC-127 was a component receiver of a triple diversity set-up that was designated as NC-127D. The NC-127 is very similar to the RAO-7 receiver with additional outputs on the rear panel for diversity hook-ups. According to Raymond Moore's book, "Communications Receiver's," only one NC-127D was built and therefore probably only three NC-127s were built.|
||U.S. Army Signal Corps NC-100ASD - The NC-100ASD was built for the Signal Corps around 1943. There was another version, the NC-100ASC, which may have been also designated as the AN/GRR-3. From available artwork it appears that the GRR-3 may have had a crystal filter thus being similar to the National NC-100XA though it may have tuned 200kc to 400kc and not tuned the AM BC band. It's likely that it had a 500Z ohm audio output and used a single audio output tube. The Signal Corps wanted a version of the NC-100A without a crystal filter (probably for ease of operation and reliability) that tuned 200kc to 400kc in addition to 1.3mc up to 30mc with this receiver designed as the NC-100ASD. Since the size of the catacomb was more or less fixed at being able to accommodate five sets of coils, National had to remove the AM BC coils in order to have the 200kc to 400kc tuning range. The articulated dial pointer was retained even though all of the later USN versions (RAO, RBH) had eliminated this feature due to it being overly-complex and somewhat delicate (dial string was used for actuation of the mechanism.) The IF was National's standard 456kc. The Signal Corps had no use for Push-Pull audio output, so a single 6V6 is installed, thus the tube total for the ASD is ten. The audio bass is not rolled-off in the ASD (as the RAO's was) so AM reproduction through the matching speaker is normal sounding. A second filter choke was installed in the power supply to take the place of the electro-dynamic speaker's field coil (acting as a choke and electromagnet) as found in the civilian version of the NC-100A.|
|Since the receiver had to have a 500Z ohm audio output it was
necessary to install a 5000Z plate to 500Z ohm audio output transformer
inside the receiver (the civilian version mounted a push-pull output
transformer on the loudspeaker.) This allowed the speaker to be
connected with just a two conductor cable with no B+ on the speaker
connections at the rear of the chassis. A matching speaker was included with the NC-100ASD and it was very
similar to the standard 8" National speaker box but with the 500 Z ohm
matching transformer connected to the 2.8 Z ohm voice coil-speaker. The
transformer is mounted to the speaker frame. Also, the speaker cable is shielded to
prevent RF pickup. There are small variations as the NC-100ASD
production evolved. Early versions have different style 4uf oil
filled capacitors with a crimp around the top while the later versions
have no crimp at all. Later versions use all-Sprague paper capacitors
while the early versions have some Aerovox capacitors in some places.
The dial scale on late ASDs will have a thin coating
of MFP applied to prevent oxidation which isn't found on the early
versions. No special military tags are usually found on
NC-100ASD receivers with the only receiver identification being the
"NC-100A" that is located on the upper right-side nomenclature plate
along with the National Company information. Most units will have Signal Corps acceptance stamp
on the back of the cabinet. Probable contract order number is
Serial Numbering on the NC-100 Series
Formats Used - Serial numbers on early NC-100 receivers appear to be
the same kind of format used on other National receivers at the time.
This consists of a letter that identifies the production run and a
number that identifies the specific receiver within that
production run. When the NC-100 was introduced in 1936, the HRO receiver
was still using serial numbers that consisted of a letter prefix
followed by a number suffix. Probably to keep the two receiver type serial number
obviously separate, the NC-100 serial numbers began with a number
prefix followed by a letter suffix (the letter identifies the specific
production run.) From the serial numbers reported so
far, it appears that the NC-100 series serial numbers begin with the "D" suffix to identify the first production run.
Interestingly, the HRO first production run also used the prefix "D" to
identify its first production run. By
1938, the HRO had used all of the letter prefix serial numbers and then
began to use a letter suffix combination, like the NC-100 had been
It appears that all NC-100 series serial numbers are sequential in their assignment within the particular production run and those production run letters are also sequential starting with run "D" in 1936. It also appears that most production runs had quantities of around 350 to maybe 400 receivers within them. From the serial numbers reported, it appears that all NC-100 Series receivers use a letter suffix in the serial number with the exception of the WWII military versions.
The NC-80 receivers have their serial numbers stamped on the rear panel of the cabinet at the lower-center just below the headphone jack opening. Since the cabinet was painted black wrinkle after the stamping, the serial numbers are sometimes difficult to see. The NC-80 receivers use a different format with a letter prefix followed by numbers. From the one reported NC-80X serial number, it appears that National used the prefix letters not used by the HRO series (A, B & C.) Since the production was low on these receivers, it's possible that all NC-80 series receivers used the "C" prefix (since the reported serial number is C-745,) although this is just speculation. >>>
|>>> The NC-200 serial numbers use a letter prefix
followed by up to three numbers. It appears that production runs C, D
and E comprise the production of NC-200 receivers. It seems unlikely
that the NC-80X series, that also used "C" prefix serial numbers, shared serial number prefixes with the NC-200
since the former was produced from 1937 up to 1939 and the latter was
introduced in late-1940. Since both model receivers used the same serial
numbers, one has to assume that National used the receiver model and
its serial number for identification.
During WWII, serial number formats vary somewhat but
usually follow the letter prefix format. Often times, the serial number
is only found on the particular receiver's data plate. Normally, if the
serial number is on a data plate it will just be a number with no
letters. The NC-100ASD serial number uses only numbers that are stamped into the chassis
between the 6C8 and the 6F8 tubes. Early post-WWII serial numbers
are still letter prefix format using the prefix F for the production run
in 1946. It seems likely from the serial numbers reported so far that
run F began after WWII ended. Sometime in 1946, National changed the serial number format to a seven digit serial number that consists
of a three digit production run ID followed by a space that is then
followed by a four digit number that identifies the receiver. So far,
runs 169 and 183 appear to be for the late NC-240D receivers.
WHRM Serial Number Log for NC-100 Series Receivers - In an effort to see what kind of serial numbering National used on the NC-100 Series, WHRM is starting up another serial number log. These logs are very effective at providing production quantity information and year of manufacture information after a fairly large quantity of serial numbers have been collected. When providing a NC-100 Series serial number for our log, please be sure to let me know the model of the receiver that the serial number belongs to. Also, note any characteristics that will help in determining the manufacturing year. Also, note any unusual production changes or professionally installed modifications. See the log below for some of the identifiers and details we're looking for.
After enough numbers have been collected it may be possible to date the year of manufacture from the serial number used. Please send your serial numbers from any NC-100, NC-100X, NC-101X, NC-80X, NC-81X, NC-100A, NC-100XA, NC-101XA, NC-200, NC-240D, NC-240CS, RAO, RBH, NC-100ASD and any Airway Communication Receivers to this e-mail:
Serial Number Locations:
Generally, on the early versions of the receivers the serial number is
stamped into the chassis top usually in front of the antenna terminal
insulator between the rear of the tuning condenser and the front of the
antenna terminal insulator. The serial number will appear upside down if
you look at it from the front of the receiver with the lid lifted. See
the first photo to the right for NC-100 Series SN location as viewed from
rear of chassis. When the larger rear mounting
flange was used on the tuning condenser, the serial number was moved
closer to the antenna terminal insulator and more towards the audio
output tubes. See photo far right for location on the NC-200 Series receivers.
Sometimes serial numbers appear only on the data plate of the specific receiver. This applies to the Airport receivers with their serial numbers only on data plate mounted on the front panel. During WWII serial number locations vary. The NC-100ASD has the three-digit serial number stamped into the chassis between the 6C8 tube and the 6F8 tube. >>>
NC-100 - 334-D (thin dividers TC*, brass hubs), 341-F (rack mount, no alum overlay on panel, eye tube),
NC-100X - 217-D (reworked from NC-100 at Nat'l), 227-D, 435-D (has original rack mounting brackets), 216-E (early TC), 253-E, 99-J,
NC-101X - 78-D(? - ham mod'd with meter), 62-E(mod'd at Nat. with S-mtr), 217-F
NC-80X, NC-81X -
NC-100A - 145-T,
NC-100XA - 58-R (thin dividers TC, brass hubs, no NL),
NC-101XA - 130-M (no NL),
*TC = Tuning
Condenser - early ones have thin metal dividers between sections
|NC-200 - C-265(CS),
C-536(SA), C-859, C-951, D-327, D-700(parts set), E-39
NC-240CS, NC-240D - F-104(240CS), F-478 (240D), 169 0061(240D), 183 0045(240D), 183 0080(240D), 183 0145 (240D), 183 0326 (240D),
RAO - 10 (RAO-7), 1412 (RAO-3),
NC-100ASD - 194, 402, 449(rep'd as A49), 426, 931, 948,
Airway (Airport) Receivers -
15 (RCD), 40 (RCD), 42 (RCD), 101(RCD), 288 (RCE),
Custom AP Rcvrs - 531-G (NC-100 mod'd .2-.4mc on highest range, separate PS)
SA = Silver Anniversary NC-200
Production and Engineering Changes
NC-100 and NC-100X introduced in August 1936
Aluminum sheet metal overlay for front panel is silk screened in black and red along with silver - used on both NC-100 and NC -100X
PW-D micrometer dial is bluish-gray on the Index Dial and red with
white numbers on the Number Dial on NC-100. PW-D is light gray Index
dial and black with white numbers on
Chassis is painted gray on both NC-100 and NC-100X
6E5 cathode ray tuning eye tube is used
Rack mount versions will not have aluminum overlay, panel is 3/16" aluminum with engraved markings, black wrinkle finish, eye tube used. Some versions may have single-ended audio output. Some rack mounted versions will use National supplied rack mounting brackets that mount to a standard table cabinet thus allowing rack mounting.
The first NC-100 and NC-100X receivers were built in production run "D"
NC-101X is introduced with ham band only coverage, black PW-D on most of production, eye tube on early versions (NC-101X introduced during "E" production run, probably Oct-Nov 1936)
Contracts for Airport "Communication Receiver" from Dept. of Commerce, Bureau of Air Commerce Air Navigation Division. DOC-BAC receivers are the RCD and RCE
S-meter replaced eye-tube in NC-101X - changeover to S-meter seems to have occurred between runs J to L (runs six, seven or eight, or around mid-1937.) Some earlier NC-101X receivers seem to have been modified to replace the eye tube with an S-meter. Some of these upgrades were obviously rework performed at National and exhibit professional-level installations. Others appear to be "hamster" modifications. For example, NC-101X sn 247-J is a sixth production run receiver but has the later Marion Electric yellow scale S-meter installed instead of an eye-tube or the earlier white face Marion Electric meter. Since some hams sent their receivers to National for repairs or upgrades, it's possible that early production run receivers that have yellow scale S-meters may have been replacements that were installed by National during a repair. It's also possible that late-style replacement S-meters were obtained from National were installed by receiver owners. In general, on original receivers that haven't been reworked, expect to see the white face S-meters from around run J up to around run N. The yellow face S-meter should be installed on receivers from around run P up to around run X (probably early-1938 up to 1940.)
Installation of the S-meter switch eliminated the green pilot lamp so all S-meters were illuminated to provide a "power on" indicator. All stock S-meters were manufactured by Marion Electric.
NC-80X and NC-81X AC-DC receivers introduced. Optional power transformer available to convert to AC-only operation. Also a battery-operated version was available.
NC-100A and NC-100XA introduced in June 1938. New direct-read dial, articulated pointer, illuminated S-meter, cabinet height increased to 10.5", weighted tuning knob on early versions.
Contracts for "Airway Communication Receivers" begin - the U.S. Civil Aeronautics Authority, the CAA, was formed in 1938. CAA receivers begin with the RCF-2 version.
NC-101XA introduced. Both versions (direct read dial or PW-D micrometer dial) are available up around mid-1940.
Noise Limiter circuit added to NC-100A versions. 6F8G (1stAF/AVC Amp) and 6C8G (Detector/NL) replaced the 6C5 (Detector) and 6J7 (AVC Amp) tubes. The Noise Limiter was also incorporated into the NC-101X receivers along with the tube changes. NL available November 1939. On NC-101X with NL, the control is between the RF Gain and the Band Change knob where the National NC diamond logo was engraved. The "NC" diamond logo is relocated on these NL receivers to the upper right corner of the front panel.
By late-1939, NC-101X receivers will use NC-100XA chassis but are built as NC-101X receivers. That is, all holes needed for the "A" direct-read dial are present on the NC-101X chassis although not needed or used. Usually, one side of the chassis will have "101-X" written in orange grease pencil to ID chassis so assemblers know not to install the "A" dial and "A" gear box.
NC-80X and NC-81X not produced after 1939. NC-80X cut-off may be 1938. Neither receiver sold well and examples are rarely encountered today.
Tuning condenser design changed to replace large bakelite insulator plates with smaller round insulators, thin metal dividers replaced with thicker metal dividers that now support the rotor contact insulators, rotor plate hub changed from brass to steel, support rods dimensions reduced, two screw rear mounting flange. Cost reduction? HRO tuning condenser went through similar changes.
Antenna/Ground thumb-screw terminals with Polystyrene insulator replaces the old "push terminals" with fiber board insulator. HRO went through same change at the time.
NC-101X production stops before May 1940. The direct-read dial version - NC-101XA - production may have continued until the NC-200 was incorporated into production (Sept-Oct.)
When the two-screw rear mounting flange was used on the tuning condenser, the serial number location had to be moved to the left side of the antenna terminal insulator (nearer to the audio output tubes.)
NC-200 introduced (October) - features both general coverage and band spread coils within the catacomb thus eliminating the need for separate "ham bands only" receivers like the NC-101X or NC-101XA.
LO tube changed to a 6J5 triode in NC-200 Series. Many other tube changes in the NC-200, see tube line up in section below "Vacuum Tubes Used in All NC-100 and NC-200 Series Receivers."
Dec. 1940 QST ad for "Silver Anniversary" NC-200. Special NC diamond insignia on receiver and matching speaker. Also, receiver equipped with brown knobs and brown S-meter case. All round control nomenclature plates, the tuning knob's skirt and the Silver Anniversary NC diamonds are finished in "gold tone." The Silver Anniversary NC-200 celebrated QST's 25th year and wasn't associated with any National anniversary.
RAO USN versions - RAO, RAO-1,2,6,7 & 9 built by National, RAO-3,4 & 5 built by Wells-Gardner. RAO-2 through 9 have double preselection. All versions have .54mc to 30mc coverage. Early versions have 500 Z ohm audio outputs, RAO 7 & 9 have 600 Z ohm audio output. RAO-2 thru 6 are 17.5" wide, RAO-7 & 9 are 19" wide. RAO-7 & 9 don't have S-meters installed but have an output for panadapter use instead. Gears used in the gearbox are changed from brass gears to cast pot metal gears for wartime material conservation.
RBH USN version of NC-100XA with special frequency coverage of 300kc to 1200kc and 1700kc to 17.0mc, IF changed to 1500kc, single ended audio, 500 Z ohm output
NC-100ASD Signal Corps version, ca: 1943 - replaced AM BC coverage with 200kc to 400kc, single 6V6 audio output with 500 Z ohm output transformer. Serial number is stamped into the chassis between the 6C8 and 6F8 tubes. Serial number is three numerical digits. Highest reported SN is 948. The ASD may have been a replacement for NC-100ASC (aka AN/GRR-3) that might have been a "militarized" NC-100XA.
NC-200FG, NC-240C and NC-240CS produced during WWII. These receivers do not have band spread function. The CS version had 200kc to 400kc band to replace the AM BC band.
NC-100A, NC-100XA and NC-200 offered in the National section of the 1945 Radio Amateur's Handbook. None of these receivers, or any other items listed, were actually available during the war and each page of the catalog states "Priorities are required for all products in this catalog until otherwise released by the War Production Board." The intention of the catalog was to show what National was probably going to have available after the war ended.
In 1945, Schuttig & Co. modified RCL and RCK Airway receivers into RCP Airway receivers.
NC-240CS sold to the civilian market in post-war 1945 as a 1946 model.
NC-240D returned the band spread function and was sold from early-1946 up to 1949. Note in SN Log that F-104 is a CS model w/o BS and that F-478 is a D model with BS. Both receivers are from the same production run "F" which indicates that this change was rapidly incorporated. In 1946, National designated the CS version as a "commercial receiver"
In 1948, National Electrical Machine Shops (NEMS) modified RCL and RCK Airway receivers into RCQ Airway receivers
In 1948, some NC-240CS receivers were produced as RCR Airway Receivers. The RCR is the last National Airway receiver produced based on a Moving Coil design.
Coil Catacomb Details
|The "Moving Coil" Catacomb - Just as the plug-in coil sets and the PW gear drive and tuning condenser are the "heart" of the HRO design, the "Moving Coil" Catacomb along with its N-PW gear drive and tuning condenser are the "heart" of the NC-100 Series. The Coil Catacomb is a two-piece cast aluminum box containing fifteen tuned coils and their respective trimmer variable capacitors. Each coil is inside a singular chamber that completely shields all of the coils from one another and from the rest of the receiver circuitry. Each coil assembly is mounted with two screws accessible by removing the catacomb cover. The coil insulator blocks each have five short, stubby pins that make the contact to the tuning condenser section to complete the tuned circuit. Each coil assembly is identified with a letter-number combination to assure proper installation. The insulator material is National's brown bakelite mixture called R-39, noted for its superior quality over standard bakelite. Robust in construction, the Coil Catacomb assured that the tuned circuit alignments stayed in adjustment and that receiver stability was solid and consistent of long periods of time. The engagement contacts are located in R-39 insulator blocks that are mounted under the tuning condenser. This allows the connections from these contacts to be very short and accomplished with TC wire. The contacts are dual entry in design allowing the Coil Catacomb pins to enter from either side. This allows the bands to be changed from any direction.|
|The two-piece engagement contacts are insulated from each other and when the connections are soldered on the top side (under the tuning condenser) both sides of the upper part of the contact are soldered together. The exception is the foremost LO contact. Since the LO only requires four connections, the unused contact is wired to act as a screen voltage switch for the RF and IF amplifiers to eliminate noise during band changes on the early models of the receiver. Later models used this contact for the RF amplifier cathode return to "mute" the receiver during band changing. The almost "sealed" nature of the catacomb construction generally protects the coils and trimmer capacitors from damage. Nowadays, most catacomb coils are still in good condition even if the rest of the receiver has not faired so well.||
|NC-100 Coil Assembly
Identification - Each coil assembly is marked with a
letter and a number that identifies its frequency of operation and its
function in the circuit. Coil assembly identification will either be
engraved or ink-stamped, depending on when it was manufactured (see
photo right.) Coil sets A through E are generally found in any early
single-preselection catacomb receivers that tune .54mc to 30mc. Coil
sets A through D and coil set H are found in National Airport receivers.
Some NC-100ASD receivers will have G coils in place of H coils. NC-200
Series coil assemblies are similarly identified.
A = 14mc to 30mc A1 = RF AMP, A2 = MIXER, A3 = LOCAL OSCILLATOR
B = 6.5mc to 14.5mc B1 = RF AMP, B2 = MIXER, B3 = LOCAL OSCILLATOR
C = 3.2mc to 6.5mc C1 = RF AMP, C2 = MIXER, C3 = LOCAL OSCILLATOR
D = 1.3mc to 3.2mc D1 = RF AMP, D2 = MIXER, D3 = LOCAL OSCILLATOR
E = .53mc to 1.3mc E1 = RF AMP, E2 = MIXER, E3 = LOCAL OSCILLATOR
F = .49mc to 1.04mc F1 = RF AMP, F2 = MIXER, F3= LO - NC-200 Series only
G = 200kc to 400kc G1 = RF AMP, G2 = MIXER, G3 = LO - Some NC-100ASD
H = 200kc to 400kc H1 = RF AMP, H2 = MIXER , H3 = LO - Airport Rcvrs, NC-100ASD
Assembly Identification - Since the NC-101X tunes ham bands
only and band spreads each range, slightly different coils and trimmers
were required to work with the "band spread" main tuning condenser. The
identifications on the NC-101X coil assemblies use only numerals and are
tied to the ham bands that are covered. Therefore, 16 denotes 160 meters
while 8 denotes 80 meters and 4 denotes 40 meters, 2 denotes 20 meters
and 1 denotes 10 meters. As with the NC-100 coils, suffix numbers 1, 2 and 3 denote RF
(1,) Mixer (2) and LO (3). Thus, 16 1 is the 160 meter RF coil or 4 3
would be the 40 meter LO coil. As with the NC-100 coils, the
identifications are stamped into the insulator-base. (see photo right.)
16 = 160 meters 16 1 = 160 RF AMP, 16 2 = 160 MIXER, 16 3 = 160 LO
8 = 80 meters 8 1 = 80 RF AMP, 8 2 = 80 MIXER, 8 3 = 80 LO
4 = 40 meters 4 1 = 40 RF AMP, 4 2 = 40 MIXER, 4 3 = 40 LO
2 = 20 meters 2 1 = 20 RF AMP, 2 2 = 20 MIXER, 2 3 = 20 LO
1 = 10 meters 1 1 = 10 RF AMP, 1 2 = 10 MIXER, 1 3 = 10 LO
Assembly Identification - The NC-80X (and the NC-81X)
uses a smaller catacomb since the receiver doesn't have an RF amplifier
section. The catacomb is driven into position using a chain drive
mechanism rather than the rack and pinion used for the NC-100 series
catacombs. Only two coil sets per band are used, those being a Mixer coil assembly and a LO coil assembly.
Since the IF used in both receivers is 1560kc, both Mixer and LO coils
are somewhat different than the NC-100 coils.
Each NX-80X coil assembly is identified using the letters W, X, Y and
Z for frequency ranges and the numerals 1 or 2 for function. The
assignment of the highest letter to the highest frequency coil set
departs from National's usual assignment of the letter A to the highest
frequency coil sets. But then, the entire NC-80X coil catacomb is
entirely different from the NC-100 catacomb.
As an example, coil
assembly X2 would be
for tuning range 1.7mc to 4.6mc and for the LO function. The identification is
stamped in the base of the coil assembly.
W = .55mc to 1.5mc W1 = MIXER W2 = LO
X = 1.7mc to 4.6mc X1 = MIXER X2 = LO
Y = 4.3mc to 12.0mc Y1 = MIXER Y2 = LO
Z = 11.4mc to 30.2mc Z1 = MIXER Z2 = LO
Shown in the photo to the right is the NC-80X coil assembly
Y2 which is for the 4.5mc to 12.0mc band and is the Local Oscillator
coil function. Note the wire loop inside the coil form that is used for
the low-end of the dial tracking adjustment. The air variable trimmer is
for the high-end of the dial tracking adjustment. Unlike the cast
aluminum cover for the catacomb used in the NC-100 series, the catacomb
cover for the NC-80X is aluminum sheet metal.
|More Coil Assembly
Identification Confusion - Coils sets employed in the later RAO and RBH receivers with double
preselection will have four coil sets per band. They are numbered 1
though 4 to identify the function and with letters to identify frequency
coverage. The RAO bands are A through E and cover 30mc down to .54mc.
The RAO coils for the 1st RF Amp covering 14mc to 30mc would be A1 and
the LO for the same band would be A4.
The RBH also has its bands identified as A through E but bands E and D cover different frequencies than E and D on the RAO. The RBH coil assemblies are also identified as A through E even though they are different from the RAO coil assemblies - they are identified just like the RAO receiver's coil assemblies. RAO IF is 455kc and the RBH IF is 1500kc which means the LO and Mixer coil assemblies are different. In fact, band D and E LO assemblies both have padder capacitors on the RBH coil assemblies. As far as any markings other than the letter-number ID, there aren't any.
Just to keep the confusion going, National decided to change the whole band ID concept of letter A designating highest frequency band with the WWII RBL Regenerative LW Receiver. Although not a catacomb receiver, it was generally paired with and looks very similar to the RAO receiver. With the RBL, band A is the lowest frequency band (15kc up to 30kc) and band E is the highest frequency coverage (about 270kc up to 600kc.)
On the NC-200 series, with it's very different coil assemblies that included four sets that had a bandspread function, still the letter-number ID remains the same. That is, the highest frequency coverage band is ID'd as Band A and the associated coil assemblies are also ID'd as A1, A2 and A3. NC-200 Series coil assemblies have six stubby pins to provide for the bandspread function on 80M, 40M, 20M and 10M.
National apparently only wanted the coil assemblies within the individual catacomb to be identified for proper placement for the particular model receiver. It was probably thought "why have several different ID combos when nobody is going to be replacing the coil assemblies anyway." If a specific coil assembly was damaged beyond repair a new one could always be ordered from National specific to the model receiver it was for. Nowadays, while it's possible to find receivers that have had some tampering to the coils within the catacomb, it is rare. Most catacombs are in good condition because of the almost "sealed" nature of the housing which results in very few problems with the coil assemblies.
|The photo above-left shows how the individual coil
assemblies are removed from the catacomb. Note the philip's head
screws (WWII and later.) These are 6-32 thread and screw into the catacomb base
for mounting. The photo above-center shows how the NC-100 series coil assembly
looks. When removing an individual coil assembly or a particular
section (coil assemblies 1, 2 & 3 of any letter designator) make
sure the catacomb is disengaged from the pin contacts by setting
the band switch in between bands. Then loosen the two screws
until they are no longer threaded into the catacomb base and
then lift out the coil assembly. Reinstall in the reverse and
then check engagement by operating the band switch. Early
versions (pre-WWII) will use round-head blade screws instead of philip's.
The photo above-right shows the alignment inductance loop that are in all of the coils A, B, C and D for adjustment of the low end of the dial tracking. This not only is used on the LO but also on the Mixer and RF Amp coils. Moving the coil loop slightly will change the inductance allowing the low end to be set exactly. During alignment, by using a plastic rod to manipulate the loop, you can have the receiver operating and set the loop position exactly for proper low end tracking (this wasn't possible on National's HRO receivers.) High end of the dial tracking is adjusted with the air variable trimmer. The lower frequency coils E or G will have a padder compression trimmer to adjust the low end dial tracking on the Local Oscillator coil assembly.
The photo to the right shows the NC-200 catacomb with its six band coverage. Note that there are many more air variable capacitors for adjustment. The two lowest frequency bands have padder capacitors on the LO and the four highest frequency bands have ceramic trimmers for the ham bandspread adjustments. To avoid confusion during alignment, National assigned the same designator for trimmers that performed the same function on each band. For example, the four bandspread RF amplifier trimmers are all designated as C-54 and the four mixer bandspread trimmers are designated C-55. Sounds confusing but it actually works fine, as long as you have the alignment instructions and drawings to guide you. LO coils are at the top, Mixer coils in the middle and RF coils at the bottom. Note LO coils on the four HF bands have three variable caps each. One for General Coverage, one for Bandspread trim and one for Bandspread padder. GC low end uses coil loops.
Dial - The NC-100 used the same PW-D micrometer dial as
the HRO receiver but with different paint colors. Initially, the NC-100
used a PW-D that had the Index Dial painted a bluish-gray and the Number
Dial was painted red with white numerals. This rather dramatic color
combination went well with the "deco" styling of the metal overlay panel
of the NC-100. Interestingly, for the NC-100X, which was supposed to be
the ham version of the receiver, the PW-D Index Dial was painted light
gray and the Number Dial was painted black with white numerals. This
seemed to be a more conservative appearance that would appeal to the
hams. When the NC-101X version was released, National wanted the
receiver to appear like the HRO, so the standard HRO Senior PW-D was
fitted to that receiver. Some of the advertising photographs of the
NC-101X show it with the light gray PW-D installed but it looks like
most of the actual production used the black PW-D, like the HRO Senior.
Though "black" describes the color of the PW-D index dial in normal room
lighting, when in sunlight or any bright light, the index dials will vary in
color from a gunmetal gray-black to and olive-bronze black. The colors
are usually only noticeable in very bright light (like a
"flash" photograph.) The gray PW-D with black Number dial was
standard for all Airway Receivers.
PW-D Micrometer Dial Servicing - NC-100(X), NC-101X, Airport Receivers
|The PW-D Micrometer dial only has two moving parts and seven stationary parts. Elegant in its simplicity of design, it is very easy to work on if you know the "tricks." About the only thing that goes wrong with the dial is that it becomes noisy in operation and might have a rough feel when tuning. Synchronization might be a problem too - if the dial was reassembled incorrectly. The inner dial has gear teeth cast into its inner perimeter and also has the bearing that fits on the elliptic hub of the tuning shaft bearing housing. The outer dial has mating gear teeth that are cast into a rim projection and a hub that is set screw attached to the tuning shaft. By having the inner dial ride on the elliptic hub cast into the tuning shaft bearing housing, the gears of the two dials engage and as the outer dial is turned, by way of the mating gears, it turns the inner dial and since the outer dial is coupled to the tuning shaft, it also moves the tuning condenser. The elliptic hub is actually round but since the tuning condenser shaft exits the hub "off-center," the hub bearing surface appears as an ellipse to the tuning condenser shaft. This eccentric placement forces engagement of the inner and outer dial gears, thus driving the inner dial with the rotation of the outer dial. >>>||>>> To disassemble the Micrometer dial, first tune the receiver to "250" then loosen the dial set screw. The dial should come off of the elliptic bearing hub and tuning condenser shaft with very little effort. With the PW-D dismounted, look at the back of the dial and, if the PW-D was synchronized prior to removal, you will notice that the three screws that hold the knob to the dial appear through cast holes in the inner dial. Also note that the two springs that hold the inner dial to the outer dial are pointing straight up. Remove the knob. Then remove the two springs - their ends fit into slots at the end of the shaft hub of the outer dial (where the set screw is.) Now the dials can be separated. Examine the cast gear teeth for condition. Usually they are in good shape and all that is present is old dried grease. Use a tooth brush and light oil to remove all of the old grease and then apply a light coating of Lubriplate (or any modern light weight grease) to the inner dial gear only. Position the inner dial to the "250" position with the cast holes lined up for the knob screws. Then install the two retaining springs. >>>|
|>>> Before putting the PW-D back on the receiver,
lightly grease the elliptic bearing hub (since the inner dial rides on
this it should be clean, smooth and lightly greased.) The PW-D should
already be set to "250" and the receiver condenser should not have been
moved. Place the PW-D back on the receiver by carefully noting how the
elliptic hub requires the inner dial to be slightly down from center. If
you have assembled the PW-D correctly, it will just slip right on to the
hub. Tighten the set screw and test the operation - it should be
ultra-smooth and very quite in operation. If your PW-D dial doesn't just
slip onto the elliptic hub then it isn't assembled correctly. It can go
together two ways, one correct and one 180º out. If your PW-D doesn't
just "slip onto" the bearing hub, read the section below
"Correct Assembly of the PW-D Micrometer Dial - Details."
Around the beginning of WWII, a guide pin was added to the inner dial, located just above the inspection/grease hole. This pin limits the vertical movement of the inner dial and was probably installed to keep the gear alignment fairly tight and allow easier installation of the PW-D onto the elliptic hub. Later production dials will have this guide pin molded in the casting of the inner dial.
photo left: Two moving parts and seven stationary parts are all that are used in the PW-D. A second set screw was used during and after WWII - total of eight stationary parts then.
Correct Assembly of the PW-D Micrometer Dial - Details - When the PW-D is assembled correctly, it will just slip onto the elliptic hub. If you are trying to force the PW-D onto the hub, then the PW-D is assembled incorrectly. There are two ways that the outer index dial can interface with the inner number dial. One is correct and one isn't. Look carefully at the photograph to the right noting the location of the smaller round hole, the two oval holes, the "NATIONAL CO., INC." embossing and the location of the screws that are for mounting the knob. If you orient the reverse side of your assembled PW-D in this position and then look at the outer dial, you should see "250" centered in the window that is directly behind the "NATIONAL CO., INC." embossing.
To check your assembly,...position PW-D as shown in photograph to the right. You should see the following:
1. "NATIONAL CO., INC" embossing should be at 12 o'clock
3. Two oval holes should be at 2 o'clock and 4 o'clock
4. Note that all three knob screws are directly centered in the small round hole and the two oval holes
5. Turn dial over from right to left and you'll see "250" centered in the window at 12 o'clock (the window should be directly behind the embossing "NATIONAL CO., INC" on the back of the inner dial.)
6. Push the inner dial so that the "250" appears closer to the bottom of the window. You should hear a slight "click" as the inner dial gear teeth mesh with the outer dial gear teeth. You may have to slightly "rock" the inner dial as you push.
If your assembled PW-D meets this criteria, then it will slip right onto the elliptic hub. Sometimes even though everything is assembled correctly, the PW-D still won't fit onto the elliptic hub. With the PW-D removed, push the number dial into the gears of the index dial while very slightly moving the position of the number dial. You'll probably hear a "click" as the gears mesh correctly. Check that the "250" is still centered in the index dial window. Now the PW-D should slip onto the elliptic hub easily. Sometimes, instead of hearing the "click", you'll just "feel" the correct meshing of the gears. You must have the gears meshed before the PW-D will fit onto the hub easily.
Dismounting a Stubborn PW-D (it's not the same as an HRO) - Sometimes removing the PW-D
Micrometer dial proves to be a very difficult operation. Almost always,
the difficulty is caused by excessive marring of the tuning shaft by the
knob set screw(s.) If everything seems it indicate that the PW-D is not
coming off of the shaft no matter what, this is the procedure necessary
to remove the dial. First, you'll need a couple of right-angle screw
drivers. I usually have to make them because everything commercial is
too big to fit between the back of the front panel and the tuning shaft
elliptic bearing hub screws. Make at least two that have different
angles for the blade engagement. You'll also have to add a slight "S"
bend to the shaft of the angled screw drivers. This applies only to the
NC-100, NC-100X, and NC-101X because these receivers are in cabinets
that have a non-removable lip that impedes easy access to the hub screws. If the
receiver is a rack mount, e.g., Airport receiver, then there is ample
access and a straight shaft angled screw driver can be used.
Now, using the right-angle screw drivers, loosen and remove the four screws that mount the elliptic bearing hub. You'll have to use a small dental mirror to see the screws for proper blade engagement. It's difficult but it can be done. The bearing hub is just slightly spring loaded with a small dual leaf spring so when the last screw is removed the hub will just slightly come forward. Now, remove the entire PW-D and the bearing hub and the main dial drive shaft out through the round hole in the receiver's front panel. Now, remove the knob from the PW-D assembly. This provides access to the front of the tuning shaft. Then, using a soft metal drift, gently tap out the shaft from the PW-D dial assembly. >>>
|>>> You will have to reassemble the gear box and set the
anti-backlash per the instructions in the "Servicing the NC-100
Series Gear Box" section above. Also, be sure to dress down
the marring on the tuning shaft so the next time the PW-D has to be
removed, it will be an easy job.
With the NC-100A Series, the main tuning shaft diameter was greatly reduced - from 5/16" diameter to 3/16" diameter. The smaller tuning shaft diameter doesn't seem to have the marring problems that the large, early style, PW-D shafts experience. Perhaps the material is harder since the diameter was reduced. Also, you'll find that the NC-100A Series receivers don't need an elliptic hub bearing. Generally, tuning knob removal on the later receivers is easy and problem-free.
NOTE: When remounting the bearing hub note that "TOP" is embossed on one of the flanges. This flange obviously must be mounted up. This is important because it positions the elliptic bearing correctly for the PW-D dial.
Servicing the NC-100 Series Gear Boxes
|Cleaning and Lubrication
- The early gear box has a
cast metal top cap that is mounted with four fillister-head screws. When this
top cap is removed, the gears are accessible. Most of the NC-100 gear
boxes don't have an excessive amount of dried-up grease inside but they
will need thorough cleaning and re-lubrication for smooth operation. I
usually use WD-40 applied with an acid brush to dissolve and loosen the
dried grease. The old grease and residual WD-40 is then removed with a "dry" acid brush
and a cotton cloth rag.
You probably won't be able to remove 100% of the old grease but it really isn't
necessary. Apply a high quality modern light grease to all of the gear teeth.
Check the operation which should be very smooth and easy to rotate the
tuning condenser via the front main tuning shaft.
Setting the Anti-Backlash - If you have to remove the elliptic hub you will unload the anti-backlash spring located on the left side drive gear. The anti-backlash setting will determine how "lightly" the gear box "feels" when tuning. A good setting is about four or five teeth. You'll feel the anti-backlash spring load at about two teeth but you do need a bit more load for proper operation. Too much load will cause gear wear and a "heavy" feel to tuning. Too little load will cause backlash and a very light feel to tuning. You'll have to hold the right side gear in place and then, using your other fingers, hold the anti-backlash gear in place while you insert the main tuning shaft and elliptic hub assembly. Besides engaging both large gears, the end of the main tuning shaft has a small thrust end pin that fits into the thrust socket bearing located in the end of the condenser drive shaft, so be sure that the assembly is fully seated. You'll feel a slight pressure being exerted by the dual leaf spring while you hold the bearing hub in place. Be sure that the "TOP" flange is up and then install the four mounting screws.
|NC-100A Versions - Gear Box Details - With the change to a direct readout tuning dial, the gear box used on the NC-100A versions required a few modifications. First would be the elimination of the elliptic bearing hub. Since a PW-D was not used there was no need for the elliptic part of the hub. The new tuning dial featured a tracking "band in use" articulated pointer that used a rather large mechanism to support the pointer. Note in the photos right and left that a large cast metal arm is mounted to the tuning condenser shaft. This arm is cast to have a bend that goes over the gear box and then is attached to the disk of the articulated pointer mechanism. This disk also "rides" on a bearing that is part of the main tuning shaft bearing hub (which isn't elliptical anymore.) To remove the cap, which is now a flat metal piece with the "NC" diamond embossed on top, the pointer mechanism arm must be moved to one side of the other of the gear box top (tune the dial to minimum or maximum frequency on any band.) When the cap is removed, it will be noted that the NC-100A version gear box is almost the same as the earlier gear boxes. The two reduction gears that are driven by the main tuning shaft have been moved slightly so that the right side gear is mounted somewhat higher than the left gear. Otherwise the gear box is identical to the early versions. Servicing is the same as for the early box.|
NC-240D Versions - Although there appears to be a gear
box in these receivers, seeing the photo to the left reveals that the
tuning shaft merely passes through the box and drives the tuning
condenser. The gear box housing is merely acting as a bearing for the
tuning shaft. The tuning of these receivers works on the large diameter
fiber disk which is rim
RAO and Other Military Versions - The RAO-2 and later versions will not have the articulated pointer mechanism since this feature was eliminated to simplify the construction and maintenance of the receiver. The cast metal arm that carried the articulated pointer mechanism was removed. To further strengthen the dial itself a bracket is mounted between the top of the gear box and the back of the main dial. You will notice that the RAO dial will have two screws near the top of the dial that are for mounting the bracket. To access the gear box on an RAO will require removal of the tuning dial bezel. This will require removing the associated knobs and control mounting nuts (if used.) With the tuning dial bezel removed, you'll have access to the two screws that mount the top part of the bracket. The mating nuts are located on the backside of the dial. Once this part of the bracket is dismounted, then the two screws that mount the lower part of the bracket to the gear box cap can be removed. The gear box cap is held in place with two hex head screws that are on the sides of the gear box. Loosen the hex head screws to remove the cap. To see how this bracket is attached, reference the photo of the RAO-3 chassis above in the section "WWII Versions." Once the gear box cap is off the remainder of the RAO gear box servicing is just like the NC-100A gear box. The early RBH gear box is similar to the NC-100A while later versions are like the RAO gear box. The NC-100ASD gear box is similar to the NC-100A box. Later WWII gearboxes will have cast pot metal gears rather than brass gears.
Vacuum Tubes Used in All NC-100 and NC-200 Series Receivers
|Unlike the HRO receiver that continued to
use glass tubes with six and seven pin bases up into WWII, the NC-100
was introduced utilizing octal base tubes with the majority of those
tubes being metal tubes. The eye-tube, the audio output tubes and the
rectifier tube are the only glass tubes used and the rectifier and the
eye-tube are the only non-octal based tubes.
It's interesting that James Millen published a QST letter in 1937 that seemed to condemn the 6.3vac glass tubes used in the HRO in favor of using the even older 2.5vac tubes and yet here was the NC-100 sporting the latest types of tubes available. Many speculate that over-stocking of older parts was the reason for the position that National had on the HRO, although maybe National didn't want to change the design of an obviously superior performer like the HRO. Eventually, in 1945, the HRO was produced with octal tubes and the performance wasn't diminished at all (HRO-5.)
RF = 6K7, Mixer = 6J7, LO = 6K7, IF1 = 6K7, IF2 = 6K7, Det = 6C5, AVC
Amp = 6J7, BFO = 6J7, P-P AF Output = 6F6(2), Eye Tube = 6E5, Rect = 80
NC-100A,100XA: RF= 6K7, Mixer = 6J7, LO = 6K7, IF = 6K7(2), Det = 6C5, AVC = 6J7, BFO = 6J7, P-P AF Output = 6F6(2) Rect = 80
NC-100XA(w/NL) 101X(w/NL,)101XA(w/NL): RF= 6K7, Mixer = 6J7, LO = 6K7, IF = 6K7(2), Det/NL = 6C8, BFO = 6J7, 1st AF/ AVC = 6F8, AF Output = 6F6(2), Rect = 80
NC-80, NC-81X: Mixer = 6L7, LO = 6J7, IF = 6K7(3), Det = 6C5, AVC = 6B8, BFO = 6J7, AF Output = 25L6, Rect = 25Z5 (not in "B" models - battery versions)
NC-200: RF = 6SK7, Mixer = 6K8, LO = 6J5, IF1 = 6K7, IF2 = 6SK7, Det/NL = 6C8, AVC = 6SJ7, BFO = 6SJ7, 1stAF/Phase Inv = 6F8, P-P AF Output = 6V6(2), Rect = 5Y3
NC-240D: RF = 6SK7, Mixer = 6K8, LO = 6J5, IF1 = 6K7, IF2 = 6SK7, Det/NL = 6SL7, AVC = 6V6, BFO = 6SJ7, 1stAF/Phase Inv = 6SN7, P-P AF Output = 6V6(2), Rect = 5Y3
NC-100ASD: RF = 6K7, Mixer = 6J7, LO = 6J7, IF = 6K7(2), Det/NL = 6C8, 1st AF/AVC = 6F8, BFO = 6J7, AF Output = 6V6, Rect = 5Z3
RAO: RF1,2 = 6K7(2), Mixer = 6J7, LO = 6J7, IF = 6K7 (2), Det/NL = 6C8, BFO = 6J7, 1st AF, AVC = 6F8, AF Output = 6V6, Rect = 5Z3
RBH: RF = 6K7, Mixer = 6J7, LO = 6J7, IF = 6K7(2), Det/NL = 6C8, 1st AF/AVC = 6F8, BFO = 6J7, AF Output = 6V6, Rect = 5Z3
Airway Receivers: RF = 6K7, Mixer = 6J7, LO = 6J7, IF = 6K7(2), Det = 6C5, AVC = 6J7, BFO = 6J7, 1st AF (Squelch) = 6C5, AF Output = 6V6, INS Control = 6J7, Rect = 80
|Note on the 6F8G Tubes - It seems that the audiophiles and the eBay tubes sellers have discovered that the duplex triode 6SN7's predecessor was the 6F8G. Consequently, the price of the 6F8G has increased significantly in the past few years. Some eBay descriptions insist that the 6F8G "sounds" better than the 6SN7 (hmmm,...really?) There are even adapters being sold on eBay to adapt the 6F8G to operate in place of a 6SN7. (Incredible!) There must be millions of 6SN7s around since every late-forties to early-fifties TV was loaded with them. The 6F8 was only used in a few applications with the National NC-100XA, NC-200 and RAO receivers being the most often encountered. So why create a pseudo-demand for a tube just because it happens to be a dual-triode and can substitute for the ultra-common 6SN7? The mystery of the audiophile mind and the eBay tube sellers, I guess. Though 6F8Gs can still be found for less than $20 each some of the current prices on eBay are insane with some BINs averaging around $50+ per tube. It is unfortunate that the NC-100A/200/RAO does require that the 6F8G (1stAF and AVC amp or NL in the 200) be in relatively good operating condition. Very weak tubes will distort the audio on strong signals since the AVC can't control the front-end gain of the receiver. It's probably is a good idea to stock-up and have a few spare 6F8Gs on hand - just in case the 6F8G goes the way of many other low-demand but insanely priced "audio tubes."|
NC-100A, XA, ASD, NC-80X, NC-101XA, RBH - Original Dial Lamp Assembly
|Almost all NC-100A receivers encountered will have the
original dial lamp assembly damaged or missing. Apparently the
fiberboard tube that held the lamp sockets deteriorated at the point
where it was held in place with the two moveable retaining clips. This
resulted in the dial lamp assembly not being able to stay mounted
correctly. Many NC-100A series receiver owners modified
the dial lamp mountings using clip-on lamp sockets that were attached to
the fiberboard tube retaining clips though other types of modifications were
probably also used.
If you want to make a replica of the original NC-100A dial lamp assembly, a photo of an original example from an NC-100ASD is shown to the right. The dimensions are as follows:
Overall Length = 8.375" Wall Thickness = 0.030"
Outer Diameter = 0.50" Slot Opening Width = 0.375"
Material is fiberboard that is painted white. The material has to be non-conductive, somewhat flexible and has to be able to withstand the heat from the two dial lamps. That probably would eliminate some types of plastics, especially the types of plastics available around 1938 to 1945. Fiberboard tubes are available from various vendors. Check eBay or McMaster-Carr for suitable material. The original lamp sockets were for screw-base lamps. I recommend using the #46 lamps that are listed in the manual as #40 lamps don't provide adequate dial illumination. Be sure to use the low current #40 lamp for the S-meter illumination since a #46 is "hot" enough to really discolor the plastic meter scale. >>>
Power Transformers for the "Moving Coil" Receivers
|The original power transformers were rated at about 100
watts (sometimes 150 watts was shown in some parts lists) which under
normal operating conditions was more than adequate to operate the
receiver for long listening sessions. However, it's not too rare to find
that the power transformer in a Moving Coil receiver has been replaced
or is, in fact, defective. It's unlikely that the original transformers
were in anyway prone to failure and probably provided service for at
least a decade without any problems. As the receiver components aged
(especially the paper-wax capacitors) and the load on the transformer
increased, if the receiver continued to be used for hours on end, a
power transformer failure was probably inevitable.
Replacement Power Transformers - National used the same basic power transformer in all of the different models of Moving Coil receivers. Today, most restorers "harvest" a usable power transformer out of a "parts set" that's the same model as the receiver they are rebuilding. Sometimes, one has to use power transformer from a similar Moving Coil receiver. All Moving Coil receiver power transformer versions have wires for the primary connections and wires for the ends of the HV winding (but not the CT.) All other connections are solder terminals. Although there are some versions that have a two-wire primary winding and others with a three-wire primary winding, nearly all transformers were originally three-wire primary windings. Close inspection of a two-wire transformer will usually reveal that the "third wire" has been cut close to the core by National assemblers. This means that a three-wire primary transformer can be used to replace a two-wire but a three-wire primary will be needed if the receiver originally had that type installed. It might be possible to splice a new wire if enough of the "stub" is left (usually they're cut really close.) All transformers versions are the same physical size. You may have to adapt the core shield connection as this was used in some receivers and not in others. Some later transformers have a separate terminal for the core shield. Some schematics show the shield connected to the 6.3vac winding CT but this CT isn't connected to chassis in some receivers. However in these receivers, one side of the 6.3vac winding is connected to chassis to provide both 6.3vac to the tube heaters return and a low resistance connection to chassis for the shield.
Photo to the right shows a typical "Moving Coil" receiver power transformer installation. At the top of the photo the two large wires exiting the transformer are the primary connections. The upper three terminals are 6.3vac-CT-6.3vac. Note that the CT is not connected to chassis in this receiver (NC-200.) On the bottom, the two large wires are the HV connections to the rectifier plates. The left-most terminal is 5vac and the terminal with the red wire connected to it is the other 5vac (rectifier filaments.) The right-most terminal is HV-CT.
Rewind? - Although rewinding the original power transformer is certainly one method of repair, it is an expensive one. The advantage is that all new materials are used in the rewinding so you are essentially getting a brand new transformer core with the original laminations and covers. Besides the expense, turn-around time can also be an issue.
|Some Observations - Check your receiver's original power transformer connections. Some production wiring is slightly different from the published schematics. Most NC-100A and NC-200 receivers don't use the 6.3vac CT even though it's present. Some receivers use a capacitor input on the B+ filtering and some receivers use a choke input. The choke input receivers will have somewhat lower B+ but the HV AC at the transformer will be the standard 270vac-0-270vac. Some transformers have an internal shield that doesn't have a terminal and relies on the shield connection to the 6.3vac winding CT or the mechanical mounting for chassis ground. Most 3-wire primary windings are for "HI" and "LO" line voltages, assume 120vac for "HI" and 110vac for "LO." If 3-wire is used in a 2-wire receiver, use the "HI" 120vac connection. The unused ("LO") wire can be insulated and tucked into the apron area of the transformer mounting or cut short next to core (as originally done.) RAO-7 and 9 use a very similar transformer but insulation on primary and HV wire is a different material and color. RAO 3, 4 and 5 were built by Wells-Gardner and use an entirely different (not National) power transformer. Reuse the original power transformer top cover and bottom skirt to preserve original appearance unless original pieces are damaged. Some Airport receiver transformers will have a vented top cover consisting of either four large holes on top of the cover or several small holes on two sides of the cover. When looking at the receiver chassis from the front, the NC diamond on the transformer top cover should be right-side-up. There was a 25 cycle version that can be identified by its height (it's much taller) and also it's ink-stamped "NC-100-25" on the bottom side of the core.||
Test Before Installing
- Original power transformers harvested from part sets should be
electrically checked before installation. First, check the DCR of each of the
windings. Also, check to make sure that the windings are isolated from
each other and from the metal laminations. Then apply AC voltage to the
primary and using an AC volt meter measure the potential at each of the
windings. The voltages will read slightly high since they are not
loaded. Leave the transformer primary connected to AC for about ten
minutes and check that the transformer isn't getting hot. It should be
cool to the touch with no load on the secondaries. If the transformer passes these tests it will more
than likely function fine in the receiver.
Expected (approximate - unloaded) secondary AC voltages with 120vac primary input
HV = 270vac - CT - 270vac (540vac across ends) FIL = 6.8vac (across ends) RECT = 5.5vac
Remember when measuring the B+ voltage after the new transformer is installed in the receiver that National specified +/-15% on almost all of the voltages. This means that if the plate on an audio tube is supposed to measure +230vdc it could measure as low as +196vdc and still be within spec. Most voltages were measured from terminal to chassis. Most receiver manuals have expected voltage charts included. Don't change the primary tap to "LO" to increase the B+. This will also increase your 6.3vac to well-over 7.0vac and the 5.0vac will be near 6.0vac.
|NC-100, NC-100X, NC-101X -
2-wire primary, CT HV secondary, CT 6.3vac, 2-terminal 5.0vac (marked
NC-100A, NC-100XA, NC-101XA - 2-wire primary, CT HV sec., CT 6.3vac, 2-term 5.0vac (marked NC-100)
Airway Receivers - Tapped 3-wire primary, CT HV sec., CT 6.3vac, 2-term 5.0vac, shield terminal (marked NC-100-F)
NC-100ASD - Tapped 3-wire primary, CT HV sec, CT 6.3vac, 2-term 5.0vac, shield terminal (60 cycle version marked NC-100-F, 25 cycle version marked NC-100-25)
RAO-2, 6 - Tapped 3-wire primary, CT HV sec, CT 6.3vac, 2-term 5.0vac, shield terminal
RAO-7, 9 - Tapped 3-wire
primary, CT HV sec (shown as 240vac-0-240vac on schematic although this
may be indicated as measured with transformer providing full load,) CT 6.3vac, 2-term 5.0vac, shield terminal
(primary and HV wires have different wire insulation material & color)
NC-200 - 2-wire primary, CT HV sec, CT 6.3vac, 2 term 5.0vac (early marked NC-100, later marked NC-200)
NC-240, NC-240D - 2-wire primary, CT HV sec, CT 6.3vac, 2 term 5.0vac (marked NC-200)
Restoration Hints and Suggestions
- There are many versions of the NC-100 Series receivers and not all versions
are covered by specific manuals. This is especially true when
engineering upgrades are incorporated into production and aren't
documented until a later version of the manual. Sometimes it will be
necessary to refer to a later version of the manual for an early
production receiver because its actual production date happened to be
post-early manual and the engineering upgrades are shown in the later
manual. Luckily, several versions of the manuals are available on the BAMA website. NOTE: The BAMA posted schematic of the early
version NC-100A has an error in the S-meter switch, meter adjustment pot
and the screen voltage divider section of the schematic. It's common to
find errors in the manuals. Even the original NC-100 manual's schematic
doesn't show that the Mixer coil primary is actually a plug-in coil.
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.
Required - Most detailed restorations are going to
require significant disassembly of the receiver. It is recommended that
the cabinet be removed just to check for corrosion which is commonly
found between the chassis and the inside of the cabinet. Any chassis
work is much easier with the cabinet removed. Most chassis work can be
performed with the catacomb left installed. Just move it to one side for
work on the receiver section and then move the catacomb to the other
side for work on the power supply section. If you have to clean the coil
pin area it's actually easier to remove each coil assembly, clean and
then reinstall. Although usually not necessary, you might need to remove the Tuning Condenser, it depends on how
the receiver was stored and how much dirt and grease have collected
under the Tuning Condenser where the tray is that is used to mount the
coil contacts. Watch the glass dial cover used on the pre-WWII "A" versions
as this is glass and can be cracked easily. Plexiglas was used in the
WWII RAO versions.
Importance of "Parts Sets" - It's possible to do any restoration without having a "parts set" as an immediate source of replacement components. However, the restoration is much easier and takes far less time when there's a "parts set" to supply needed replacement parts and to use as a reference when a question comes up regarding component placement or originality. Unfortunately, the expense of shipping heavy communication receivers has made it almost "cost prohibitive" to obtain a receiver just as a "parts set" anymore. Despite the cost, it will probably be worth the expense though since your restoration will be able to use the best parts from either receiver.
Restoring the NC-100ASD
This restoration is a combination of "museum quality" electronic restoration and some cosmetic rejuvenation. It involved using two NC-100ASD receivers to end-up with one receiver that functioned correctly with a nice, original appearance.
|Sometimes shunned by collectors because of its "often-seen"
status and military
origins, I've always had a "soft spot" for the National NC-100ASD.
Probably because my first exposure to an "ASD" was at a neighboring
teenager's ham shack. That was WA6LHL, Dennis Antilla, and his station
consisted of an NC-100ASD and an EICO 720 with Heathkit VFO and homebrew
modulator. Dennis was an early "Elmer" of mine although he
joined the Navy before I got my license (I was 14 at the time.)
I've owned a few NC-100ASD receivers since then but none of them were in very good condition. Most were actually "junkers." However, recently I saw an ASD for sale on Craig's List and since it was only 15 miles away in Carson City, I went over, inspected it and ended up purchasing it. While the chassis was in excellent cosmetic condition, the cabinet had a non-original hole to mount an ON-OFF switch for the dial lamps (hmmm, why was this so popular?) There was also a mod to replace the 6F8 tube with a 6SN7. Luckily, no extra holes were needed for that mod.
The Other"Parts Set"ASD - Many years ago, while I had the Western Historic Radio Museum open in Virginia City, I had a fellow stop by with a small pickup full of radios. He wanted to sell them for his neighbor. Among the radios was an NC-100ASD. This "radio-selling guy" turned out to be the famous Bob Wilkins. As a teenager I had watched Bob Wilkins host "Creature Features" on KTVU out of the SF Bay Area. When Wilkins retired he moved to Reno and was a neighbor of my old friend W7MS in Reno.
It turned out that the NC-100ASD I got from Bob Wilkins had spent a lot of time in a basement under a leaking water pipe. The rust had eaten through the top lid hinges and the chassis was a total mess. Amazingly, this NC-100ASD did work. It actually worked pretty well but the cosmetics were terrible and I ended up storing the receiver for some future need - like a "parts set."
It was lucky that I did save the "Wilkins' ASD" because it could supply the front panel-sides cabinet piece and replace the same piece in the latest ASD with the non-original hole. A combination of cabinet parts would result in the best condition original ASD cabinet. I was to also going to find out as the restoration proceeded that I was going to need some very important parts that only this "parts set" could supply.
Original Paper-Wax Capacitors - I wanted to actually use
this NC-100ASD set-up with an ART-13A transmitter for a vintage WWII
station. This required that the receiver be reliable since it would be
in operation for extended period of time. There really isn't an
option when the receiver has paper-wax capacitors and long-term
operation is desired. You have to replace the capacitors. However, I
wanted to also preserve the "under the chassis" original appearance as
much as possible. This leaves "restuffing the caps" as the only option.
In the past, I've melted out the old cap, hot-melt glued the new capacitor in the old paper shell and then filled the ends with brown sealing wax. I left the paper shell clean and somewhat "wax-free." This looked really nice but not authentic since the original capacitors were dipped in bee's wax. I decided this time to try a slightly different approach.
I still removed the capacitors "one at a time" to restuff. This avoids confusion or any difficulty remembering where the caps are to be installed and their proper orientation. I use a heat gun to apply enough heat to remove the old cap from the paper shell. I melt the excess old wax into a metal wastepaper basket (it is a messy process.) I hold one lead of the old capacitor with needle nose pliers and when everything is hot I pull the paper shell off with a paper towel acting as a "heat protector" for my fingers. I wipe the excess wax off of the shell and let it cool for about one minute. Next, the new cap is inserted and held in place with hot melt glue. Leave about 0.250" space on each end. Let the hot melt cool for a few minutes. Now this was where I changed what I had been doing. This time I end-filled the caps with bee's wax. I just use a soldering iron to melt the wax and fill the ends a "drop at a time." When the bee's wax is cool enough, I then melted more bee's wax over the outside of shell. When this was almost cool, I wiped the excess wax off with my fingers. The rebuilt cap was then installed into the circuit. This procedure was followed until all 26 paper caps were rebuilt.
Shown in the photo to the right is the end result. It's difficult to tell that the paper capacitors have been rebuilt. The coating of bee's wax is the final touch that really imparts an authentic appearance. In fact, at first glance it was so difficult to tell the caps that had been rebuilt from those remaining, I had to place a small square of white tape to each rebuilt cap. The white tape pieces (~ 0.250" squares) were removed after the job was finished.
photo above: The completed capacitor rebuilding is difficult to distinguish from original. On the NC-100ASD, the filter capacitors are oil-filled paper capacitors. These are checked for value at voltage and used if they check-out okay. The electrolytic in the upper left corner was also restuffed-rebuilt. This requires sawing the can apart (mark it for proper reassembly) then installing a new electrolytic. The can is reassembled with a paper sleeve inside to hold epoxy to insure that the bond has a lot of glue. Note the "600-900-1200" written on the catacomb. This was a portent to future surprises when aligning this receiver.
Restringing the Dial Pointer on the NC-100A Receivers -
The action of the articulated dial pointer is controlled by a dial cord
that is tied to the band change shaft, then is routed up to a dual
pulley, then strung around the tuning shaft, routed to the dial pointer
mechanism pulley, then back over the tuning shaft and finally routed
down to an anchor tie point on the receiver chassis. A single dial
string is required that is around 20" long and is about 0.025" inches in
diameter (actually most dial cord is now measured metrically, so
slightly less than 1mm.) The length is much longer than necessary but
gives you enough for the knots required at each end of the run (and then
Note in the photo to the right that the dial cord wraps over the shaft in a clockwise direction when viewed as shown. You have to tie a knot in one end of the dial cord and then route it thru the hole in the shaft making sure that you end up with the cord going over the shaft as shown. Be sure that the coil box is set to the lowest frequency band (box all the way to the right as viewed from the bottom-front.) Route the dial cord thru the hole in the chassis as shown.
Next, look at the photo below-left and note that the dial cord exits the chassis hole and is routed up to the large sheeve of the dual pulley. Be sure you wrap the dial cord counter-clockwise (viewed as shown.) You will have to wrap the cord at minimum one and a half turns. You will see that there are two holes in the dual pulley that allow you to thread the cord thru the upper hole to a lower hole to allow the cord to wrap around the small sheeve of the dual pulley. The cord is only going to wrap over the small sheeve and then be routed to the top of tuning dial shaft. Looking closely at both bottom photos, you can see that the cord wraps over the tuning shaft and down to the small pulley that works the dial pointer lifter. Loop the cord around this small pulley and back up to the tuning shaft. Route the dial cord over the tuning shaft and down to the tie point that is mounted to the chassis.
photo above: Note how the dial cord is routed in this photo. The dial cord use here is "waxed twine" that is 1mm in diameter. Don't use any larger diameter than 1mm. Slightly smaller diameter would be easier to route thru the two holes in the pulley.
|Before you knot the dial cord test the operation of the
dial cord by changing bands and noting that the articulated pointer
moves as the band changing shaft is rotated. You'll have to hold the
dial cord at the tie point with your fingers while testing the
Where the dial pointer "points" is dependent on the mounting of the dial scale and the tension of the dial cord which is dependent on the last knot at the tie point. You can string the dial cord with the dial scale removed but final setting of the pointer position will require that the dial scale be mounted (at least temporarily.) Test how the pointer operates and if it points to the correct bands on the dial scale. When satisfied, tie the final knot at the tie point. Be sure to knot at least three knots and glue the knots with Duco cement.
photo above: Note the routing of the dial cord thru the two small holes of the pulley by observing the cord "loop" at the back of the pulley. Also, the missing paint on the chassis was touched-up after the photo was taken. See chassis photo below.
- The RF Gain control is mounted in a bracket that requires the
control to have an extension. You should be able to mount the pot
with the front panel/sides part of the cabinet somewhat in position.
Once you have the RF Gain pot mounted and its extension installed,
then you can slide the cabinet into position. Next, install the
remaining pots and switches with their appropriate hardware. Mount
the S-meter using the S-meter bracket. This must be done before
mounting the dial panel because the bracket screws are hidden when
the dial panel is installed. With the S-meter mounted, then install
the dial panel with its glass dial cover in place. Be sure you have
a gasket around the glass perimeter to protect the edges from
chipping. I use a 0.25" wide piece of friction tape to make a gasket
and since its tape it stays in place while the glass and panel are
being installed. Be careful when tightening the dial panel screws
and be sure the glass is not binding or flexing anywhere. Proceed
with the tightening if no binding is encountered. Next, install the
mounting hardware for the controls that mount to the dial panel. Be
sure to have installed washers on the controls to act as spacers
between the cabinet in the dial panel. Only one washer per control
is needed and they should be around .030" thick. The RF Gain control
just exits the panel with no other hardware. Be sure to install the
cabinet to chassis spacer which is a rectangular piece about 0.190"
thick that is held in place by the two lower front panel screws.
Install all of the other cabinet screws, washers and nuts and
tighten. Be sure to install all screws. Position the rear panel/top
lid piece of the cabinet in mount with the appropriate screws. All
of the cabinet screws that hold the rear panel/ top lid piece in
place are short sheet metal screws. Install all of the knobs and
bottom panel. This completes reassembly.
photo right: This is the chassis of sn: 194 after cleaning. Note the older style filter capacitors. The tops of the filter capacitors were eventually painted silver so they didn't look so much like derelicts.
Testing Reveals Other Problems - The NC-100ASD powered-up with no issues and
seemed to be fairly well in alignment. Most stations were close to
their proper frequencies on the dial. I then switched over to Band
200kc to 400kc MW band, and heard AM BC stations - that can't be
right! I tuned across the dial and found that 200kc to 400kc
actually tuned .5 to 1.2mc. I removed the catacomb top cover and
inspected the coils and found that an "H" set was
installed but had been modified to cover the AM BC band by removing
the adjustable padding capacitor. Probably, a former owner wanted to
listen to AM BC and trimmed the LO coil assembly until the AM BC was
tuned in. Although "H" is the correct coil set, ASDs were also
fitted with "G" coil sets that also cover 200kc to 400kc. Fortunately, I had the
"Wilkins" ASD parts set that could provide an original set of "G" coils. When removing or
installing the coil assemblies just be sure that the pins are
disengaged. Just switch the catacomb "between bands" and then the
pins on all coil sets are disengaged. Once the swap was made, I
checked the mechanical alignment by switching bands to see how the
pins engaged. You just about can't get the coil assemblies mounted
any other way than correct with proper mechanical alignment.
Receiver operation on
200kc to 400kc tested "as expected" with the "G" coils installed.
A second problem involved audio distortion on strong signals. Very strong signals would "cut-out" as if the receiver was in MVC and was being "over-loaded." The problem turned out to be a Russian metal octal "look-alike" 6K7 in the 2nd IF amp. The tube tested like a 6K7 and looked (at a quick glance) just like a 6K7. So I installed it into the 2nd IF position. The distortion problem was found by tube substitution. Close examination of the Russian tube showed that it was a "6X7" with the "X" actually being the Russian letter that looks like two "K" letters back-to-back. The nomenclature was all engraved and black so close examination was required to read everything (and that showed that it was all in Russian.) Anyway, a standard 6K7 cured the problem. >>>
|>>> A third problem involved strong images on band
B. Normally, with a single TRF stage, one wouldn't expect to hear
images until around 15mc. However, 10mc WWV was tuned at both 10mc
and 9.1mc at about the same strength. The image on 15mc WWV was
barely heard which seemed to indicate something was wrong with the B
coil sets. The problem appeared to be caused by a defective Mixer
coil assembly which had a damaged "ticker" coil - probably done when
trying to adjust the inductance loop. Luckily, the parts set
provided a complete set of "B" coils. After installation of the "B"
coils, alignment of that band was necessary. After alignment the
10mc WWV image was barely perceptible at 9.1mc.
Alignment - The IF section is straight forward and is aligned to 455kc. I just inject an RF signal at the Mixer grid that is modulated with 400 hz and use the S-meter as an indicator. The receiver will have to be operated in AVC with the RF gain at 10. Adjust the gain by using the RF signal generator's attenuator to have the S-meter reading about mid-scale. Adjust each IF transformer for maximum indication on the S-meter. Since there's no Crystal Filter, the IF is not ultra-critical but try to adjust as close to 455kc as possible since this will affect RF tracking somewhat.
RF alignment uses an air-trimmer for the high end of the band and the famous National "coil loop" in each of the inductors for adjustment of the low end of the band. I use a plastic tool to move the coil loop to adjust the low end and adjust the air trimmer for the high end until tracking is correct. With the catacomb receivers you can adjust the loop with the receiver in operation (unlike the HRO.) This means you can observe the S-meter (or AF Output Meter for non-A receivers) and adjust the low end inductance exactly (much easier than the HRO.) The ASD has a padder capacitor for low end adjustment on the 200kc to 400kc band. Once you have good tracking adjusting the LO section then adjust the Mixer and RF Amp sections at the high end of the range for maximum indication on the S-meter. You can check the low-end Mixer and RF Amp adjustment by moving the coil loop while observing the S-meter. Normally, these seem to be set correctly and the movement of the loop only confirms that they were in adjustment. This completes alignment.
I've set up sn:194 with one of my ART-13A transmitters as a vintage WWII
military station. Granted, the ART-13 is an "aircraft" transmitter
and wouldn't have been paired with a "ground" receiver like the ASD,
but still, they are of the same vintage and go nicely together. In
such a use, the ASD has to be able to receive reasonably weak
signals through various types of QRM and propagation conditions.
Although almost any receiver can provide decent reception when the
operating frequency is in the 3.8 to 4.0 mc part of the spectrum
when conditions are good, a usable receiver will be able to cope
with all of the different types of reception problems and provide the user with Q-5
>>> The ASD is easy to set up as a station receiver. The remote stand-by uses a screw terminal strip on the rear chassis. Having the matching speaker helps the audio since the 500Z ohm matching transformer is mounted on the speaker itself. The eight inch Jensen does sound pretty good and with the 8kc bandwidth the signals won't have too many highs and thus will have a mellow sound. The ASD doesn't "roll-off" the bass like the USN RAO receivers do. The ASD audio is natural sounding with respectable bass response.
|>>> The ASD does have its limitations. The tuning rate is very
fast as all general coverage NC-100s are. It makes CW and SSB
difficult to tune until you develop the "micrometer finger" touch to
your tuning ability. You can use the BFO to fine tune SSB or CW
signals. Lack of a Crystal Filter leaves you with only the receiver's IF
bandwidth to combat QRM. Bandwidth is around 8kc at 20db down.
That's fairly selective but might require some fine tuning to either
sideband on AM if QRM is encountered.
Also, one might find that going to MVC and reducing the RF Gain will
help with QRM or heavy static. The Limiter normally works best in
the AM mode with the receiver in AVC with the RF Gain at maximum.
After using the NC-100ASD as the station receiver with the ART-13A for several months I have to say that I'm impressed with the receiver's performance on 75M. The audio is very natural sounding and selectivity is more than adequate for the normal adjacent QRM. In fact, our Vintage Military Radio Net has been plagued for the last couple of months with a pair of SSB stations "parked" 2kc up from our net frequency. I have to say that the NC-100ASD does a good job handling the adjacent frequency QRM but I do have to tune down slightly (~ 2kc) to cope with the interference. The fact that a WWII-era receiver with no crystal filter can effectively null QRM is impressive. However, I was really impressed a few weeks ago when KA7NGT from Mineral, Washington (using an ART-13 xmtr) checked into our Vintage Military Net and his signal was Q-5 on the NC-100ASD. So, I'd have to rate the NC-100ASD sensitivity on 75M as excellent and it ability to cope with QRM, impressive. Naturally, as the operating frequency is increased the difficulties encountered will also increase but I'm sure any operations on 160M, 80M or 40M would present no problems.
The ASD is relatively small and the weight is around 60 lbs. It's easy to find a place for it and it's not too heavy to move it there. It's a nice looking receiver that's easy to use and has pleasant, natural sounding audio on AM.
Restoration of the Silver Anniversary NC-200
This project started out as a "return to original" condition because of a "hamster-job rework" performed by a former owner. I hadn't really decided whether to leave the receiver in good cosmetic condition doing duty as a "shelf-queen," or whether to do a complete "museum quality" restoration job so the receiver would also be a reliable performer. Curious temptation for a "quickie power-up test" resulted in finding a defective power transformer and this was the deciding factor to go ahead with the full restoration. Also covered in this write-up is how to replace the plastic dial cover with a new repro dial cover. This method can also be used on the NC-240 receivers.
|I acquired this Silver Anniversary NC-200 and matching
speaker as a trade that involved a repaired and calibrated TV-7A tube
tester. The receiver was mostly in excellent, original condition as far
as the cosmetics were concerned. The can electrolytic capacitors had be
rebuilt by a former owner in a most unusual way using orange plastic
pill bottles on the interior holding the new capacitors. The tops of
both cans were missing and the metal back-covers from potentiometers
were installed as tops instead. Under the chassis, three capacitors had
been replace with modern "orange drops." Everything else was still
original. The receiver had never been "powered-up" by the former owner.
My initial goal was to just re-do the electrolytic capacitors to appear original, re-install original Sprague paper-wax capacitors where needed and to do a cleaning. I thought I'd just have the Silver Anniversary as a "shelf-queen" because,...who needed another restored "moving coil" receiver anyway?
Rebuilding the Electrolytic Cans - This required removal of the plastic pill bottles and new electrolytic caps and accurately trimming the cut that had removed the tops. I then looked in the junk box for some defective can electrolytics (easy to do) and accurately cut the tops off of two of the cans. Since both the original cans and the new tops were the same outside diameter, as long as the cuts were accurately accomplished the seam joining the two pieces together should be minimal. I installed a dual 10uf 450wvdc in the can that originally was a dual 8uf and I installed a 40uf 300wvdc in the can that originally was a 40uf at 200wvdc. The new tops were then mounted using epoxy with a cardboard tube inside (to hold the epoxy.) The finished caps looked nearly perfect and, other than the new caps inside and the new tops, they were essentially the original cans for the receiver.
|Cleaning - The only way to really clean the chassis is to remove the cabinet. The bottom is mounted with several sheet metal screws, including two on each side of the cabinet that must be removed. Then the back and lid assembly is mounted using sheet metal screws. The knobs and control mounting nuts have to be removed to dismount the front and sides assembly. Four 10-32 machine screws and nuts are used to also secure the front panel to the chassis which also must be removed. The front and sides assembly then pulls forward and up to dismount. The chassis is now easy to access for cleaning. Like many older receivers, this one had been somewhere exposed to greasy or oily dirt. WD-40 removes this kind of grime easily. You'll usually find this dirt is an excellent preservative and the chassis paint will clean up nicely. Once the grease is removed, I then clean the chassis with Glass Plus to remove the WD-40 residue. Don't scrub the aluminum shields as this will tend to polish them. They should be matte finish. Lightly clean with WD-40 followed by Glass Plus. Use a soft paint brush to lightly remove the dirt then dab dry. This will preserve the matte finish and clean the aluminum surface. Most pre-war National equipment will have acceptance markings on the IF cans that consists of orange grease pencil "OK" or maybe initials. These orange grease pencil markings shouldn't be removed during cleaning.||Cleaning the Wrinkle Finish - Use Glass Plus and a soft brass brush (sometimes sold as brushes for suede shoes - they are kind of hard to find.) Spray a heavy coat of Glass Plus on the wrinkle surface and then use a circular motion with the brass brush for several minutes. Don't scrub hard but use gentle to moderate pressure in a circular motion to clean the dirt out of the convolutions of the wrinkle paint. The original wrinkle paint is very tough and can take this treatment. Wipe off the dirty Glass Plus with dry paper towels. You'll have some dirt and a little color showing on the paper towels. Do one more application. The paper towel should be pretty light on dirt and color this time. Move on the the next surface and use the same procedure. When all sides have been cleaned, let the wrinkle finish piece dry completely. Then apply 10W machine oil with a clean cotton cloth and rub into the wrinkle finish. You don't need a lot of oil since very little covers a fairly large area. Wipe off the excess with a clean, dry cloth. The oil works much better than Armor All and doesn't leave the surface feeling "slippery." Also, the oil provides a nice luster and doesn't have the "glossy shine" that Armor All has.|
||Cleaning Knobs and the
Meter Case - Soak in warm soapy water for about 30
minutes. Scrub with a soft tooth brush to remove the dirt and then dry.
Now polish each knob with Wenol's or Semichrome. The meter case can be
cleaned with Glass Plus and then polished with Wenol's. With this
receiver the S-meter scale was extremely dark from having a high-current
lamp installed for illumination. I swapped scales from another National
(Marion Electric) meter that was only slightly darkened. Originally,
these scales were light yellow and the red nomenclature was bright and
easy to read but UV light and high heat illumination darkens the plastic
and fades the red printing.
Replace Power Cord - I just couldn't leave a Kawasaki power cord installed on this receiver. I didn't even know Kawasaki made electrical parts. I looked around in the junk pile to find a vintage radio that still had it's original power cord. I found an old junk broadcast radio that had its original power cord and molded plug (and that was still pliable.) When installed, this cord looked original and was in good useable condition.
Repair NC Diamond - I was really lucky that the former owner had kept the Silver Anniversary NC diamond after it had become detached from its mount. To remove the mount required that the cabinet be apart (which it was, for cleaning.) I used a jeweler's file to clean the back of the NC diamond and the top of the screw mount. I then used high-tensile strength epoxy to "glue" the diamond to the mount. I think originally the diamond was soldered to the mount and then the cloisenne NC and the plating was done. I wasn't sure that the heat of a solder mounting wouldn't damage the cloisenne so I opt'd for the epoxy which worked fine.
Re-doing the Gold Tone - This finish was almost totally worn off from years of use. It wasn't an electroplating but rather was a gold tone lacquer that wasn't very durable. I initially thought the gold tone was smoker's residue but tobacco deposits are water soluble and this toned lacquer, of course, wasn't. My "tip off" to the true nature of the gold tone came when I removed the large brown bakelite tuning knob from its skirt. There, protected by the knob, was the original color finish - a light gold color (about the color of beer.) I was lucky to have a special yellow toning nitrocellulose lacquer that had been mixed for me several years ago for duplicating amber lacquer finishes. I had found that diluted a little, this lacquer also duplicated MFP coatings (without the smell.) I had also used this lacquer to duplicate the gold finish on HRO crystal filter boxes, so I knew this would work fine for the gold tone necessary for the Silver Anniversary. Mixed about 3 to 1 with thinner and lightly sprayed, the yellow lacquer duplicated the color found under the tuning knob exactly. I also shot all of the nomenclature plates since the two Crystal Filter plates still had their gold tone present indicating that all of these plates were also gold tone.
|Functional Test Finds Bad Power Transformer - The Silver Anniversary NC-200 was reassembled and then photographed for this website. I thought that was going to be the extent of the work performed. I had asked the former owner if he had ever powered up the receiver. He said that he hadn't but that the fellow he got it from had said it worked. Hmmm. I couldn't resist temptation. Since I had the original matching speaker, I connected up everything and carefully applied AC power. I watched the 5Y3 rectifier tube to see if any blue glow would indicate a severe overload but everything looked fine. In about 25 seconds, receiver background noise came over the loudspeaker. I connected an antenna and signals were coming in strong. I tuned around on all bands and everything was working fairly well. After about ten minutes, I though I better feel the power transformer. It was very hot. I felt the chokes and they were still cold. Hmmm. This indicated that there wasn't a high current flow thru the chokes as would be the case with a high leakage capacitor load on the B+. I turned off the receiver. The next day, I pulled all the tubes along with the three lamps and again powered up the receiver. After about ten minutes the transformer was again hot and beginning to smell. A sure indication that the power transformer had a problem. The removal of the rectifier tube removes the entire HV winding and the 5vac winding from that load on the transformer. With the remaining tubes and the lamps removed (the 6.3vac load) there was "no load" on the transformer. I also checked the HV winding and found that it across the windings was over 700vac (it should have been around 550vac.) This indicated that the turns ratio had changed and was probably due to some shorted turns in the primary winding which would also cause the over-heating problem.|
Power Transformer - I have several "moving coil" receiver
spare transformers so I just selected what was the closest which
happened to be from an NC-100ASD receiver. Only a slight modification is
needed since the "ASD" uses a three-wire primary and the NC-200 uses a
two-wire primary. National assemblers would cut the third wire when
required since all of the power transformers originally came with the
three-wire primary. All other connections are the same. These
transformers are usually ink-stamped "NC-100-F" on the bottom part of
the core. Since almost all of the "Moving Coil" receivers have the same
power transformer and same basic power supply, if the wires are
carefully unsoldered from the "part set", then the wires should be long
enough when installed in the new receiver. This was the case with the
ASD to NC-200 swap. I couldn't use the top cover and the bottom skirt
from the replacement transformer since they were rusty and pitted. I
replaced these parts with the cover and skirt from the bad NC-200
Since the NC-100ASD uses a capacitor input filter and the NC-200 uses a choke input filter the resulting B+ will be somewhat lower. The power transformer HV windings are the same with 270vac-0-270vac being the expected voltage. So, the NC-100ASD has the B+ at about +260vdc on the audio output screen while the NC-200 runs about +200vdc. The different voltages are due to the type of B+ filters used in each receiver. Also, National spec'd the expected voltages at +/-15% which means that if a voltage should measure +230vdc it could measure +196vdc and still be within spec.
Original Capacitors - As long as I had gone this far, I
decided to "re-cap" the receiver too. Since this was a fairly rare set,
I opt'd to "restuff" the original paper-wax caps so the underneath of
the chassis maintained its original appearance. In the past, I'd usually
rebuild one capacitor at a time. Since I wasn't going to be able to do
the wax-work at the bench, this time I had to come up with a way to do
several capacitors at a time. This involved making a rough sketch of the
capacitor locations first (with values and orientations noted.) I then
would clip-out about seven or so capacitors and take them over to the
rebuilding bench. This bench had the Steelcase waist paper basket, the
heat gun, the hot-melt glue gun, the bee's wax and the soldering iron.
This bench was in an area that allowed for better ventilation since the
waxing can smell a little.
The Steelcase basket is used in melting the original caps using the heat gun. This is a messy job with a lot of excess wax blowing off of the old capacitor shell, so the metal waist paper basket works great for this. The old cap is heated, then the original insides are pulled out and dropped into the basket. The shell is wiped down with a paper towel to clean off the excess old wax. All seven (or however many) caps have this step performed first. Next, the new capacitors are inserted into the old shells and secured with hot-melt glue. Although metalized-film capacitors can be oriented in either direction, I still orient all the new capacitors in the same direction as I'm rebuilding the old caps. My method is, with the capacitor nomenclature right-side up, the left side of the cap is the outer wrap. It really doesn't matter but that's how I do it. Next, the cap ends are waxed with bee's wax and finally the other shell is coated thinly with bee's wax. The seven (or so) caps are then taken to the receiver bench and installed. Then the next seven are removed and the process started again until all the caps have been rebuilt. The outer coating of bee's wax is the "key" to having the rebuilt capacitors look absolutely original. The photo to the left shows the NC-200 chassis after the capacitor rebuild. It's unfortunate that a former owner removed the original 10uf chassis mounted 1st AF amp cathode bypass capacitor. Without the original capacitor I had to leave the modern electrolytic in the circuit until I can find a parts set that can donate the original cathode bypass capacitor.
- I tested all of the tubes and found several that were weak but none
that were defective. I replaced the weak tubes making sure that the two
metal 6V6 tubes were a close match on their respective Gm. National
notes in the manual that if you don't use the metal 6V6 tubes and have
to use the glass version that "Goat" tube shields should be installed
since the glass audio tubes could oscillate.
As I was installing the now "tested" tubes, I dropped one of the 6SK7 metal tubes. Its fall to the floor was broken by it first hitting my foot, so I thought "no problem." I didn't bother retesting the tube.
It came as a surprise that when powered up, the NC-200 was silent. I touched the grid cap of the 6F8 which is the first AF amplified and got a loud hum - okay, audio was fine. I touched the IF amp grid (6K7) and also got some hum. I touched the grid cap of the Mixer tube (6K8) and started to pickup a strong AM BC station. Hmmm. I pulled the 6SK7 RF amp tube (the one I dropped) and the receiver started to weakly pickup the AM BC station. I installed another 6SK7 RF amp tube and soon the NC-200 started playing strong. Now,...I've dropped lots of metal octals on the floor before but I'd never had one go bad from the fall. This 6SK7 didn't even hit the floor directly but still ended up defective,...weird.
|Installing the Plastic Dial
Cover - The dial cover on this NC-200 had "yellowed"
quite a bit. I polished it with Wenol's which did lighten it somewhat
but it still was difficult to read the red scales on the dial itself.
Years ago, I used to order repro plastic dial covers but they all came
with flat fronts, not convex fronts like the originals. As a result, I
quit ordering repro plastic dial covers and just tolerated the cracked,
split or yellowed originals. It had been so long since I had ordered a
repro dial cover, I thought I'd try out Mark Palmquist at dialcovers.com.
The NC-200 dial cover from Mark was excellent with the original type
convex front and a perfect fit. If you need a plastic dial cover, try
dialcovers.com. Mark has an online catalog but can also do custom dial
To install the new dial cover requires the upper chrome strip be loose. From inside the cabinet, push the four wire retainers thru their mounting holes. This will allow the chrome strip to be pulled out from the front panel. You don't have to totally dismount the strip. Dismount each dial cover side retainer strips that are held in place with two screws on each strip. Now pull the old dial cover up and behind the upper chrome strip. The new dial cover will have a protective plastic cover that should be left on the front but remove the back side protective plastic. Now insert the new dial cover behind the chrome strip and move it into place. You'll have to pull out the bottom chrome strip a little to get the edge of the new dial cover to slip behind it. Move the dial cover around until it is squarely in place and even on each side. Now mark the mounting holes with a fine point Sharpie. Remove the dial cover and, using a pin-vise, drill the mounting holes. Start with a .060" drill and finish the hole at no larger than .125" diameter. Now, reinsert the dial cover back in place in the same manner. When the four holes are aligned, push the upper chrome strip back in place. Be sure the protective plastic is slightly removed so it's not under the chrome strip. Entirely remove the protective plastic. Then mount each side retainer strip with the mounting screws. Do not over-tighten the four side retainer strip screws as this can crack the plastic of the dial cover. This completes mounting the new dial cover. The same method can also be used for the NC-240 receivers.
|Test-run Reveals Other Problems - I ran the NC-200 on all bands over a period of a few days to "ring-out" any problems. I was then planning to do a full IF-RF alignment. After several listening sessions with no problems I started the alignment. When I got to the RF tracking on Band E, the receiver stopped operating and after a minute a small curl of smoke was noticed behind the catacomb. I shut down the receiver and began checking. It turned out that I had a direct short from B+ to chassis at the second IF transformer. I disconnected the four wires that connect the IF transformer into the circuit. I still had the short from the IF transformer B+ wire to chassis indicating the problem was inside the IF can. I removed the IF can from the chassis and checked but I still had the shorted B+ to the can. I removed the transformer from the can and, of course, then the short was gone. The problem appeared to be one of the rotor contacts terminals on the plate side trimmer was longer than normal and slightly protruded beyond the ceramic mount. Also, the aluminum mount for the ceramic trimmer mount was slightly bent causing the can to slightly distort when the mounting screws were tight and this caused the rotor contact terminal to press against the inside of the can. I put a slight bend on the end of each of the rotor contact terminals and straightened the aluminum mount. I then reassembled the can and with the mounting screws tight there was no B+ to can short. I then reinstalled the IF transformer back into the receiver. I checked the IF plate load resistor that apparently smoked a bit but it was still well within spec for value. As to why this type of mechanical short would develop after years in a non-contacting position is a mystery but it does seem to happen once in a while.||
IF Alignment - The IF was pretty far off. It seemed to
be aligned to 445kc. I tested the crystal in the crystal filter and
found it to be 453.6kc which indicates that the IF was probably 455kc.
Some of the very early NC-200 receivers might have used 456kc but the
frequency was changed very early in production. This receiver is a
little over 500 units into production and has a 455kc IF. However,
that's just the design IF, you actually use the crystal frequency for
alignment. After the IF adjustment, the BFO control should be set to "0"
and the BFO trimmer adjusted for zero beat with the IF.
RF Tracking - The RF Tracking alignment has a lot of adjustments but the trimmers and padders are logically placed to avoid confusion. If a metal screw driver is used for the adjustments there will be some interaction because of the steel blade. You can use a fiber tool but often the trimmers require a higher amount of torque than the fiber tool can provide without breaking. If this is the case, you can compensate for the steel blade by slightly "over-shooting" the peak and see if the adjustment is correct when the screwdriver is removed. The higher the frequency, the more interaction will be encountered. The General Coverage must be performed first, then the Band Spread. General Coverage uses coil loops for the low-end adjustments (variable L) requiring a fiber tool to move the loop around for adjustment. The two lowest frequency bands use padder variable capacitors for adjusting the low-end. Band Spread uses adjustable trimmers for high-end and adjustable padders for the low-end. When adjusting Band A and the 10M band spread, be sure to check that the image is 915kc below the tuned frequency. This assures that the LO is correctly adjusted to above the tuned frequency. The only problem I encountered was with the dial pointer. After I had finished the two low frequency bands, I noticed that the pointer seemed bent (to the right.) I had to remove the plastic dial cover, straighten the pointer, replace the dial cover and then realign the two low frequency bands again.
|Performance - The NC-200 is like having a NC-100XA receiver and a NC-101XA receiver in one package. You can use it as a ham bands-only receiver where it has great dial accuracy across the entire band. Or, just pull out the tuning knob, turn and select a general coverage band, push the knob back in and tune in your favorite SW BC or utility stations. The dial accuracy is good on general coverage but the ham band spread is, of course, more accurate since less frequency span is covered. Sensitivity is comparable to any of the later moving coil receivers and the crystal filter works quite well. I'm lucky to have the matching speaker for this Silver Anniversary NC-200 and the sound reproduction is impressive from the 10" PM speaker. The audio circuit uses a phase inverter rather than an interstage transformer and the reproduction seems to benefit from that change. Lots of bass from the push-pull 6V6 tubes. Great audio when listening to AM BC or to some of the better SW BC stations. Some ham AM stations also sound fabulous. The band indicator flags are very useful and help you find the desired band with this ten-band receiver. My only complaint is that the dial is practically unreadable if you want to just use the dial lamps for illumination (with #44 lamps installed.) The black scales are difficult to read and the red scales are virtually impossible to read. A desk lamp or ceiling lamp is really necessary to adequately read the dial unless you're listening during the day with the window curtains open. A minor complaint. I've set this Silver Anniversary NC-200 up with my National RCR Airways receiver since they are similar receivers, the RCR being a NC-240CS.|
RCE SN: 302 - Airport Communication Receiver
This restoration was more mechanical and cosmetic than it was electronic. It involved finding and installing correct vintage parts along with correcting other mechanical problems. Additionally, front panel paint "touch-up" and the mechanical approach to "body work" is covered. Further, the methods for operating a vintage receiver on "original parts" is presented.
|>>> The knobs are correct but it's odd that the RF Gain, Audio
Gain and CW Osc knobs are long-boss versions and the Tone is a
short-boss type. The knobs appear to have the same patina and probably
are original. This implies that National inter-mixed the knob styles on
production. This seems normal for National and the fact the the
difference is hardly noticeable further convinces me that the knobs are
UPDATE - July 21, 2014 - Someone in the past "forced" the PW-D to turn when the tuning condenser or part of the gear box was "frozen." This stripped the main condenser drive gear and the right side reduction gear of several teeth and deformed both gears. The end result was although the PW-D appeared to rotate it only did so for about one turn and then "locked up" due to the deformed gear teeth. Fortunately, I had a few extra gear boxes and I thought that a gear box replacement would fix the problem. However, the original gear box shaft is designed to fit the tuning condenser shaft that is about .375" in diameter. The "after-market" gear boxes were the type National sold for projects and were designed to fit a .250" shaft. Additionally, the original gear box output shaft was "pinned" to the tuning condenser shaft. Needless to say, the two types of gear boxes weren't directly interchangeable.
As I thought about this project, it seemed like the way to go was to
find a complete NC-100 gearbox and tuning condenser assembly. That would
solve the severe rust problem on this tuning condenser and would solve
the broken gearbox problem too. About a nine months later (April 2015,) I was able to purchase a
complete NC-100 N-PW gearbox with the three gang tuning condenser
assembly attached. Also, included was a good condition PW-D. Thanks to
Capacitor Transplant -
May 31, 2015 - This operation required removal of
the front panel which is easily accomplished. With as much corrosion
present, it was quite a surprise that the PW-D wasn't a problem to remove. The BFO
knob required some light oil and heat to break its set screw loose but
all other knobs came off easily. I gave the chassis a quick cleaning and
although there are quite a lot of small rust pits, it's not so bad that the
chassis needs repainting. Cleaning has really helped the overall appearance
(see photo below.)
The first thing to do in the removal of the old tuning capacitor is to desolder the seven TC (tinned copper wire) connections at the solder lugs on the tuning capacitor. The lugs aren't difficult to access if you have a small soldering iron ( I use a 25W Weller.) National didn't wrap the TC wire connections probably because they knew if removal became necessary it would be difficult to "unwrap" the connections. Luckily, the TC wires just slide out when the solder is molten. The LO grid leak is on a tie point that is soldered to the stator lug. Just desolder the entire tie point from the stator lug. Once the seven connections are off, the capacitor can be dismounted by removing the mounting screws.
The tuning capacitor has four screws under the chassis that mount the gear box. These need to be removed. You have to move the catacomb from one end to the other to access all of the screws. There is also a bracket at the rear of the tuning capacitor. Remove the chassis screw and leave the bracket mounted to the capacitor (for now.) At this point you should be able to lift the capacitor/gear box assembly off of the chassis. This reveals the tray under the capacitor that has the three insulator blocks with the terminal connections exposed. This area will probably be very dirty and filled with all sorts of solder bits, wire ends, dust and grease - mine was (see photo right.) Clean the tray area using Q-tips to clean the insulator blocks and hard to access areas of the tray. I use WD-40 as a degreaser followed with Glass Plus to remove the WD-40 residue.
Be sure the seven lugs on the replacement tuning capacitor are tinned and the holes are fully open. On the terminal blocks, you'll note that there are three short wires and three longer wires plus the wire that has the LO grid leak. The short wires connect to the stators and the long wires connect to the rotors. You have to route the longer wires with a slight bend under the stators and then up to the rotor lugs. Originally, these wires were not sleeved but, if you are worried, you can sleeve them since sleeving was used on some of the later Moving Coil receivers. Guide the TC wires into place as you lower the replacement capacitor into place. You will need a slotted soldering aid tool and long thin needle-nose pliers to guide the TC wire through the holes in the soldering lugs.
Once you've soldered the seven wires you can then mount the gear box and the bracket. If your replacement capacitor doesn't have the bracket (mine didn't) use the bracket from the original capacitor. After the capacitor is mounted, disengage the catacomb (place between bands) and check with an ohm meter if you have any shorts from the stators to chassis. Check rotor to stator which should also not have any shorts. Mount the PW-D and check operation of the entire assembly. No problems should be encountered.
Restoration - The front panel on the RCE was bent and had
several nicks to the black wrinkle paint. The panel needed to be
straightened out first. I "eyeball" down the edges to see where
the bends are. The panel is then placed on a large pine board that is
perfectly flat (well, as flat as wood is going to be.) I then use two
inch square oak wooden blocks about four inches long and a weighted
hammer to gently bend the panel into proper alignment. If carefully
done, the result will be a straightened panel with no damage to the paint. More delicate types of
paint will require paper or cloth be placed on the pine board for
additional protection but old wrinkle finish is tough and quite
resistant to scratching or chipping and can be placed directly on the
After the panel is straight I clean it with Glass Plus. I then use nitrocellulose black lacquer that has been thinned down (~ 3 to 1) with lacquer thinner. I use this paint to touch up any significant nicks or scratches. Of course, when the "touching up" is finished, the panel paint looks uneven and spotted. The next step is to use several cotton pads - the round 2" diameter flat pads work best. Wearing nitrile gloves, dab the cotton pad with thinned black lacquer and rub the entire panel to even out the finish. Don't saturate the engraving but don't be too worried if some of the engraving is darkened by the thinned paint. >>>
|>>> The lacquer will dry in a few minutes and then the engraving has to
be "reconditioned." I don't want to scratch the metal so I use a long
round wooden skewer (like you use for barbequing) that I've sharpened
the end of. I use this tool to go over the engraving to remove any
paint, dirt or corrosion that's in the engraving. I use a head-mounted
magnifier to enhance my view of the engraved nomenclature as I'm working. The
result will be nice looking engraving that doesn't look "scratched out"
but just looks clean and somewhat bright.
Dust Cover Body Work - The rear panel of the dust cover was really bent up. I used the same body work method of a flat pine board and oak wooden blocks with a weighted hammer to straighten the panel. The rest of the dust cover just needed a little "tweaking" to get all sides and corners square. The back of the dust cover was pretty scratched and the top wasn't much better. Sides were okay. I decided to give the dust cover a light coat of "Satin Black" paint to even out it's overall appearance.
- The original PW-D was "lumpy" with corrosion under the paint. This is
difficult to remove since the base metal is damaged. I decided to just
use another PW-D that was in better condition but still needed a
repaint. The Airport receivers used a light-gray paint on the Index dial
and then black with white numbers on the Number dial. I had an excellent
condition gray PW-D that was on my RCF-2 receiver. This was used to
"match" some automotive lacquer paint from All Auto Paints in Carson
City. They will do the match and then fill a spray can with the paint
The old paint has to be removed from the Index dial. This can be accomplished with Jasco Epoxy Stripper (methol chloride - so do it outside using Platex and nitrile gloves.) Clean with steel wool and alcohol and then clean the Index dial with lacquer finish afterwards. This prep is enough to then apply the lacquer without any primer (that's the way the originals were painted.) Don't spray more than two coats since modern auto paint has some fillers in it which might "fill" the index lines too much. Two coats should be enough. Let the paint set overnight. Then use a mixture Artist's Acrylic paint of white and raw sienna to create a manila color (beige color) to fill the index lines. Don't use pure white - it will look way too bright. Apply the manila paint a few index lines at a time (five to ten works well.) Let the paint set for one minute and then wipe off with a damp paper towel piece (dampened with Glass Plus works best.) You might have to do two applications of paint to get a really good looking fill. Let the index lines dry for a day. Wipe the Index dial with a cotton cloth. Reassemble the PW-D if the Number dial is is good shape.
- Since the commercial and military versions of the NC-100 receivers use
oil-filled paper dielectric filter capacitors, frequently they can be
operated with all original parts. I really wasn't planning on using the
RCE as a station receiver and the set would only be powered up for
testing and maybe a casual listen around the bands. When long-term use
is planned (like the NC-100ASD profiled above) it is necessary that all
paper-wax capacitors be replaced since their leakage current is only
going to get worse as the receiver is operated. But, I felt that the RCE
could be safely operated with all of the original components if the
filter capacitors tested okay. I just checked these parts for shorts and
for value and found them to be okay. Before power-up, it's a good idea
to just check the DCR of the B+ to chassis. It should be very high
resistance which would indicate that there are no "power-off" shorts to
Upon power-up, I watch the rectifier tube carefully. If there is a short or other type of excessive load on the B+, the rectifier tube will glow purple inside the filament to plate area. If the rectifier appears normal, then B+ should come up to voltage in about 20 seconds or so. I measure it to be sure it's in spec and not loaded down. I watch for any indications of sudden heating which would indicate a heavy current flow through a resistor due to a shorted or nearly shorted bypass capacitor. Luckily, none of these things happened and the RCE powered up fine and did work somewhat. Of course, since I had replaced the tuning capacitor, it was pretty much out-of-alignment but still a few AM BC stations could be tuned in which let me know the receiver basically was operational. I performed an IF alignment and found that was pretty close. The RF tracking was way out, as expected. I kept having trouble with the contacts of the catacomb with the receiver on its side (necessary for RF tracking alignment.) The catacomb had to come out for further checking of the pins and receiving contacts. Mechanically, the catacomb seemed to be "flexing" the chassis when bands were changed which also warranted a thorough inspection. Catacomb removal is covered in the NC-100AX restoration article below.
Movement Issues - The catacomb problem was due to some corrosion spots here and there
on the shaft. These were removed with 600 grit Alu-Oxide paper and oil.
The bearings in the catacomb were also oiled. The flexing of the chassis
was due to the shaft mounting screws not being fully tightened. The
front bearing for the pinion gear needed another spacer to be added to have the rack
and pinion gears fully mesh and to have the shaft bearing fully within
the catacomb slot. A light coating of grease was applied to both the
shaft and to the rack.
On the pins and split-contacts, they were of course dry which increased the force necessary to have all fifteen pins to engage the contacts. Originally clean grease was applied to the pins although nowadays this has dried up or has turned into a "dirt-trap." I cleaned all of the pins and split-contacts with DeOxit and Q-tips, reapplied new clean grease (very light coating) and then reassembled. The bandswitching was now a fairly light action and there was no more flexing of the chassis.
Back to Aligning the RF Tracking
- The LO kept dropping out at the
low end of the bands on bands A, B and C. Bands D and H seemed okay. The
entire operation was somewhat erratic. I checked the connections to the
tuning condenser. I found that during soldering of the TC connections I
had not soldered the lower TC wire to the LO stator. The wire was thru
the hole in the lug but just not soldered. This was enough for the LO to
not operate correctly on the higher frequencies. Soldering the TC wire
corrected the problem and the alignment was completed on all bands.
Final Testing - Since the RCE was operating on all original paper capacitors, I was particularly careful about checking the heat build-up in the power transformer. This is a crude method of determining how much of a load there is on the B+ due to leaking capacitors (one can also measure the current draw of the B+ - it should be around 70mA nominal.) During an alignment, the receiver is going to be in operation for at least a half-hour. Checking the power transformer and the chokes several times for heat, I was surprised that nothing got hot. Again, I would stress though,...I'm not planning on operating the RCE for extended periods of time. If I was going to do that then it would be almost mandatory that the paper caps be replaced. For short test periods, original parts are okay if they not defective (other than the expected minor leakage current.)
- I'm quite surprised that this RCE actually works better than my much
nicer condition RCF-2. I guess that goes to show that appearances can be
deceiving. Anyway, the RCE now looks pretty good but how does it
I used a 7000Z ohm to 8Z ohm transformer on a 10" speaker that is mounted in the larger style National speaker housing for the RCE. When I tested the National RCF-2, I used a military LS-3 speaker that had a 600Z ohm transformer installed. However, the DCR of the output transformer on the RCE measures 280 ohms which seems too high for a 600Z ohm output which would normally be around 50 to 100 ohms DCR. The 7K set-up seemed to provide a normal reproduction level. AVC has a rather slow attack, probably a capacitor problem. Sensitivity is respectable with 40M ham signals coming in strong. I tried 20M and also heard a few hams there. The surprise was 11M CB with many strong AM signals in that region of the spectrum. No ham 10M sigs were heard. The AM BC signals (above 1300kc) were strong but the audio not very good. The DGPS "screamers" around 300kc were quite strong on band H. I didn't hear any NDBs because I was testing performance during the daytime.
I'm sure that if I went ahead an did a full "re-cap" and replaced any "drifting" resistors followed by another complete alignment, the RCE would be a really great performer,... when compared to other similar pre-WWII receivers.
Detailing the NC-101X Receiver
Many times the receivers we find don't really require a total restoration or rebuild. Sometimes only a little cleaning and repair of a few minor problems will result in a usable receiver that is performing quite well. I call this "detailing" a receiver. Mainly a cosmetic job with only a little electronic work necessary. Here's how we did this type of task on a NC-101X receiver recently.
The chassis "as received" is shown in the photo to the right. The dirt is
obvious and as mentioned above actually acted as a preservative for the
paint. The first thing noticed is that only one can-type electrolytic
capacitor is present. Although it is very similar to the originals, it
actually isn't an original type. The original filters were all 10uf
450wvdc individual capacitors. The one installed is a dual capacitor
which is not correct.
The next thing noticeable is that the power transformer appears rather tall. A quick look under the chassis showed that the power transformer was indeed marked "NC-100" but the complete ink stamp was "NC-100-25" which indicated that this transformer was for 25 cycle AC. All power transformers for 25 cycle operation will have extra laminations installed to increase the iron and prevent over-heating on such a low frequency AC. Operating a 25 cycle transformer on 60 cycles will cause no problems and many so-called 25 cycle transformers are actually rated "25 to 60 cycles." Note the transformer cover is from an Airways receiver.
The tuning condenser appeared to be slightly rusty however this wasn't the case. What appeared to be rust was actually greasy dirt probably mixed with tobacco smoke that turned the metal pieces a rusty color. The rotor hubs are brass but it's difficult to tell through the dirt.
The tuning seemed to work fine so that indicated that the gear box probably only needed to be cleaned and lubricated.
I was lucky that nobody had tried to clean the aluminum IF transformer cans and BFO can. Improper cleaning of this soft aluminum will end up polishing the matte finish.
|Under the Chassis - This area had been recapped several years ago using Sprague black plastic molded capacitors (with red nomenclature - not Black Beauties.) The installation appeared to be a professional job with all of the capacitors matching in manufacturer and type used. Soldering appeared professional. Many hams WERE professionals in the electronics business besides being hams. All of the resistors were Centralab types that National had changed over to by 1937. These Centralab resistors are excellent in quality with carbon drifting almost never being encountered.||Cosmetic Inspection - The NC-101X only had some minor blemishes to the original black wrinkle finish. The skirted knobs were very dirty with tobacco deposits and some tarnish. The PW-D dial, while in pretty nice original condition, did show some wear especially at the point where the Index dial angles off. Here the paint was worn through to the metal. There were no gouges or scratches as is normally found on poor condition PW-D dials. There were no extra holes drilled anywhere on the chassis or the cabinet.|
Getting the NC-101X Operational
- This isn't restoring, it's just getting the receiver to a point where
I could use it and do some evaluation of its performance. The new
filters got the NC-101X running with no hum. The receiver came with 6V6
tubes installed in the audio output. Original tubes were 6F6 and while
you can substitute the 6V6 and it will draw only 2 amps of heater
current versus the 3 amps required for the 6F6, there are internal
structural differences between the two tube types. The 6V6 is a
beam-power pentode while the 6F6 is a regular pentode. To some ears, the
6V6 will sound a bit harsh compared to the 6F6 if the circuit and
transformers were designed for 6F6 tubes. This was the case with the
NC-101X, the 6F6 tubes sound a little bit mellower and slightly more
bassy - at least with the original 10" Rola electrodynamic speaker.
At this point the NC-101X was performing well enough to try using it as a station receiver. I used it on both 80M and on 40M on the AM nets available on those bands. The NC-101X was surprisingly effective as a station receiver. Sensitivity was ample although successful 80M or 40M reception doesn't require too much of a receiver anyway. The important thing was how the receiver sounded and how immune to adjacent SSB activity it would be. Performance was good enough that I decided to proceed and do the complete "detailing" followed by a complete IF/RF alignment.
|Disassembly - Without removing the cabinet it's next to impossible to access all of the areas of the chassis for complete cleaning. The cabinet is easy to remove. First, remove all of the tubes. Then take off all of the knobs. Then note that the RF Gain control is mounted to the front panel as is the Tone Control. Remove the mounting nuts on these two controls. Remove the mounting nut for the phone jack assembly. Dismount the pilot lamp assembly. Remove the back cover and lid assembly by removing eight mounting screws. Now remove the four large screws, nuts and lock washers that mount the cabinet and front panel assembly to the chassis. The cabinet and front panel assembly should now slide forward to clear the control shafts and then can be lifted off. Cabinet removal exposes all of the chassis and chassis mounted components for thorough cleaning.||Cleaning the Chassis - As mentioned, the dirt and grunge was a dust combined with grease and tobacco deposits. Since grease was involved I used WD-40 as a cleaner. WD-40 generally will not react with older types of paint, which in this case was lacquer. The dirt and grunge came off very easily and left the chassis paint clean and having somewhat of a shine. The application of the WD-40 was with a medium size paint brush and sometimes Q-tips when the area was too small for the brush. Paper towel strips were used for WD-40 removal. This was followed by another cleaning using Glass Plus to remove the WD-40 residue. Glass Plus doesn't contain ammonia and won't react with painted surfaces (don't use Windex.) The ceramic tube sockets were cleaned with De-Oxit and a clean tooth brush and wiped down with a paper towel strip. As expected, the grunge had acted as a paint preservative and the chassis turned out to be in excellent condition.|
|Cleaning the Tuning Condenser
- Again, WD-40 was used as a cleaning agent due to the greasy nature of
the grunge. Small paint brushes and Q-tips were used for the most part.
The nickel plated spacers were cleaned with a brass tooth brush (brass
won't scratch the Nickel plating) and
WD-40 which removed the discoloration completely. The ceramic spacers
had to be cleaned with Q-tips. After all of the grunge was removed, the
tuning condenser was further cleaned using Glass Plus to remove the
WD-40 residue. Paper towel strips or Q-tips were used to remove excess
Glass Plus. After the tuning condenser had time to dry (overnight) it
was lubricated with DeOxit on the rotor contacts and 10W oil on the
Cleaning Under the Tuning Condenser - This area seems to really attract dirt. I didn't want to remove the tuning condenser so I used long handled small paint brushes and bent Q-tips to clean this area with WD-40 followed by Glass Plus. You have to fit the brushes and Q-tips under and sometimes through the tuning condenser.
Photo Right: This shows the chassis after half of it has been cleaned with WD-40 and Glass Plus. The tuning condenser has also been cleaned in this shot. Note that the three electrolytics were removed for cleaning. Actually, two were missing and the third wasn't original and wasn't connected up anyway. With the electrolytics out of the way, cleaning is easy around the tuning condenser and reinstallation of these capacitors is not difficult. I didn't do any polishing of the chassis paint. This is how certain kinds of dirt and grunge end up protecting the original paint finish. The WD-40 cuts through the grease and removes it and the associated dirt. Then the Glass Plus is used to remove the WD-40 residue.
|The PW-D - The original PW-D had a badly worn Number Dial that "chattered" if the dial was turned too fast. Even lubricating the elliptic hub didn't stop the "chattering." I swapped another Number Dial to correct the problem temporarily. The Index Dial was in pretty good shape with no scratches but the paint was worn through around the tuning knob. Eventually, I wanted to find a good condition replacement. It ended up I didn't have to wait all that long. A couple of weeks after acquiring the NC-101X, I was at the annual Minden, Nevada swap meet put on by N7RCA. What should turn up but an excellent condition PW-D in a protective plastic bag - nice. The paint was very close to the original NC-101X's PW-D in that it was the dark bronze-black shading even though the new PW-D was actually a WWII vintage piece. All the new PW-D needed was to be disassembled, cleaned, lubricated and reassembled to end up functioning perfectly - and looking incredibly nice.||Cleaning the Skirted Knobs - The knobs on this NC-101X were the early style knobs with the short boss on the bakelite bar knob section. These knobs were advertised by National as having skirts that were "solid German Silver." German Silver is an alloy of Nickel, Copper and Zinc that generally is used in plating but also can be used as a solid metal alloy. In this case, the metal is the same all the way through so it can be polished heavily without wearing through any plating. The skirts on these knobs were pretty tarnished and I initially cleaned off the oxidation, tobacco deposits and grunge by carefully using NaOH (Sodium Hydroxide) commonly found in Easy Off Oven Cleaner. Using a small paint brush, I carefully painted some EOOC (that I had sprayed into a paper cup) on the skirt. Be sure to avoid getting EOOC on the bakelite bar knob. Work the EOOC around on the skirt and then rinse under cold water. This will remove all crud and oxidation. Follow this by polishing the skirt and the bar knob with Wenol's Polish (similar to Semichrome.) Since the skirt on these early knobs is solid German Silver, the polishing works great and if the EOOC has removed any of the nomenclature fill, the polishing residue is "jet black" and ends up filling the nomenclature perfectly.|
Electrolytic Capacitors - This wasn't going to be one of my "museum
quality" restorations where I restuffed caps and made the chassis look
totally original both on top and underneath. Since it had been
professionally recapped in the past with excellent components and
workmanship, I decided to leave the underneath "as-is." I had already
connected three new 10uf 450wvdc electrolytic capacitors under the
chassis so the receiver was functional. I did want the top of the
chassis to look correct however. So, I dug through the "junk boxes"
until I found three close matching can electrolytics. These had to not
only match each other but had to be similar to what was originally in
the NC-101X. Luckily, I was able to come up with three good candidates.
Since these were dummy-cans, I removed the wires from the bottom of the
can. I also repainted to top silver (although some NC-100s are found
with red or brown tops.) When mounted, everything looked convincing
correct. Now, anytime in the future, if I want to do a "museum quality"
restoration, these cans can be rebuilt into functional components.
Photo Right: This shows the NC-101X chassis top after detailing. This is after the painted surfaces have been given the final cleaning with Glass Plus and polished with a soft paper towel. Note that tuning condenser hubs now look like they should - brass. Also, the nickel plated spacers look like nickel. Don't get too aggressive when cleaning the tops of the IF transformers, the top of the BFO can or the interstage transformer can. These aluminum cans are pretty soft and will "polish" quickly if cleaned or rubbed aggressively. Use WD-40 first with a soft brush to gently remove any grunge then follow with Glass Plus. Dab with a soft cloth to dry - don't rub. This should leave the cans appearing as a matte aluminum finish - as original.
|Cabinet Detailing - The cabinet pieces were in very good condition and only required a little touch up painting. I use Artist's Acrylic Mars Black for most touch ups but sometimes I also will use nitrocellulose jet black lacquer thinned and applied with a small brush. It depends on the blemish. In this case, the inner surface of the cabinet where it contacted the chassis had some surface rust. Cleaning this up removed the original black paint (which was lacquer.) This was touched up with the lacquer. The wrinkle finish blemishes were touched up with the acrylic paint applied with a small brush or Q-tip.||Front Panel Detailing - The front panel was in very good condition and only required cleaning and touch up. I used Glass Plus applied with a 2" paint brush and then the Glass Plus was worked into the panel using the paint brush. After a couple of minutes cleaning the dirty residue was wiped off with a paper towel. Then a second spray down with Glass Plus and wipe down with a clean paper towel. This gentle cleaning will usually get most of the grunge off. I then touched up the panel and let the acrylic dry overnight. The next day I used Armor-All to polish the panel and also gave the cabinet the same treatment. If I was doing this today (2016) I'd use 10W machine oil instead of Armor-All. I find the results less harsh-looking.|
Test - About the only thing to mention on reassembly is to be
sure to set the PW-D correctly as described in the section "PW-D
Micrometer Dial Servicing."
Basically, you have to set the tuning condenser to "half mesh" and then install the
PW-D which will be set to "250." When installed correctly you should be
able to cover 0 to 500 with an overlap on each end that is around 10
Alignment - The IF is aligned first and you have to determine the frequency of the crystal that's installed in the crystal filter. Using this exact frequency in your IF alignment will assure that the crystal filter is a useful tool for combating QRM. Switch on the crystal filter and adjust the Selectivity and Phasing controls for a narrow bandwidth. With the RF signal generator connected to the grid of the mixer tube, sweep the generator frequency through 456kc and listen for a peaking noise as you pass the crystal's frequency. With a digital frequency counter connected to the RF signal generator note the exact frequency that the crystal responds to. This will be the IF used for alignment. In my case, the crystal was 456kc. Adjust the IF capacitors for a peak indication using the "eye-tube." You can also connect a VTVM up to the AVC line and monitor that voltage for alignment purposes. Be sure to use a 400 hz modulated signal on your RF generator and have the receiver in AVC.
The RF tracking alignment requires the receiver to be on its side with access to the coil catacomb. Generally, the inductance which adjusts the lower end of the band was set at the factory and shouldn't change unless there's severe environmental problems with the storage that allowed moisture to ingress into the coils. Most of the time the catacombs are sealed pretty well and are protected by their location under the chassis. Usually, only the trimmer capacitors needs to be adjusted for each band. Connect the RF signal generator to the Antenna Input and use a 300 ohm load (a 300 ohm carbon resistor in series with the input.) You'll have to reference your NC-101X calibration charts for each band since these provide the frequencies that should be used at 450 on the PW-D for the high end and 50 for the low end of each band. Set the PW-D at 450 and the proper frequency input on the RF signal generator. You'll probably find that the LO is pretty close but tweak it in at the exact frequency. >>>
|Performance - I'm impressed by the NC-101X. It's certainly sensitive enough on 80M and 40M. I've listened on 20M and it seems to do very well on that band too. I haven't tried 10M but I think it would also be adequate there. Certainly, if 10M performance was an issue, an external RF Preselector could be added. IF bandwidth is probably about 12kc or so without the Crystal Filter. QRM hasn't been a problem but I'm sure the Crystal Filter would be able to eliminate all but on-frequency interference. The audio is really nice. I'm using the matching 10" Rola electrodynamic speaker that came installed in the larger National speaker box. This seems to reproduce AM signals quite well and there is quite a bit of bass in some signals. The Tone control is not very responsive and does most of its action in the last 10% of rotation. On the station antenna, which is a 135 foot center-fed tuned inverted vee, the signals on 80M are so strong that I have to reduce the RF gain to about 7, even with the AVC on. This is to get the "eye-tube" to show some reaction since with the RF gain at full the eye-tube will always remain closed. Since my use of the NC-101X has been on 80M and 40M AM, I haven't noticed all that much drift. The bandspread action of the NC-101X will make drift show up more since a slight drift of a few kc would be indicated with as many as 8 or 10 divisions on the PW-D. The most drift I've noticed is about three divisions over an entire net time (about an hour.) Remember though, this is in the AM mode. Of course if you're monitoring a SSB net or doing CW, the drift in the BFO will also be a factor and more drift will probably be noticeable. Overall, I'm impressed enough with the NC-101X that I've set it up with the Collins 32V-2 transmitter and have been using it as the station receiver (off and on) for several weeks now.||Using the Eye-Tube - I've been using the NC-101X as a station receiver for a while now. I've found that for AM on either 80M or 40M, I can't really run the RF gain "wide open" (receiving on the station antenna) and have the eye-tube show anything other than fully closed. Backing the RF gain down is necessary and provides some interesting possibilities. I've been listening to the AM "round-table" stations and adjusting the RF gain to have the eye-tube fully closed on the strongest station. This usually has the RF gain set at about 7 or 8. Now all of the other stations are normally going to show something less than fully closed on the eye-tube when they transmit. After a couple of sessions you get to where you can sort of estimate signal strength relative to the strongest station. It might have been something that the original owners of early NC-101X receivers did - at least if they operated AM.|
Detailing the NC-80X Receiver
This isn't a restoration write-up. It's detailing a receiver that had already been recapped but was not completed as far as mechanics and assembly. Also, this write-up provides a way to present more detailed information on this "seldom-encountered" receiver, including photos of the chain drive assembly for moving the catacomb.
- The cabinet is held together with sheet metal screws. The front and
sides are one sheet metal piece. The back is one sheet metal piece. The
top and bottom are sheet metal pieces. The only unusual feature is the
eleven screws and shoulder-nuts that isolate the chassis from the
Serial Number - I looked and looked for the serial number on this NC-80X. Most National receivers have the serial number stamped into the chassis but not the NC-80X. Complete cleaning and some disassembly proved to me that there wasn't a serial number anywhere on the chassis. Sometime into the project, I was thinking, "I wonder if National stamped the cabinet instead." Looking at the back panel in the proper light and there it was,...stamped just below the opening for the headphone jack,...C 475. Difficult to see since the black wrinkle paint was applied after the stamping of the serial number.
The Bezel and the Station Markers - The bezel was brown plastic and there were six curved metal clips that were inserted into a lengthwise slot at the bottom of the bezel. What did these clips do? At first I thought it was some sort of mechanical method to mount the bezel - it wasn't. Looking at the advertising artwork, I could see that the points of the clips were next to dial scale. By reverse-mounting the clips, they then revealed their purpose - they were station indicators. The clips have a portion that protrudes out the bottom slot and one can then grab this "tab" and slide the clip in its slot until the "pointer" is aligned with where your favorite station was located on the dial.
The bezel was missing its clear plastic dial cover. I assume it was plastic because whatever was original was held in place with drive pins. I think glass would have broken when the drive pins were installed but plastic would work fine. I used .030" thick clear plastic cut to fit and mounted with Duco cement. Only one drive pin was present so the Duco cement was an option that worked fine.
Drive System - The small coil catacomb of the NC-80X only
requires a chain drive to move it into position for the four bands. The
catacomb seemed to be locked in place so it was disassembled. There
wasn't anything wrong but, as with the NC-80X's big brother the NC-100, the
pins and the receiving contacts must be clean and the rod bearing must
be lubricated for smooth, easy operation of the band switching system.
There are only 10 pins to engage (rather than the 15 pins on the NC-100)
so the chain drive is adequate for moving the catacomb into position.
Thorough cleaning got everything working smoothly.
Coil Assemblies - These are identified as W, X, Y and Z for frequency coverage and 1 for Mixer function and 2 for Local Oscillator function. They mount in the same manner as the NC-100 coil assemblies. Since the IF is 1560kc, these coils are different than the NC-100 series coils.
Chassis Cleaning - The chassis was cleaned as described in the NC-101X write-up above. I use WD-40 to cut the grease and Glass Plus to clean up the residue. This chassis had mostly soot deposits - weird. Anyway, they cleaned up with just the Glass Plus. Complete disassembly allows for a thorough cleaning job.
Cabinet - Four pieces comprise the cabinet. The rear panel, the front/sides piece, the top and the bottom. All pieces were cleaned and then touched-up with black nitrocellulose lacquer. Afterward the pieces were rubbed down with 10W machine oil (this does work great - better than ArmorAll.) The two nomenclature panels are held in place with the control mounting hardware so their removal and reinstallation is very easy. The "ground wire" lug for the wire for the antenna input coil return is riveted to the cabinet. This was missing and a new lug, wire and spade lug needed to be installed.
Bezel Mounting - The bezel can only be mounted or dismounted after the cabinet has been taken off of the chassis. This allows access to the back side of the front panel. Four 6-32 screws hold the bezel in place. They screw in from the inside of the bezel behind the clear dial cover. A gasket is needed so the bezel will mount "tight" against the cabinet. I used a thin piece of friction tape to make a gasket that accomplished the proper spacing.
Loud Speaker - The NC-80X doesn't have an internal audio output transformer. The pin jacks connect to B+ and to the 25L6 plate. The speaker that was intended for use with the NC-80X was the standard National 8" table speaker that used a PM speaker with an audio output transformer mounted on the speaker. This is the same speaker used with the HRO Senior. Primary impedance is about 7000Z and the secondary is 8.0Z.
photo left: This shows the favorite station clips and how they can slide along the slot in the bezel
- Originally the NC-80X used two 40uf 150wvdc electrolytic capacitors
that mounted to the chassis via large threaded bases. The negative of
each capacitor is connected to chassis (B-.) I left the original caps in
place and removed their connecting wires. I mounted two Sprague 50uf
150wvdc axial mount electrolytics (yes, they were old ones from 1983.) I
tested and reformed the caps before installing.
AC-DC and Chassis Isolation - Everyone knows that in a receiver with no power transformer, the circuit is directly connected to the line. B- is usually chassis but in some cases, with improper power plug orientation, the chassis can have AC voltage present. If this were also on the metal cabinet, then the operator would likely receive a line voltage shock if he was touching the cabinet and another part of his body was at ground. Realizing this possibility, all designers of AC-DC radios isolate the chassis from the metal cabinet. National did this using eleven rubber grommets and special shouldered nuts and screws to provide complete isolation of the chassis. Additionally, realizing that someone could lift the lid on the receiver and touch the chassis parts while the receiver was powered up, National didn't provide a lift-lid. Instead the lid is screw-mounted.
All of the rubber grommets were natural rubber and had disintegrated or were in that process. All grommets had to be replaced. Modern black rubber grommets were used since the brown natural rubber ones are difficult to find these days.
- I tested all of the tubes and found that the 6B8 was weak but all
other tubes tested fine. I installed a vintage power plug on the AC line
cord and was ready to test the NC-80X. But wait a minute,...I didn't
want to just plug this into the AC line direct. So, I had to drag out
the hefty isolation transformer that I have (good for 20 amps) and
actually have something between the house AC line and the NC-80X.
Anyway, I was going to need the isolation transformer for the alignment
since if you try to hook up test gear to an AC-DC radio there will
usually be a conflict between grounds and how the radio is connected to
the AC line. Best to use an isolation transformer to eliminate any
Power-up - With power applied the NC-80X lamps didn't come on. This was just a loose lamp. The receiver came to life tuned to the broadcast band. Very good audio from the 8" National speaker. Just a 25 foot wire for an antenna. Bands W and X functioned fine and calibration was fairly close. I tuned into some 75 meter AM signals which sounded quite nice. Bands Y and Z only tuned in signals on a portion of their coverage which indicated a LO problem.
Problem - There was no B+ going to the plate of the LO
tube. Originally there had been a jumper between pins 3 and 4 which allowed the
plate to connect to the screen for the B+ connection. For some reason
the jumper was missing and installing a new jumper got the LO working on all bands
Alignment - Since the NC-80X doesn't have a TRF stage it might be possible to align the LO or the Mixer to an image, especially on the two highest frequency bands. However, this would require an extremely high level input from the signal generator. Since the IF is at 1560kc, an image would appear 3120kc lower than its actual frequency. Images are caused when an "untuned" signal gets pass a non-selective RF or Antenna stage and mixes with the LO to produce the signal at the IF. Normally the LO would be operating above the tuned signal but mixing can also take place with the LO below the received signal and that's what happens with an image. It's usually the non-tuned signal mixing with the LO below its frequency. That's why images are normally two times the IF below the actual frequency of the signal that is "leaking thru." Since the NC-80X's images are 3120kc down, the normal selectivity of the antenna coils are sufficient to practically eliminate image response.
IF alignment is straight forward except that the frequency is 1560kc. Be sure to use the actual crystal frequency for IF alignment and then your crystal filter will function correctly. You can use either an audio output level meter or use a VTVM monitoring the AVC line as a "peak" indicator.
Surprising! I was not expecting the NC-80X to be a very good performer
but it has plenty of sensitivity and pulls in signals even up to 10
meters. The selectivity is via the crystal filter and it works like the
HRO crystal filter in that there is a "peak" adjustment on the
selectivity control that should be set, even when the crystal filter is
off. I used an eight inch HRO speaker box and the audio on AM BC is very
good. SW BC also sounds good on strong signals. Dial accuracy is fair
and the resolution is very limited. The BFO provides a fairly stable
oscillation and allows decent CW or even SSB however drift would be an
issue if you wanted to monitor a SSB net over a long period of time.
Drift is certainly normal for the vintage of the NC-80X and not
The down side of the NC-80X is mechanical. The chain driven band change feels like the chassis flexes whenever it is operated. The band in use indicator is very crude and although it does its function it's basically a string and a wire spring operated device. The clips for indicating favorite stations are crude and always seems to be leaning one way or the other. It's obvious that National put most of the cost cutting into the mechanical side of the NC-80X.
The NC-80X is a sensitive receiver that is certainly capable of functioning as a station receiver. However, the AC-DC power and some of the mechanical issues might limit its compatibility with other equipment.
Restoration of the Wells-Gardner RAO-3
This 2006 restoration was more cosmetic than electronic. While more involved than a "detailing" job, it still can't be considered a "museum-quality" restoration. What is interesting is that I've restored this RAO-3 more than once. "Rios Radio Revisited" is a re-telling of how I found this receiver in 1969.
How I Found the
|>>> Phil had been in the Radio-TV repair business since 1946
and I had worked for him during the summers when I was in high school. I
knew he would have the tubes that were missing and some suggestions on
restoration. Of course, in 1969, restoring a WWII relic was more of "get
it running" than anything else. With the rocks removed, I saw that about
half of the tubes were still present and we needed the easy ones, the
6K7, 6L7, etc. After "tubing up" the RAO, we found a power cord that fit and a
speaker (that was the wrong impedance - but who knew that then.) The RAO
came to life and played pretty well with just a connection to the test
TV antenna at the shop. Phil said, "Sounds pretty good. Run it for a
week or so and then do an alignment and it should be a good set."
It wasn't too long before the audio output transformer opened up. I had taken the receiver down to Rios' shop and had the bottom off. After poking around for a while with no progress, Phil must have felt sorry for my inexperience and started issuing orders. "Measure the 6K6 pin 3. Okay, now measure pin 4. Okay, you have an open audio output transformer." Phil knew all the connections in his head from years of experience. I went over to Schirmer's Electronics, the local parts house, and bought a small universal audio output transformer. Who knew that the RAO was supposed to have a 500 ohm audio output impedance? Well, I installed the new transformer inside the old housing and everything looked original but of course the output impedance was now 8 ohms. The RAO came back to life and was ready for use as my new ham receiver. How I made any contacts on 15M CW I'll never know, but I did. As far away as VE3BAW. Exciting stuff then.
I later replaced the RAO with a Collins 75A-4 which was a vast difference in performance to a fairly new ham. I didn't get rid of the RAO though. I kept it in storage through several moves. In 2006, after more than thirty years of storage, I thought that the old RAO needed to be "revisited" again. Poor ol' Phil was long gone having been hit by a car while crossing the street in front of his shop. That was in 1989. I was a different person now, with 25 years of Industrial Electronics experience in engineering prototyping, test fixture design and field service repairing behind me. I also now had about 12 years (at that time) experience with owning and operating the Western Historic Radio Museum. The things I found that I had done to the RAO over thirty years ago were amazing - maybe even embarrassing! The goal now was to bring the RAO back to working condition and original appearance. This was going to require a total repaint and locating several correct type parts for the restoration.
*"Rios Radio Revisited" was an article that I wrote for "Antique Radio Classified" that was published in that magazine around 1990. It combined a "found radio story" about the RAO-3 with a bio of Phil Rios.
|Plexiglass Dial Cover -
Who knew this cover was supposed
to be plexiglass? Not me, obviously, since I had installed a pane of
glass. I guess that assumption was based on seeing an old NC-100ASD at the
neighboring ham's shack when I was a kid (that was Dennis Antilla,
WA6LHL, another mentor of mine mentioned in the NC-100ASD restoration
section above.) I remember that receiver's
dial cover was cracked, so it had to be glass (it's supposed to be
glass in the ASD.) Like most RAOs, the
plexiglass on this one was warped and discolored, so glass went in.
Thirty years later, after realizing for some time that the
glass was incorrect for an RAO, during this rebuild I replaced the glass with a
correct plexiglass dial cover.
Interesting Painting Problems - At first I thought I was going to be able to just spray over the original wrinkle finish with new wrinkle finish paint and the cabinet would look okay. Luckily, I tested the idea on the dial escutcheon first. Wells-Gardner used some sort of dark olive drab primer on the sheet metal that reacted with the new paint by "lifting" - almost like paint stripper was being used. So, with that idea eliminated, I had to strip each of the cabinet sheet metal pieces first. This was probably better anyway, since the paint job will be higher quality.
The RAO-3 was cabinet pieces were prepped by light sanding with 400 grit Al-Ox paper and washing with lacquer thinner. You have to paint one side at a time because the wrinkle paint has to be hot to "wrinkle." I use heat lamps along with a heat gun to assure that the wrinkle is complete. Since the lamps are in a fixed position, only one side at a time can be painted and then put under the lamps. Apply at least three heavy coats of wrinkle paint with a time spacing of about 1 or 2 minutes between each coat. Then place that painted side under the lamps and standby with the heat gun. Wrinkling will take place in about 5 to 10 minutes. Use the heat gun on the corners and edges where the lights don't provide enough heat. Don't use too much heat from the heat gun or you'll "gloss" the wrinkle and it won't match. You don't need much heat, just a little to assure the you get full wrinkling on the entire panel-surface. If you're careful painting each side to not "over-shoot" the edge you'll find that each side will blend and when finished the cabinet looks like it was all painted at one time. Be sure to allow a little time for curing before assembly. Overnight is okay but a few days is better.
Shown above is the main cabinet piece after wrinkle finish was applied. NOTE - this application is using Krylon Black Wrinkle Finish paint, not the VHT paint I now use. At the time I painted the RAO in 2006, only Krylon had wrinkle finish paint and when the Krylon was compared to true "black" paint, one will see that the Krylon is sort of a charcoal black, not deep, jet black.
- I had to make a cover for the small housing for the 1st RF Amplifier
tube. This box is mounted to the rear panel of the cabinet and due to
the necessity for complete shielding, the box does have a small cover.
Additionally, the dial lamp harness was in terrible shape so I rebuilt
it using correct period lamp sockets since the originals were missing
their mounting brackets and the dial lamps were just shoved into the
lamp holes. The four small control panels were straightened and then
repainted. Mounting these panels, which were originally riveted,
required making "fake rivets." These are 4-40 slotted head screws that
have the heads turned down to look like rivets. The panels are then
mounted with "fake rivets" and nuts. I usually remove any excess length
of screw that extends past the nut to prevent any interference problems
Observing the photo above in the RAO section of this article which shows the RAO-3 after detailing of the chassis, it can be noted that there are some minor problems remaining. Due to the cleaning most of the silk screened nomenclature is missing on the IF cans and from the chassis. Remember, this receiver was filled with rocks when I found it! Note that W-G used a different power transformer cover than National did. I'm not sure that this cover is correct since it was merely one that fit and the original cover was missing. The filter capacitors are not electrolytics. They are oil-filled paper dielectric capacitors which is why most of the RAO receivers will usually work somewhat with mostly original parts. Replacement of the paper-wax capacitors is recommended for best and safest performance, however.
On the Air, Again - After many years of storage and then a second restoration, I finally did use my old RAO-3 "on the air." It was on the Saturday morning West Coast AM Net on 3870kc at 8AM. The transmitter was a Collins 32V-2. All I can say is that the RAO-3 performed quite well with its typical great sensitivity. Of note though, the RAO receivers do not have any significant bass reproduction in the stock configuration. The audio reproduction was pretty thin sounding - but, that's normal for the RAO. My initial use in 1970 was as a CW receiver, which really was it's most common mode of operation with the Navy. The Net operations went fine and the RAO-3 finished the entire Net without having to resort to using a back-up receiver. Although I wasn't configured for remote stand-by, that option is available on the RAO-3. I used the B+ OFF switch as the remote stand-by. Although I haven't used the RAO-3 with any military transmitters on the Vintage Military Radio Net, I have used my RAO-7 on several occasions and it also is a dependable receiver, providing plenty of sensitivity and stability, although not much bass response.
The header photo in this section shows the finished RAO-3 mounted on an original RAO-3 shock-mount.
1. Original Manuals for NC-100, NC-100ASD, NC-200, NC-2-40D, RAO-7/9, RCF, RCQ, RCR - Original manuals or copies of original manuals will provide circuit descriptions and operational procedures that reflect the manufacturers' design intent.
2. QST Magazine 1936 to 1948 - Original advertisements are good for providing a time line in the evolution of the design and which of the features the manufacturer believed were important at the time.
3. Communications Receivers, 4th Ed. - Raymond Moore - The best reference book for production details on early receivers.
4. The Coil Catacomb Radios and
Variations on a Theme by:
Lawrence R. Ware -
AWA Old Timers Bulletin - 1998
- This article covers all of the receivers that used the moving coil
system with lots of photos of seldom encountered versions.
4. BAMA, Boatanchor Manual Archive - aka BAMA Mirror site (or BAMA edebris) is a great online source for many old manuals. Great for reference on rigs you don't own but maybe want to.
5. Thanks to all of the National Co., Inc. radio enthusiasts - who have provided photos, serial numbers, circuit and performance data and other information via the National Radio Reflector and via e-mails.
Henry Rogers/Western Historic Radio Museum © 2012-2016 - info on NC-101X, updates on NC-100XA project, new photo additions - June-July 2014. Catacomb Coil Assembly info added, NC-100A dial lamp assembly info added, NC-100ASD restoration added, RCE restoration added - June 2015. Added NC-80X restoration, photos,...Added RCR photo and info, Feb 2016,...Added additional information and photo NC-200, May 2016,...Added RCQ photo and info, Jun 2016,...re-edit and expand Airport Receiver section, June 2016,...added NC-101X SN:141-N photo, July 2016,...added info on "Silver Anniversary" NC-200, Dec. 2016,...
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