Navy Department - Bureau of Ships
RAO Series of Radio Receiving Equipment
National Company, Inc. - Wells Gardner & Company
by: Henry Rogers WA7YBS
|The famous U.S. Navy RAO receivers can trace their origin back to National Company, Inc. and their 1936 "Moving Coil" receiver, the NC-100. The Navy recognized that the extremely durable nature of a coil enclosure made of cast aluminum that was rack and pinion gear driven into position was a better solution than the alternatives of plug-in coil sets or band switches. The first section of this write-up is about the design of the "Coil Catacomb Receivers" (as they later became known) and why they were developed and evolved into what ultimately led to the robust and reliable RAO receiver. Following the history is a detailed description with photos of each RAO version and then a couple "over-the-top" restorations.
History of the National Company, Inc. "Moving Coil" or "Coil Catacomb" Design
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
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 (changed later to RF/IF cathode connection.)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 that was about half the width of the chassis.
photo above: Note that the rack and pinion is at the front of the catacomb.
|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 and knob. Photo left shows rack and pinion drive are in front of
catacomb on the early catacomb receivers.
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.
The initial NC-100 receivers used a PW gear drive and a PW-D micrometer dial (PW-D was National's identification of their "micrometer" dial.) Since the NC-100 was intended for the SWL or intermediate-level ham, a different type of signal strength or tuning indicator was utilized. Since a cathode ray tuning indicator was part of the NC-100 design it was going to require strong and consistent AVC voltage. This required the incorporation of an Amplified AVC stage in the receiver. Since nearly all hams were running on CW in the thirties and their receivers were always operated with the BFO on and the AVC off, the use of an amplified AVC stage implies that National was designing the NC-100 more for voice reception of AM signals from Shortwave Broadcasters. IF is National's standard 456kc. 12 tubes were used in the NC-100 circuit. The NC-100 tuned from .54 to 30 mc in five tuning ranges.
Finally, there was the Push-Pull Audio output stage using a pair of 6F6 tubes driving a 10" Rola Type K-10 electro-dynamic speaker. Even an optional 12" electro-dynamic speaker was available. Again this great audio section implies the reception of voice and music, not the "dots and dashes" of International Morse. Since the intended market for the NC-100 was not necessarily hams, the receiver was introduced with a fabulous "art deco" front panel that featured geometric-linear black panels and black octagon control nomenclature layout combined with the natural aluminum finish along with a red highlighted central panel for band in use indication. The PW-D combined a bluish-gray index dial with a red number dial along with a green pilot lamp and the green "eye tube" (when in operation) finished off the striking "art deco" design of the receiver. The NC-100 and the NC-100X were introduced in August 1936 and were available from dealers in September 1936, with the initial pricing at $105 and $127 respectively.
These early version NC-100 and NC-100X receivers were soon followed by the "ham bands only" version, the NC-101X. The ham band versions tuned 160M, 80M, 40M, 20M and 10M. Still, the micrometer dial was retained requiring the operator to use some sort of correlation between the tuned frequency and micrometer reading. That correlation took the form of tuning graphs that were included in the operating manual.
The use of an indirect-readout device such as the PW-D probably accounted for more than a few complaints to National from casual users who had to constantly refer to the manual for a graph that provided frequency versus dial readout correlations. The competition's direct-readout dials also could have been a factor that resulted in National revamping the entire NC-100 line to replace the PW-D micrometer dial with a direct readout, illuminated tuning dial. The suffix "A" was added to all NC-100 receivers that had the new dial installed. The introductory ad appeared in the June 1938 issue of QST. For sometime, the NC-101X was available with either the micrometer dial or with the "A" version dial as the NC-101XA. After all, the NC-101X was a ham receiver and many hams enjoyed the correlation exercises involved with using the PW-D.
One of the interesting features of the new "A" version dial was its articulated pointer. When the band was changed, the dial pointer would automatically increase or reduce its apparent length so that its red tip would line up with the selected band's tuning scale, thus indicating the "band in use." This required the pointer "lifter" mechanism to track the tuning while maintaining the proper length of the pointer. A dial cord that was anchored to the band changing shaft and then routed via a pulley system to pivot against the tuning condenser drive shaft so that the action of tuning the receiver didn't affect the apparent length of the pointer while it tracked around the dial. The pointer-lifter mechanism and the entire articulation system seemed overly complex for the simple task of a band select indicator. Small wonder that during WWII this feature was eliminated and replaced with a gear-driven indicator dial mounted to the band change shaft. A logging scale was included to allow for precision frequency resetability that was comparable to the PW-D dial's accuracy. Initially, the dial cover was a pane of glass. Most later military versions had the glass replaced with plexiglass.
In addition to the new tuning dial, the cabinet itself was increased in height from the nine inch height of the NC-100 to ten and a half inches of the NC-100A. The eye-tube was replaced with a "behind the panel" S-meter that was installed on all versions now. A push-pull switch allowed disabling the S-meter when CW was being received, just like the HRO receiver. On the earliest versions, the white scale S-meter was installed but these were soon replaced with the yellow scale meters. With the S-meter installed, the total tube count of the NC-100A circuit was reduced to eleven tubes since the cathode ray tuning eye tube was no longer used. >>>
|A Crystal Filter was added to the
NC-100A with the designation changed to NC-100XA. The Crystal Filter is
the same circuit that was used on the NC-100X and the NC-101X receivers
(and the HRO receiver) in that a
cam-operated switch on the Phasing control places the Crystal Filter
into the circuit. Selectivity is not a stepped switch but rather is a
variable condenser that is infinitely adjustable while the Phasing
control allows some adjustability of the "notch" within the passband.
When the Crystal Filter is switched out, the Selectivity control must be
"peaked" for maximum signal response since the variable condenser is
still in the IF circuit.
The Tone control on early NC-100A versions used a large inductor in the circuit. Push-Pull 6F6 tubes were used in the audio output with the output transformer located on the external speaker. The speaker included was a 10" diameter Rola and the speaker-cabinet assembly was designated as MCS-10.
With the NC-100XA, some of the control locations were moved with the addition of the Crystal Filter (still at 456kc.) The S-meter switch was moved from adjacent to the meter to the lower part of the panel. Different nomenclature was required so the small panels were changed to reflect the additional controls needed on the "XA" version. The selling price for the NC-100XA was $147.00 from Allied Radio in 1939. Earliest reported "A" version is a NC-101XA with SN 130-M which would date from around May or June 1938.
Throughout the NC-100A production, the circuit went through several minor changes. Later in production, an adjustable Noise Limiter circuit was added (Nov. 1939.) The NL circuit changed the 6C5 second detector to a 6C8 duplex triode to provide the second detector function and the Noise Limiter function (keeping the tube total at eleven tubes.) The 6J7 AVC Amp was replaced with a 6F8 duplex triode to provide AVC amplification and also to add an audio preamp (1st AF Amp.) Also, with the NL addition, the location of some of the controls was rearranged. The Audio Gain control was changed to a standard grid input on the new 1st AF amp and the Tone control was changed to an RC type control which eliminated the audio choke that was used in the earlier versions. Some of the NC-100A and NC-100XA versions featured a "weighted" tuning knob that added a fly-wheel affect to tuning the receiver. The weighted knob wasn't used on the military versions. Production of the NC-100A and NC-100XA probably lasted until the NC-200 was fully in production (Oct. 1940) but, by that time, National was working with both the Navy and the USCG to supply them with NC-100A receivers that were modified specifically for their needs.
RAO and other Military Catacomb Receivers - Production Begins
U.S. Navy RAO -1 - The contract for the RAO-1 is dated Dec 11, 1939. The contractor is General Electric Supply Corporation in Washington D.C. with National Company being the manufacturer. National began supplying the US Navy with the initial, single-preselection RAO and RAO-1 probably sometime in 1940. The double preselection RAO-2 was built on a contract issued April 30, 1942.
U.S. Coast Guard R-116 Receiver - The R-116 double pre-selection receiver was built on a contract dated 15 May 1941. The R-116 has a lot of the later RAO-2 characteristics but it also has several major differences. For example, the R-116 uses a separate power supply with both the power supply and the receiver in a rack mount configuration. Additionally, the receiver actually has six tuning ranges covering frequencies from 1.5mc to 27mc. With six bands and less total frequency coverage than the NC-100XA or RAO, the R-116 tuning rate has much more "spread" and makes tuning in CW stations easier. The coils used in the R-116 are wound on ceramic forms where the RAO used R-39 material for forms. The catacomb used was from the NC-200 with its 18 coil compartments. But the R-116, with the addition of the six 1st RF coils in the smaller catacomb, brings the total coils used in the R-116 to 24. Due to the second catacomb and the total length of the dual catacomb system the rack gear was moved from the front of the catacomb to the center of the larger catacomb and a second rack was installed at the rear of the smaller catacomb. The band switch knob operated a gear drive and a long shaft with pinion gears that engaged the catacomb racks and allowed band switching. In the RAO and RBH receivers, the system parts were somewhat simplified though the drive remained basically the same. The R-116 layout of controls is quite different from the RAO, the dial escutcheon is reduced in size and several different types of vacuum tubes are used in the circuit. The Crystal Filter only has two positions. The R-116 was normally rack mounted using a "picture frame" adapter that bolted to the corners of the receiver cabinet. The R-116 is possibly the first "catacomb" receiver with the double pre-selection modification since its contract date is May 15, 1941 and the RAO-2 (first RAO with double pre-selection) contract is dated April 30, 1942.
U.S. Navy RBH Receiver - The RBH was built for the Navy almost concurrently with the RAO. It's very similar to the RAO but had a different frequency coverage, that being 300kc to 500kc and 1.7mc to 30mc. The RBH utilized a 1500kc IF that then allowed continuous tuning from 300kc to 500kc. The RBH followed a similar evolution as the RAO in that the initial RBH was single preselection while the RBH-1 and later versions were double preselection to reduce LO leakage. The National RBH was also identified as the NC-156 with the Navy identification CNA-46114 built on a contract dated June 28, 1941. The later RBH-1 and subsequent versions were identified as NC-156-1 with the Navy identification of CNA-46188.
Note that the proceeding receivers had their initial contracts issued before the USA became involved in WWII.
U.S. Army Signal Corps NC-100ASD - The Army also wanted a NC-100-based receiver and in a 1943 contract was supplied with 1000 NC-100ASD receivers. These receivers are very close to the civilian NC-100A and even have the articulated dial pointer. The push-pull audio and field coil speaker circuits were changed to PM speaker and single-ended 500Z ohm audio. The Army found little use for the NC-100ASD and there was a large quantity available post-WWII. Newark Electric sold the NC-100ASD for $115 and that included the matching loudspeaker and a manual that was serialized to the receiver (probably removed from the original crate when sold.) There is some evidence that a Signal Corps AN/GRR-1 was built that was an Army version of the NC-100XA but no known examples have turned up. Some NC-100ASD receivers were used in Army DF equipment.
More R-116 Details
- The R-116 lacks an internal power supply requiring that the receiver
operates from a separate AC power supply (rack mount dual supply) or
from a DC source such as batteries, motor-generator/battery combo or
other types of shipboard power sources. Voltages required are 6 volts
for the tube heaters and +230vdc for the B+. A four wire power cable
exits the rear of the receiver. I used a National 697 "dog house" power
supply as a power source. Audio outputs are 300Z and 600Z. Serial number
on the R-116 shown is 46.
Components - Photo to the left shows the front panel of the R-116. The dial escutcheon is reduced in size and the controls use small round nomenclature plates. The photo right shows the chassis of the R-116. The circuit below the tuning condenser (in the photo) is very similar to the NC-100XA or RAO with the exception of the oil filled bypass capacitors that are mounted to the chassis. Above the tuning condenser, where the AC power supply normally is on the RAO or NC-100XA, are several unusual components. The large box is an audio filter that can be switched in from the front panel. The left choke is also part of the filter. The transformer to the right of the choke is the audio output transformer. To the left of the two tubes in front of the filter is the Relay. It appears that this might be a remote speaker on/off relay.
Operation - With no electrolytic filter capacitors and the majority of bypass capacitors being oil-filled, SN: 46 does function and seems to be a very stable and sensitive performer.
RAO & RAO-1 - National Co., Inc.
photo left: Although this photo is of the initial pre-WWII (June 1941) version of the Navy RBH receiver (National NC-156) it has the same general appearance that the initial RAO-1 receiver had.
|The RAO (with no numeral suffix) was
a 19" rack mounted receiver. Two audio outputs were provided, a 20K
capable of 2 watts for loudspeaker use and a 500 ohm impedance at the phone jack
capable of 500mW. There was a rack mounted loudspeaker with a 20K matching transformer provided.
The RAO-1 was a table version that added a Noise Limiter circuit and provided 5000Z audio for loudspeaker and 500Z for phones. A table loudspeaker was included that had a 5000Z matching transformer. Note that the matching loudspeaker is shown with a National Company cloisonné <NC> emblem installed (see artwork to the right.)
The US Navy RAO-7/9 manual along with the Navy "Catalogue of Electronic Equipment" are both very explicit in stating that the RAO and RAO-1 are not to be used onboard ship and are only suitable for "shore use" as they do not meet specifications. This undoubtedly was referring to the level of LO signal leakage coupling to the antenna, a problem that was corrected with the RAO-2 and later receivers by adding an extra stage of preselection (TRF amplifier stage) and additional shielding.
photo right: Artwork from the RAO-1 manual, contract NOs-70248
RAO-2 - National Co., Inc. - Long before WWII began, the Navy wanted minimal radiation from any of the shipboard superheterodyne receivers' Local Oscillator appearing on the antenna. This was primarily to allow the receivers to be used in the presence of other shipboard radio equipment without causing interference. Also, the receivers had to operate without interference from the other shipboard electronic equipment. Most Navy ships had multiple radio receivers and transmitters in close proximity. Most of the large Navy ships had multiple radio rooms and many ships were equipped with radar. It was imperative that all radio equipment could operate simultaneously without mutual interference. Additionally, as surveillance, signal intercept and shore communication operations increased at Naval facilities, with the many receivers operating simultaneously, mutual interference had to be eliminated for reliable and accurate reception.
There was also the highly publicized reason for "low radiation receivers." There was the remote possibility that an enemy could "Direction Find" (DF) the radio's position from receiving the LO radiation if it was allowed to couple to the antenna. It would be possible to also discern at what frequency the receiver was tuned. In the late-thirties, many of the Regenerative Medium Wave receivers used on commercial ships operating with autodyne detectors could easily be received at a distance of up to five miles or more - and this was from other commercial ships, not the enemy! Most of these types of regenerative receivers had been removed from ships prior to 1940. The Navy believed that if the Germans put their minds to it, they could probably receive and DF inadvertent superheterodyne LO leakage radiation on the ship's radio antenna from up to 100 miles away. Consequently, the Navy came up with a specification of <400pW as the maximum of LO energy that could be measured at the antenna input terminals of the receiver. This assured that the receiver operation would not cause interference and it would be impossible to receive the LO leakage as radiation from a ship antenna at any significant distance away from the ship.
Beginning with the second of the numbered suffixes, the RAO-2, National added an extra RF Amplifier stage for isolation of the antenna from the LO radiation. This was accomplished with an additional coil catacomb and tuning condenser housed in a bolt-on rear chassis and bolt-on cover. This added RF Amplifier upgrade dates from just before WWII with the upgrade possibly first installed on a U.S. Coast Guard receiver, the R-116. The R-116 contract was dated May 15, 1941 and, although there are a lot of similarities to the RAO-2, the mechanical design of the coil catacomb is somewhat different allowing for six tuning ranges by utilizing the National Co. NC-200 receiver catacomb. The circuit of the R116 has some significant differences from the RAO-2 the most obvious being that it is operated by a separate power supply. The RAO-2 simplified the coil catacomb gear drive, simplified circuit construction by eliminating the many chassis-mounted bypass capacitors and, of course, using the combination of NC-100 catacomb, the later NC-100 circuit (post -1939 with NL circuit) and using the NC-200 Crystal Filter. >>>
>>> 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. Antenna input shielding was also improved but the RAO-2 retained the exposed Antenna and Ground terminals (changed to a coaxial input with the RAO-3.) This additional Antenna Input/RF amplifier section (sometimes called "the back porch") increased the depth dimension by about four inches and increased the weight significantly, running the scale up to about 75 lbs for most of the RAO-2 and subsequent receivers (see photo of the underneath of the RAO-3 chassis in the next section below.) Tube total in RAO-2 (and later) is eleven. The contract for the RAO-2 was NXs-4681 and the date issued was April 30, 1942. The USN designation for the receiver was CNA-46187.
The RAO-2 borrowed several features that National had been using in their NC-200 receiver (introduced in October 1940.) As mentioned, the Crystal Filter circuit was the same that had been used in the civilian NC-200 receiver in that a stepped-switch Selectivity control provided five positions of increasingly narrow bandwidths and the variable Phasing control adjusted the "notch" position within the passband. Using this Crystal Filter required using 455kc as the IF but that made sense since 455kc had become an industry standard (more or less.) The LO tube was changed from a 6K7 to a 6J5 as in the NC-200.
Other RAO-2 changes were that the dial scale background was changed from silver to off-white (cream color) paint and the delicate articulated dial pointer was eliminated. Instead, a rotating dial gear-driven by the band change shaft indicated which tuning range is selected. RAO-2 and 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 S-meter was from Marion Electric and the scale has the National diamond logo on the face. The chassis and most component housings are painted National gray on the RAO-2, only the IF, the BFO and the filter capacitor housings were matte aluminum finish. For reliability, no electrolytic capacitors were used in any of the RAO receiver versions. The power supply filters are 4uf oil-filled paper-dielectric types and 1uf paper-wax capacitors are used for filtering within the receiver circuitry.
The early RAO receivers (pre-RAO-7) are 17.5" wide, with an integral panel-cabinet-chassis construction that requires major disassembly if more than tube replacement, alignment or minor repair is needed. These early versions of the RAO used a cradle-type shock mount that is seldom found with the receiver today. Note in the "snapshot" radio room photo above-right and the photo to the left that the RAO receivers are mounted in their proper cradle-type mounting bases. Photo of the RAO-3 below also shows the cradle-type mounting base. The mounting base was identified as CNA-10125 when supplied with the RAO-2.
|RAO-3, 4 &
- Wells Gardner & Co.
- Wells Gardner & Co. was founded in 1925 as Wells Radio Co. in Chicago.
In the early thirties, Wells Gardner was selected by Zenith Radio
Corporation to build (as a contractor) their low-cost "Zenettes" and their "Challenger"
line of radios (1933 to 1935 models.) It's believed that Zenith learned how to build
their radios inexpensively from their collaboration with Wells Gardner. During the late-thirties and
into the early-forties, Wells
Gardner built most of the radios that Montgomery Wards sold as "Airline"
models. The Wards' model number will have "WG" as a prefix if Wells
Gardner was the builder. During WWII, Wells Gardner built a tremendous quantity of BC-348-Q model
aircraft radios (over 40,000 radios in one contract alone.) Wells
Gardner did business almost exclusively as a "contractor" building
electronic equipment for other companies. Later, as radio and
television manufacturing moved overseas, Wells Gardner began to manufacture other
types of electronic
products and nowadays builds video display units for the gaming industry
from their current location in Las Vegas, Nevada.
Wells Gardner & Company (Chicago, IL) was the second contractor for the RAO series, building the RAO-3, 4 & 5. The RAO-3 and RAO-4 were built on contract NXss21446 dated Jan. 11, 1943. The RAO-3 Navy identification is CWQ-46187-A and the RAO-4 was CWQ-46187-B. The RAO-5 was identified as CWQ-46229 and was built on contract NXsr38492. The RAO-5 appearance is almost exactly like the RAO-4 but an accessory loudspeaker is listed on the data plate. The mounting base was identified as CWQ-10125-A for RAO-3, 4 & 5. It's the same number as the RAO-2 mounting base but with the Wells Gardner identification CWQ prefix and an "A" suffix.
All WG versions have the extra RF amplifier for double preselection and all versions have S-meters installed. Though WG used National Co. "designed" parts, where specified, many other components were strictly WG manufactured or purchased parts. The IF transformers, power transformer, chokes and the chassis all appear to be WG components (that is, built to spec or purchased by WG but differing in appearance when compared to the same National component) while the knobs, dial, tuning condenser and coil catacomb appear to be directly from National. However, close examination of these "National" parts will reveal slight differences. For example, the catacomb contact insulators used in National receivers were made of a material called R-39 but the WG insulators are molded bakelite. When a National "bar knob" is placed next to the Wells Gardner "bar knob" it's obvious that the WG knob is slightly smaller, more squared and the pointer is much narrower.
|Wells Gardner had all of the National-designed parts built by subcontractors and suppliers located around the Chicago area. All paper capacitors were supplied by John E. Fast & Co. (Chicago.) Crowe Industries (Chicago) made several of the dial parts. In some cases the parts were purchased from the same supplier that National Co. used. For example, the S-meter was supplied by Marion Electric and is similar to the National RAO S-meter but note in the photo below-right that the WG receiver S-meter doesn't have the National diamond logo on the face. The NAVSHIPS-900,359-B manual has all of the receiver parts cross-referenced with the component suppliers. National Co., Inc. is very conspicuous by its absence from that list.
There was a myth that Wells Gardner RAO versions were not considered to be "heavy-duty military-type" construction by the Navy and that the Navy directed that WG RAOs could only be used at shore stations. But, the WG Navy manual specifically states the receiver is for shore stations OR for shipboard use since its LO leakage to the antenna is <400pW. Also, any examination of a WG RAO will reveal that its construction is just as robust as the National RAO-2 or RAO-6. The myth probably originated because nearly all RAOs receivers were used on shore for surveillance and communications. Certainly when examining WWII vintage photographs of radio set ups, the RAO receivers are seldom seen on ships but are very common at shore stations. Certainly, the lack of any RAO having medium wave Marine Band coverage might have been the reason for their limited shipboard use (the RAO was usually paired with the RBL for MW-LF-VLF coverage onboard ship.) For signal intercept, frequency surveillance and communications relay the RAO was ideal with wide frequency coverage per band, the ability to tune from the low end to the high end of a band quickly along with good dial accuracy and stability. Perhaps a more accurate version of the myth might be that all versions of RAO receivers were better suited for the types of duties found at Navy shore stations than they were as "shipboard" receivers during wartime.
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. The audio output impedance was specified as "total load of 600 ohms" referring to the possibility of multiple headsets being connected to the receiver. The RAO-5 included a loudspeaker with 600Z matching transformer (supplied by Jensen, CJS-49493.) Interestingly, the WG versions of the receiver had rivet mounted metal glides or "feet" on the bottom covers that allowed using the receiver on a table top without the shock mount assembly. When "in the crate," the shock mount, the manual and a spares kit (an actual metal box of extra parts) were packed with the receiver.
photo above: Showing the top chassis of the Wells Gardner & Co. RAO-3. Note the small box mounted on the rear wall of the main cabinet and its "wing nut" retained cover. This box contains the 1st RF amplifier tube. WG versions of the RAO will have a cadmium-plated chassis, a more standard power transformer cover, different IF transformer housings and different BFO housing (different from National Co. style housings.) The chassis serial number is written in pencil and is located on the chassis just below the left side of the dial plate. On the one example seen, the chassis SN and the data plate serial number were the same.
The photo to the right shows underneath
the chassis of the RAO-3. All of the parts used in Wells Gardner & Co. RAO receivers, even the coil catacombs, were built
by Chicago-area contractors and not by National Co., Inc. However, both National
and WG catacombs are virtually the same and operate in the same manner. 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
between the two coil catacombs is via a long shaft that passes under
both catacombs and through a hole in the two chassis. The shaft has two
pinion gears that engage a rack gear on each coil catacomb. The rack on
the large catacomb is near the center while the small catacomb's rack is
at the rear of the box. Both catacombs ride on their own round rod
shaft and also on a cast slot (in the large catacomb) and guide pin to keep
the catacomb level. Rotating the
band switch knob rotates its gear that meshes with the gear mounted at
the front of the catacomb driveshaft which in turn moves the coil catacombs into
Also note that all paper-wax capacitors, other than all .01uf values and one .05uf and one .1uf, utilize a clamp to mechanically secure these physically larger capacitors to the chassis. All carbon composition resistors were supplied by Allen-Bradley and are in the JAN style. These resistors are known for the long-term stability and reliability. All tube sockets are isolantite types (ceramic tube sockets.) The RAO-3 is the first RAO version to use a concentric cable antenna connector rather than a thumb screw terminal block.
Surveillance, Intercept and Communications Relay for the RAO Receivers
In actual WWII use, most of the RAO
receivers ended up being used primarily for surveillance, intercept or
shore-based relay communications but that wasn't due to any lack of
durability. The fast tuning, wide frequency coverage per tuning range,
and stability made the RAO ideal for this type of frequency monitoring
or for intercept purposes.
By late-1943, it was becoming apparent to the USN that the primary use for the RAO receivers was either for enemy signal intercept or for frequency surveillance. The RAO receivers produced later in WWII were built specifically for use with a panoramic adaptor for signal frequency surveillance, beginning with the RAO-6.
photo left: Banks of RAO receivers used at a shore relay communications or intercept station. The receiver in the foreground shows the "back porch" addition of the 1st RF amp very well. Also, note the length of the USN coaxial plug installed in the antenna input receptacles. Since these receivers all have the coaxial antenna input and don't have the panadaptor inputs on the rear and are in the multi-piece cabinet, these are all Wells Gardner & Co. versions of the RAO receiver.
National Co., Inc.
Company continued on with the Navy contracts building the RAO-6, a receiver that
was similar to the earlier RAO-2 using the same multi-piece cabinet and
cradle-type shock mount.
Like the RAO-2, the RAO-6 had "NC-120" shown on the Crystal Filter
nomenclature plate as the receiver identification. The single data plate
mounted on the top lid and the RAO-6 designation was there.
Apparently, at the beginning of the production run for RAO-6 receivers, the USN decided they needed to change the RAO-6 and optimize it for surveillance and specifically to function with a panoramic adaptor. The panoramic adaptor worked from the Mixer plate and allowed instant visual monitoring of wide segments of the spectrum which increased the ability of the radio operator to intercept enemy signals. The S-meter was considered superfluous since the panoramic adaptor also showed relative signal strength so the S-meter was to be eliminated.
The USN decision for a panoramic adaptor version RAO-6 should have required another different contract but it's uncertain whether that happened. It would also seem likely that a different Navy identification would have been used. The later panoramic adaptor RAO-6 receivers have a USN identification of CNA-46187-D, which is the same numerical designation as the RAO-2 but with a "D" suffix. One would assume that the S-meter version RAO-6 used suffix "C" and that suffixes "A" and "B" were for various contracts for the RAO-2 (but this is just an assumption.)
It should be noted that these "early" RAO-6 receivers would be very similar to the RAO-2 in features and if the data plates were missing it would be very easy to confuse a receiver's true identity. As listed in the Navy "Catalogue of Electronic Equipment," the initial (serial numbers 1 to 200) RAO-6 receivers were the S-meter versions which would make the early RAO-6 a rarity. The RAO-7 manual (NAVSHIPS 900,554) also mentions that the RAO-6 was equipped with a S-meter. The later, panoramic adaptor version RAO-6 is normally the receiver type that's encountered. Serial numbers 201 to 1200 (Navy Catalogue) were assigned to these later RAO-6 receivers. A different POWER SUPPLY B+ nomenclature plate is used on these later RAO-6 receivers and the data plate used on the top lid is smaller. The RAO-6 chassis and some components, such as chokes, power transformer cover, etc., are painted National gray.
RAO-7 and RAO-9
The RAO receiver underwent significant changes with the RAO-7 and RAO-9 (the
designation RAO-8 was not used.) The RAO-7 used CNA-46233 for
its Navy designation and the RAO-9 used CNA-46263. These receivers were of
robust construction and had increased shielding to further allow their
use with other shipboard equipment without mutual interference. The audio
output Z was the standard 600 ohms and the design eliminated the S-meter in
favor of a panoramic adaptor connection. The RAO-7 and RAO-9 simplified the
maintenance of the receiver by designing the chassis so that it was
easily removable from the cabinet (handles were added to the receiver
front panel to assist removal.) The receiver front panel was changed to
a heavy 8 gauge steel panel although the dial escutcheon remained about
the same as earlier models. The cabinet itself was redesigned for
better shielding and easier mechanical construction by making it a one
While the early RAOs used a separate mounting base, the new
RAO-7 and RAO-9 cabinets mounted the shock feet directly to the bottom of the cabinet
further easing construction and maintenance. The RAO-7 and RAO-9 receivers are
physically larger than the earlier versions with full 19" rack width
panels although the receivers are not specifically designed for rack
mounting (since the cabinet provided necessary shielding.)
photo left: National RAO-7 sn:10 from the first contract, NXsr-38306 Sept.22, 1943. Received by the Navy in September 30, 1944 but never put into service. Chassis sn: is H720 which is National's actual production run serial number. The data plate serial number and the chassis serial number will never match on these receivers - they aren't even the same format. The chassis serial number was assigned before assembly was complete and is a letter-prefix and number suffix format. The data plate serial number was assigned when the receiver was used to fulfill the Navy contract and is strictly numerical. Other than the obvious major changes, note that the new RAO-7/9 also uses different style bar knobs that employ a skirt
Other minor changes were to the bar knobs that now had skirts added, some of the nomenclature panels were removed and replaced with individual circular control nomenclature metal plates. The data plates were moved from the top lid of the cabinet to the front panel of the receiver. The gear box was slightly changed to have an additional idler gear and internal stops.
Electronically, there is no difference between the RAO-7 and the RAO-9 with the exception that, in the RAO-9, the two RF amplifier inputs each have a RC filter installed between the grid and the coil selected. These RC filters were low-pass types to keep VHF radar from interfering with the receiver's operation onboard ship.
Original receivers will have the Navy-style coaxial connector. Since these connectors are relatively difficult to find, many RAO receivers will have SO-239 coaxial connectors installed. If you still have the original Navy coaxial connector on your RAO, use a double female UHF coax adapter. The adapter may fit better if the threads are slightly filed until the adapter fits fully into the Navy connector and contacts the center pin. Then just connect the PL-259 up for the antenna connection. That preserves the rare, original Navy connector. Audio output is available either at the rear terminals or the front panel PHONES jack. Both are direct connections to the 600Z output transformer.
RAO-7 Serial Numbers and Contracts - There were two contracts for the RAO-7 (NXsr-38306-RAO-7 and NXsr-55614) and two contracts for the RAO-9 (NXsr-38306-RAO-9 and NXsr-85045.) A total of 1838 RAO-7 receivers were built and a total of 202 RAO-9 receivers were built. All contracts were issued between Sept.1943 and Dec.1944. Serial numbers on the data plates were assigned when the fully assembled receiver was used to fulfill a USN contract. The serial number on the chassis was assigned and stamped prior to production assembly of the receiver. These two serial numbers never match and shouldn't. They are different formats with the data plate SN being just numerical and the chassis SN being a letter prefix followed by numbers.
photo above: RAO-7 SN: J444 - this receiver was destined for the South Pacific so it was heavily coated with MFP, inside and out. Even the knobs and data plates were given a coat of MFP.
Data plate serial numbers appear to be non-sequential on these receivers. Serial numbers within the sequence may have been assigned to different accessories within the contract (e.g., the spares kit.) In fact, some data plate serial numbers are higher than the total quantity of receivers built for the contact. From this, one can infer that the numbers within the sequence must have been assigned to other items. To further the confusion, the two "J" production run RAO-7 receivers examined appear to have had their chassis serial number restamped (with the original stamping obliterated.) The build quantity of each production run is unknown but NAVSHIPS 900,356 indicates that 765 RAO-7s were on the first contract and 1073 RAO-7s were on the second contact. However, it appears that production runs aren't directly related to the contract quantities since it appears that National built somewhat ahead and then used completed receivers to fulfill the contracts.
More confusion can result in the possibility that the data plates found on a particular RAO receiver may not be original to that receiver. Of four RAO-7 receivers that have been examined closely, two positively have data plates that came from other receivers, another receiver's data plate originality is suspect and only one of the four (SN:10/H720) is positively the original data plate. The upshot of all of this is,...consider yourself lucky if your RAO-7 even has a data plate since many were swapped to different receivers during WWII for various reasons and many more were entirely removed post-WWII. Contemporary collector's have also swapped data plates from junk receivers to receivers being restored.
The stamped chassis serial number will provide some indication of the receiver's approximate build date. From reported serial numbers, it appears that production run prefixes start with "H" and end with "K" and this may help with possible dating with the early prefixes dating from late-1943 and the late prefixes being around early-1945.
photo left: RAO-7 SN: 10/H720. Note that the separate chassis for the 1st RF catacomb is still in use (note the junction "seam" just at the edge of the power input panel bracket.) The side panels provide rigidity for the chassis since it is now mounted in a complete cabinet rather than the sheet metal "pieces" that make up the earlier type cabinet. Other improvements are to the antenna input shielding and the addition of a panadaptor output - the SO-239 connector. Note the almost "sealed" RF amplifier box which requires removing the top cover to access the first RF amplifier tube. This receiver interior was never given a MFP treatment. Note that the RAO-7/9 chassis is cadmium plated.
|Post-WWII Surplus Market - The RAO receivers didn't make it to the surplus market immediately after WWII ended. Probably because the receivers were still useful to the Navy for various purposes. The companion regenerative LF receiver, the RBL, did hit the surplus market by 1947 for a price of about $70. The Signal Corps version, the NC-100ASD was available from Newark Electric in 1948 for $115 (included matching loudspeaker.) A contemporary of the RAO (but not a superhet) was the RAK and the RAL TRF regenerative detector receivers. These were $225 for the pair complete with power supplies and cables in 1947. If the RAO could have been found surplus it probably would have been priced around $125. I looked through Radio News magazines that were "loaded" with ads from military surplus dealers all around the country. The magazines dated from 1946 up into 1950. Although there were a lots of mil surplus electronic gear available, I didn't find any dealer that was advertising the RAO receiver during that time period. However, it's obvious that at sometime after 1950, the Navy did dump their RAO receivers onto the surplus market. When I find an advertisement showing a surplus dealer offering a RAO receiver I'll add it to this section with the date and price.
Performance - All of the versions of
the RAO receivers have impressive sensitivity and freedom from images. Dial
accuracy is usually quite good given that the resolution is limited. Use
the logging scale for accurate reset ability. The
tuning rate, which is standard for the National NC-100XA Series, seems
fairly fast but it's still easy to tune in CW signals and SSB can be
"fine tuned" using the BFO control, if desired. The audio output was modified
(requested by the Navy) for
the RAO series to eliminate the P-P output in favor of a single audio
tube stage that was
primarily designed for CW reception using earphones (although an
optional loud speaker is shown in the manual for some versions.) Audio output of the RAO, especially the later
have the bass rolled off significantly since this enhanced the copy of CW signals. The audio peak response is at 800hz to 1000hz which favored CW
copy. Overall audio response is -3db down at 290hz and at 3000hz which
became the standard for "communications audio." The bass response
in the RAO-7 is
-10db down at 125hz with the Tone control set to "N." The
RAO-3 spec is -6db down at 125hz at "N." The AVC action is
quite strong and will require advancing the AF Gain substantially when
switched in. The Navy radio ops rarely used the receiver in the AM mode
and the amplified AVC bias was probably designed and intended for AM
Broadcasts. Using the correct 600Z ohm
load (use a 600Z to 8Z matching
transformer if necessary) will help the audio response in the AM mode somewhat.
Stability is very good with negligible drift after a 15 minute warm-up.
All RAO receivers (RAO-2 and later) don't have remote standby
capability. The rear panel auxiliary plug was provided to allow battery
connections for the possible necessity of emergency operation (a USN shipboard requirement.)
Using any stock original RAO receiver paired with a transmitter will require using the B+ OFF position of the POWER SUPPLY switch for standby. As a station receiver the RAO will have very good sensitivity up to about 20mc or so (a directional "gain" antenna such as a yagi or quad should be used for higher frequencies,) excellent selectivity (use the crystal filter,) freedom from images, very good stability after a short warm up and good communications audio if a properly matched speaker is utilized. Since the dial readout is vague due to limited resolution, use a frequency meter (or RF signal generator) in the initial setup to exactly tune desired receive frequencies. The logging dial and scale were provided for accurate reset ability so write down the logging numbers and afterward use the receiver's logging dial and scale for accurate receive frequency set up. Best performance will be attained using a matched, resonant antenna. Antenna Input Z is approximately 70 ohms. Using a directional gain antenna such as a yagi will allow operation up to 30mc. I used my RAO-3 with a rotatable dipole as a Novice for 15M crystal-controlled CW operation in 1970. I was able to work as far away as VE3BAW in Ontario, Canada (on 21.212mc.) However, most vintage military radio nets operate in the 75M phone band nowadays where the RAO does a fine job with a typical wire dipole antenna although QRM will sometimes require using the Crystal Filter or "off frequency" tuning.
The RAO-2 and later versions do not use any electrolytic capacitors in the circuit. This has resulted in a typical, good condition, "as found" RAO receiver that will function fairly well on "all original parts." The paper-wax capacitors were the best that could be purchased at the time and the resistors were generally JAN-types that seem to be very stable as far as value drift. Most of the problems with the RAO receiver today are due to poor storage conditions, dings, dents and crunches that probably resulted from the weight of the receiver and trying to move it around in a storage environment. The cellulose-acetate dial cover will warp, darken and shrink in poor storage conditions and that is a very common problem but, luckily, it's very easy to fix. Careless or incompetent workmanship is also encountered fairly often and certainly "hamster" modifications also do show up from time to time. I think the overall size and weight of the RAO has greatly limited the amount of ham-induced problems,...it was just too big and exhibited way too much black wrinkle paint for a station receiver for most hams when it became available surplus in the 1950s. Excellent condition RAO receivers that have always been stored inside in a favorable environment and do operate should be checked over to be sure that nothing is getting hot during operation and that the voltages are about where they should be. At that point, the receiver should be aligned for best performance.
When I restored my RAO-3 with new rebuilt capacitors, I did notice an improvement in the audio but there were so many non-original parts in the circuit, it's difficult to tell whether the capacitor rebuild did much other than remove a "worry factor" about operating an electronic device with three quarters-of-a-century old paper dielectric capacitors. I don't intend to "re-cap" my all-original RAO-7 even though its components are just as old. It was stored inside at the Alameda Navy Air Station for years where even the outside climate doesn't change much. This type of consistent temperature storage environment has certainly helped to preserve the components and the cosmetics. Certainly poor storage in outside sheds or unheated garages will result in the most damage and these types of receivers will certainly benefit greatly from a thorough rebuild that includes new polyfilm caps to replace the paper-wax originals. On the 4uf oil-filled, paper dielectric filter capacitors,...check them for value and look for any seeping of the oil. If the value is correct and the seal is good, then these capacitors are very stable and will function correctly indefinitely. Be sure to check these capacitors at full B+ voltage and monitor the leakage current using an analog current meter. If you see any sudden meter needle movements that usually indicates arcing inside the capacitor. It doesn't happen too often but it should be checked to be sure the oil-filled nature of the dielectric is functioning correctly at voltage.
Navy Department - Panoramic Radio Corp. - RCX Panoramic Adaptor SN:46 - Using a Panoramic Adaptor with the RAO-7 (455kc IF) makes for an interesting receiver set-up. Panoramic Adaptor coupling is thru a series 50K resistor inside the RAO-7 internal coaxial cable from the Mixer plate. There is a SO-239 connector on the rear of the RAO-7 and also a SO-239 on the rear of the RCX panoramic adaptor. All that's required is to connect the two SO-239 connectors together with a RG-58 cable that has PL-259 plugs on each end.
WWII panoramic adaptors aided surveillance by allowing the radio operator to see instantly if any unknown signals appeared within the slice of the spectrum he was monitoring. This eliminated the constant tuning that was necessary for receiver-only surveillance or for "guarding frequencies" using multiple receiver set-ups. The panoramic adaptor was primarily used for intercept of unknown signals. It wasn't necessarily used for signal analysis other than it was obvious to the radio op if the signal was CW or if it was somehow modulated. Surveillance and intercept was the panoramic adaptor's main role in WWII.
Shown in the photo to the right is Navy RAO-7 SN:10/H720 operating with Navy RCX Panoramic Adaptor SN:46. The RAO-7 is tuned to a weak SW BC station that is about 20kc down frequency from Radio Havana which was operating on 11.75mc in the 25M Shortwave Band. Radio Havana presents a strong AM signal as shown by the large peak at the right of center of the display. The total slice of spectrum showing is about 100kc or +/- 50kc each side of center. By tuning the RAO-7 up frequency 20kc, the large peak would move to the left on the CRT and, when centered, Radio Havana would then be within the RAO-7 IF passband and would be receivable. Scale is -50 -0+ +50 in kilocycles. Other graduated scales were available.
The RCX Gain is usually adjusted to just show a slight indication of noise at the bottom of the display with the receiver not tuned to a transmitting station. This will generally show any signal above the noise level. Extremely strong signals may require a slight reduction in the RCX Gain to show the peak of the signal. Also, receiver RF gain can be reduced if necessary. If the sweep is expanded, more details of individual signals can be observed. At +/- 50kc sweep only general characteristics can be observed. Modulation can be easily seen although its rapidly changing characteristics on the display doesn't photograph well. Comparative amplitudes can be easily seen.
The adjustments behind the "toilet seats" are for setting the display to be positioned correctly within the graduated scale. Intensity and Focus adjust the quality of the trace. Center Frequency allows other receivers with somewhat different IFs to be used. The RAO-7 IF is 455kc but the HRO receivers used 456kc and the Hammarlund Super-Pro receivers used 465kc. Any of those receivers are well within the adjustment range of the Center Frequency control (450kc to 475kc.) Sweep allows setting the approximate spectrum range of display from 0kc up to 200kc. The setting of ~ 100kc allows the -50-0-+50 scale installed to be easily used to measure where other signals are located in reference to the tuned signal.
|RCX Rework - Many panoramic adaptors will work on all original parts but some, like RCX SN:46, need a little help in the form of troubleshooting/repair to get them functioning correctly. Problems with SN:46 were as follows,...the AC power cable should be a three-contact "twist-lock" type with the RCX's welled-socket male receptacle requiring a female "twist-lock" plug. A former owner had (ugh!) soldered a two conductor "zip cord" to the connector pins for the power cable. I had to remove all of the solder residue (lots of solder-wick) so that an original "twist-lock" plug would fit into the socket receptacle correctly. I made up an AC power cable with the correct style plugs. A quick check of the chassis indicated it was safe to power-up the RCX. The resulting trace was very distorted and wouldn't respond to adjustments. Tubes were tested with only two measuring "at minimum acceptable" all other tubes tested good. It was noted that on the component boards the resistors all had somewhat extended, wrapped leads allowing easy "disconnecting" of one lead for an accurate resistance measurement. Seven resistors were significantly out of tolerance with four being over 100% out and three being over 400% out of tolerance. These resistors were made by Erie Resistance Corporation and this manufacturer's resistors are very prone to drift in value (especially after about 75 years.) Replacement of the seven resistors (plus a check of all capacitors) got the RCX functional with adjustments working as expected. Additional rework involved replacing the rotted rubber boot the surrounds the front part of the 3JP1 CRT and installing a correct type green filter with graduated scale (actually from a RBV, hence the -/+50 scaling.) Also, all four shock mounts were missing but luckily correct type replacements were found in the junk boxes. Cosmetic condition was very good so only minor cleaning and touch-up was required. The RCX is a very nice addition to the RAO-7 receiver and it does provide some interesting visual information, although it can be irritating to not only hear adjacent frequency QRM but to also "see it."
The National Co., Inc. RAO-2 and RAO-6 used mounting base CNA-10125. The Wells Gardner & Co. RAO-3, 4 and 5 used the same mounting base but with the designation of CWQ-10125-A. The receivers will have in each lower corner, both front and back, inserts that are mounted to the inside of the cabinet sheet metal and provide a threaded barrel for the four shock mount thumb screws to utilize for securing the mounting base to the receiver. The heavy duty "shock" feet have metal spacers that are suspended in rubber. When the RAO receiver was installed onto the desk where it was to be used and that installation was onboard a ship, then the mounting base was bolted to the desk. If installed in a shore station, whether the mounting base was bolted to the operating desk would depend on the situation. The photo left shows the top of the mount where the receiver is placed. The photo right shows the underneath of the mount and the shock feet (that were over sprayed with black paint - they should have a silver-color metal housing.)
The CJS-49493 loudspeaker was 6" in diameter, mounted in a black wrinkle finish box and the grille consisted of square wire mesh. Internally, there was a multi-tapped matching transformer that allowed selecting various nominal impedances from a low of 6Z up to a high of 20,000Z by way of a terminal board and wire connections. 600Z is one of the selectable impedances. Also included in the lower right corner of the front panel was a Volume control which was connected in series with the transformer secondary (lo-Z to the voice coil.) If the listening environment required the radio op to switch-over to phones, he could plug his phones into the RAO phone jack and then reduce the CJS-49493 Volume to the lowest setting (fully CCW) since plugging in phones didn't disconnect the loudspeaker. Typical of Navy loudspeakers, the connecting cable is a two-conductor shielded but un-jacketed cable. There is a grounding screw on the RAO receivers that is located near the SPKR terminals for connecting the loudspeaker shield to the receiver chassis-ground.
The RAO-7 and RAO-9 only list a loudspeaker as "equipment not supplied with receiver." The manual only shows a line drawing representation of a loudspeaker in one section and only the 600Z ohm impedance is mentioned.
The photo above-right shows the CJS-49493 Loudspeaker. The data plate is missing on this example but the Circuit Label inside the cabinet showing how to route the wires for the various impedances also shows the speaker's USN identification. The internal label is shown to the left.
Nearly all vintage photos showing RAO receiver set ups appear to have multiple operators using multiple receivers. All radio ops are using phones since multiple receiving stations precluded using loudspeakers. Today, any loudspeaker that has an input load of 600Z ohm will function correctly with any RAO-2, 3, 4, 5, 6, 7 or 9. The early RAO had a matching panel speaker with a 20K impedance matching transformer and RAO-1 used a 5000Z ohm output transformer and table cabinet speakers. Each would require a proper nominal impedance load to operate with the best efficiency.
Vacuum Tubes Used in the RAO and RBH Receivers
subsequent versions: RF1,2 =
6K7(2), Mixer = 6J7, LO = 6J5, IF = 6K7 (2), Det/NL = 6C8, BFO = 6J7,
1st AF, AVC = 6F8, AF Output = 6K6, Rect = 5Z3
RBH, RBH-1 and subsequent versions: RF =
6K7, Mixer = 6J7, LO = 6J7, IF = 6K7(2), Det/NL = 6C8, 1st AF/AVC = 6F8,
BFO = 6J7, AF Output = 6V6, Rect = 5Z3
Serial Number Log - Collected Serial Numbers for RAO Series and other Military Catacomb Coil Box Receivers
These serial numbers have been provided by other collectors that have responded to the "National Moving Coil Series of Receivers - NC-100 Series" article, specifically for the Serial Number Log. If you would like to send your RAO, RBH, NC-100ASD or R-116 serial numbers in, I'll list them here, in this log. Send your serial numbers to: RAO, RBH, NC-100ASD, R-116 SERIAL NUMBERS
RAO, RAO-1 Initial Models - National Company -
RAO-2 Series - National Company Build (RAO-2, 6, 7 or 9) - 10/H720 (RAO-7), nodp/J444 (RAO-7), 1642/J944 (RAO-7), nodp/K155 (RAO-7), 843 (RAO-2),
RAO-3 Series - Wells Gardner & Co. Build (RAO-3, 4 or 5) - 1064/na (RAO-3), 1412/na (RAO-3),
RBH Initial Model - 840,
RBH-1 Series -
NC-100ASD - Signal Corps - 194, 402, 426, 931, 948, A49**, A578
R-116 - U.S.C.G. - 46, 103,
xxx/xxxx = this format shows the data plate serial number first and the chassis stamped serial number second
nodp = no data plate,...serial number from chassis stamping
(xxx) = specific model receiver that sn assigned to
na = not available,...either not provided or not accessible or possibly not used
** = confirmed sn, although sn is in an unexpected format. A possibility is that "A" indicated "1000" and "A49" would have been receiver number 1000 + 49 or 1049. A578 would be "1578" - maybe,...just a hypothesis.
Restoring and Rebuilding RAO Receivers
|When I was a 19 year old kid, I did a short stint at the local Post
Office that involved substitute deliveries. I generally was at a
different post office and different route each day. While delivering the
mail on one of those "new to
me" routes I came upon a house that had an old National receiver setting
in the drive-way. A lot of weeds around the easement and grease in the driveway but the receiver
looked pretty good from a distance. A walked over to the derelict
National and noticed that it had "Wells Gardner & Co." on the badge instead of
National. I lifted up the lid on it and saw the inside was full of small
rocks and pebbles. Not too many tubes but lots of rocks. I knocked on
the door, handed the resident his mail and pointed over at the
receiver in the drive-way but I didn't get a chance to say anything
before the guy blurted out, "Oh, you want that radio? Go ahead and take
it. It's your's." I thanked him and loaded it into the back of the mail
truck. I finished that route in record time so I could get off work and
go down to see my old friend, Phil Rios.
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, 6J7, etc. After "tubing up" the RAO, we found a power cord that sort of 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 cover 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 600 ohm audio output impedance? Well, I installed the new transformer inside the old transformer's 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 (for one thing, I used a 15M rotatable dipole for the antenna.) Exciting stuff then. One thing to consider,...in 1969 this RAO-3 was only a 26 year old receiver (the paper caps have aged over half-a-century since I used this receiver as a Novice.) I doubt the performance today on all original parts (in 2021) is anything like it was in 1969. 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've always kept it in storage through several moves.
|The 2006 Cosmetic Refurbishment
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 Anttila, WA6LHL, another mentor of mine.) I remember that his receiver's dial cover was cracked, so it had to be glass (glass IS correct for the ASD.) Like most RAOs, the plexiglass on this one was severely scratched up and discolored, so glass went in. Thirty years later, after realizing for some time that the glass was incorrect for the RAO, during this rebuild I replaced the glass with a correct plexiglass dial cover. NOTE: The new material used is Lexan, which is available in the correct .090" thickness. Lexan is highly resistant to cracking or discoloring. Original RAO dial covers were cellulose acetate, a very unstable plastic that was photosensitive resulting in darkening to a semi-transparent brown color. It could also easily shrink in size, warp and could become somewhat opaque if exposed to a lot of moisture.
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 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 a lot of heat, just moderate heat 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 and a month is about how long the paint needs to fully harden.
- 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 originally had 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 rectangular control nomenclature panels were
straightened and then repainted. Mounting of these panels, which was 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
during reassembly. The fake rivet heads were painted black to look like
originals. If I were doing this today, I'd use the correct
The original RAO-3 shock mount turned up when a local collector-ham bought the shock mount thinking it would work for his RBL receiver but the RBL cabinet has less depth so the shock mount thumbscrews couldn't be tightened at all. Since it wouldn't work for the RBL, the shock mount was given to me for the RAO-3. I believe this was around 2015.
The filter capacitors are not electrolytics - there are no electrolytic capacitors used in the RAO receivers. The filter caps 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 would be recommended for best and safest performance, however I didn't do that since I wasn't planning on using the RAO-3 as a station receiver. Maybe that will change in 2021,...and it did.
Further Inspection of the "Old RAO-3"
1. All tubes tested good except 6J5 LO
tested odd. Decreasing gm as test button held down which usually
indicates a tube at the end of its life or possibly a gassy tube. Might have caused
the decreasing sensitivity (it did.)
Jan 24 - Started rebuilding the paper-wax capacitors. I'm removing a small quantity of original capacitors from the "parts set" and then rebuilding those. Once that quantity is finished I then remove the same value caps from the old RAO and then install the rebuilt ones. I rebuilt the six paper-wax caps on the power supply side of the chassis. These along with the three under the RF box total nine caps rebuilt and installed so far. Both the "parts set" RAO and the old RAO have a few non-original capacitors so a few of the same value caps from the old RAO will need to be rebuilt and then reinstalled into the positions that had non-original maker caps. The end-result will be that all paper-wax capacitors will appear to be original CBV types but will have modern polyfilm capacitors inside.
Jan 28 - Replaced the three newer style dipped mica capacitors with original style, tested vintage mica caps (this was for under-chassis appearance only.) Some of the original mica capacitors used in both WG RAO-3 receivers seemed to have defective leads. The lead wire seems to be very brittle and easily breaks when trying to extract the capacitor. One of the original receiver problems back in 1969 was due to a broken lead on a mica capacitor. It's probably why the three dipped mica caps were installed. My thought is that if an original mica cap can be extracted from the "parts set" without damage then the leads are probably not defective. About 50% of the mica caps in the WG RAO-3 receivers seem to be affected. Part of the "test" besides measuring the value was to determine that the lead wasn't brittle.
Replaced the grid leads to the 6C8G and the 6F8G tubes. The "parts set" wires were in better condition than those in the old RAO. I was going to replace the S-meter lamp wiring because the two wires were "friction taped." But, the wires are within a laced harness and removal of the wires would probably break the laces. Instead, I replaced the friction tape with original black spaghetti tubing which looks more convincing as original. The wire from the S-meter switch to the meter also had friction tape on it so I used the good condition wire from the "parts set" as a replacement. Grid connection wire to the 2nd RF amp had friction tape so repaired with black spaghetti tubing. Installed the NOS S-meter and installed original lugs for the meter connections, also used original meter stud nuts and washers. Installed the original type of S-meter lamp socket. I had a screw base #40 lamp installed, should have been bayonet base #47. The rubber grommets on the lamp socket are to prevent pushing the lamp into the S-meter too far which can overheat the meter scale and discolor it.
I added two fiber washers between the back of the logging dial hub and the front of the gearbox hub to take up some clearance that had been allowing the tuning shaft to have about .090" thrust end play. Mechanically set the logging dial to be accurate.
Functional Test - Installed the tubes and cleaned the pins with brushed on DeOxit and then plugged in and out a few times. Connected 600Z ohm loudspeaker, the AC power cord and a test antenna. Power applied with variac with no problems. Full scale S-meter indication in AVC and no signal (AVC at max negative bias, apparently.) Loud signals in MVC, tested three bands with loud ambient noise levels, tested Crystal Filter OK, tested BFO OK, tested Tone OK, tested NL OK, RF and AF gain OK. Receiver is almost fully functional with just an AVC problem. When the AVC problem is repaired, then the receiver will be reassembled.
Jan 29 - Even though the coupling capacitor from the detector/3rd IF transformer to the grid input of the AVC amplifier tube (a 50pf mica) seemed to measure okay on the cap meter, it apparently was open. Installed another vintage 50pf mica and that got the AVC functioning. Synchronized the band indicator to the catacomb position - it has to mesh with the pinion gear on the correct "tooth" of the drive gear in order for the correct band indicator to be in the 12 o'clock position when that band is engaged at the catacomb pin receiving contact blocks. Installed the combination front panel-side panels part of the cabinet. Installed the lid, the band change bezel, the dial escutcheon and the "back porch" cover. All original screws and lock washers used in all of the mountings. Most of the black wrinkle finish truss-head screws needed cleaning and touched-up. Installed the four threaded adapters that are for mounting the receiver to the shock mount. Lightly lubed (with grease) the two round rods that the two catacombs ride on.
Jan 31 - Operated the RAO-3 yesterday and this morning to allow any unusual issues to turn up but the receiver performed quite well. I was thinking about not doing an alignment because the sensitivity was okay and the tracking was very good. Luckily, I did do a full IF/RF alignment in the afternoon. The IF was slightly improved with the alignment. Tracking also was very close but Band C did need the lower end slightly touched-up to be accurate on the 40M band. Audio is very good although it is "communications grade" but the bass response isn't lacking, it just isn't emphasized. The problem of the slight frequency change in the BFO as the RF Gain is adjusted in MVC is now only encountered on Band A and even there it's just barely noticeable compared to what it was prior to the rebuild.
Return to Original, Mechanical Restoration of RAO-7 SN: J444
August - September 2018
Attention to detail is very important in any restoration. Correct mounting hardware and period finishes along with careful workmanship will result in an original appearance to any restoration. Without having a totally complete and original RAO-7 (SN:10/H720) as an easily accessible reference as to "what's correct?" I don't think I'd have been able to impart as much exacting detail into the mechanics of this restoration. Maybe this write-up might be "too much detail" on the mechanical aspects of the restoration but, if you're interested in minutia and appreciate the excruciating OCD-aspect of "attention to detail," then all of the work that was necessary in this restoration is covered in the sections that follow.
|Another RAO-7 made it into the shop in August 2018.
It seemed that a fellow military radio enthusiast had convinced a friend
of his to allow him to "store" this RAO-7 in the back of that friend's
Subaru Outback. After a few months of this sort of "temporary" storage,
the friend had back surgery and afterwards he wanted the "heavy" RAO-7
out of the back of his Subaru. As an unfortunate coincidence, about that
same time, the RAO-7 owner himself had hernia surgery. With both guys
"laid-up" and unable to lift anything, I was contacted (since
I was a mutual friend.) I was told that if I
would move the RAO-7 out of the Subaru, I could have it. Now, I'm not
sure if this RAO-7 was responsible for the former owner's hernia or his friend's back
problems but the friend was adamant that the RAO-7 had to be removed
from his car. I met with the
Subaru owner and I moved the receiver from the Subaru into my Toyota FJ
- a feat that made everyone involved happy, I guess.
The weight of the RAO-7 is 75 pounds but 20
pounds can be shaved-off by taking the receiver out of the cabinet.
Two restoration attempts had been made
by two different former owners over a period of a couple of decades. The
former owners never seriously got into the electronics so the receiver
chassis was all original. Having never been molested, the receiver
actually functioned fairly well. Chassis serial number was J444.
|Fixing Mod-Hole Damage and
Other Hamster Mods - The original panel was actually in very
good condition. Really, all that was required was to repair some minor
modification damage. Luckily, none of the modifications were ever
installed, only some of the preliminary "hacking" was done. There were
two .375" drilled holes, two .250" holes and a "ring" of center-punched
markings where an S-meter was going to be mounted.
I used All-Aluminum Body Filler (ABF) to patch all of the holes. This was then sanded, primed and painted with black wrinkle finish (BWF.) I hand painted the ABF patches with BWF and then used a heat-gun to actuate the wrinkle. After this BWF paint had set for a few days, the entire panel was wiped down with thinned black nitrocellulose lacquer to even-out the color and bring out the luster. All of the patched holes except one were covered by data plates when the receiver was fully assembled.
The escutcheon had a .030" plastic dial cover partially held in place with duct tape. The original dial covers were .090" thick plastic (cellulose acetate) and didn't need duct tape to hold them in place. The logging scale pointer was bent incorrectly to allow for a non-original mounting of the thin plastic dial cover. This had the pointer mounted with a screw and nut through the plastic of the dial cover. The original mounting had the plastic dial cover "notched" to provide clearance for the pointer which was then bent to have the pointer against the backside of the dial cover plastic. The pointer should be mounted to the escutcheon using a flat-head rivet.
A non-original hole was drilled into the escutcheon and into the lower part of the tuning shaft grounding fork to allow a screw and nut to mount the fork into a non-moving position for some reason. This was removed allowing the fork to be able to pivot on its mounting rivet. The drilled hole was filled with ABF and painted.
|Finding the Correct Hardware
There are literally dozens of major hardware dealers on eBay
that specialize in supplying various types of fasteners. I'm surprised
that nearly always, with enough searching, the exact size and type of
screws or rivets can be found. The down side is that there will be a
waiting period of a few days to a week for delivery. Planning the rework
required in advance so you know what needs to be ordered ahead of time
helps to reduce acquisition delays.
Weird Screw Sizes and Odd Rivets - Original escutcheon mounting screws are 4-40 oval head phillips machine screws 5/16" long. These had to be purchased. These screw heads had to be painted flat black before installation (originals were black oxide finish.) Original data plate mounting screws are 4-36 fillister head slotted machine screws 3/16"" long. I had six originals and needed two more. I made two fillister head 4-36 screws out of round head 4-36 screws. I turned the screw in a drill press and used a fine file to shape the round head into a fillister head.
The nomenclature plates on the dial escutcheon had been incorrectly mounted with screws and nuts. I had to order the correct 3/32" diameter by 1/8" length semi-tubular rivets to mount these plates as they originally were mounted. These rivets were vintage and had the correct "raven finish" (black.) I used a .125" sheet metal punch as a rivet set tool and a solid copper plate for the backing tool.
As mentioned, the logging scale pointer had been mounted with a screw and nut. The pointer itself needed to be reshaped correctly. It was then mounted as original with a flat-head rivet that was 3/32" diameter and 3/16" long. This was a solid aluminum rivet so a center punch was used as a setting tool and a copper plate used for the backing tool.
All of the round nomenclature plates had been mounted with screws into tapped holes on the repro panel. On the original panel these had to be correctly mounted with #2 x 3/16" drive screws. The drive screws were ordered so these plates could be mounted as original. The drive screws were round head so before they could be installed the head had to be reshaped into a pan head. This was accomplished with a file. After the drive screws were installed their heads were painted black, as original. (I ordered round heads so these drive screws could be used on other restorations. You can always make a round head into a pan head but you can't go from a pan head to a round head.)
The bezel surrounding the band select dial was mounted with incorrect 4-40 screws on the repro panel. The screw holes in the original panel are tapped for 3-48 screws. The original screws were 3-48 flat head slotted machine screws. I found the local Ace Hardware had 3-48 FH screws but in phillips. Since the main tuning dial escutcheon is original-correctly mounted with oval head phillips screws, I decided to go ahead and use the 3-48 FH phillips. The heads of these screws had to be painted black before installation.
Screw Assemblies - The thumb screws are 12-24 thread with a
knurled and slotted head that is 9/16" diameter and with a shoulder and
thread length of 15/16". These mount into a standoff that is 1/2" in
diameter and about 7/16" tall. The standoff has a smaller diameter
shoulder that is press-fit into the panel mounting hole and is then peened in place. All of the original captive thumb screw
assemblies were missing. New captive thumb screw standoffs had to be
made from 1/2" diameter brass stock. I had Chuck, KŘDWC, machine nine
standoffs each with a clearance hole for a 12-24 screw. The back of each
standoff was drilled out to 5/16" diameter to a 3/8"depth. I
press-fitted these standoffs into the panel and then the backs were
peened with a rectangular punch (as original.) The standoffs were then
painted with black wrinkle finish paint that was activated using a heat
The original size 12-24 thumb screws can't be found as new items. I really couldn't find anything except original RAO-7 thumb screws that were the correct dimensions and would function as "captive" screws. Since the panel-mounted replica captive thumb screw standoffs were built exactly like the originals, if original RAO thumb screws are found, they can be fit into the standoffs. This isn't as easy as it sounds. Each RAO thumb screw has a boss just behind where the 12-24 threads stop. This is to retain a small "C" clip. The thumb screw is inserted into the captive screw standoff from the front. From behind the standoff, the "C" clip is installed onto the thumb screw threads and then pushed over the boss. The boss keeps the clip from sliding off of the threads and the clip is large enough to keep the thumb screw from being pulled out the front on the standoff.
The only solution is to have nine RAO thumb screw replicas made. Probably an expensive solution to the missing parts. For the time being, I can use the five RAO thumb screws that had been somewhat modified by a former owner and ruined for authentic restoration use. Although they will look original and function to hold the panel to the cabinet, they won't actually be "captive" thumb screws.
|MFP without the
Odor - The receiver ID data plate, the "Notice" tag and the
acceptance tag were not MFP'd. This was because these three tags must
have come from a different RAO-7 receiver. Since the data plate serial
number and the chassis serial numbers never match on these receivers
(they are different formats and shouldn't match,) these data plates will
be acceptable with a little MFP touch-up. I used a yellow-tinted
nitrocellulose lacquer that has the appearance of MFP when applied as a
spray. This is applied highly thinned so the correct color can be
gradually attained by thin, successive coats. The data plate screws also
have to be MFP'd since the data plates were sprayed after they were
mounted. All of the nomenclature tags and the one nomenclature panels
were in good condition with their original MFP. The second nomenclature
panel needed a minor touch-up due to finger wear from operating the B+
switch. Thinned black lacquer applied with a brush and then sprayed a
very light application of MFP (actually, my yellow-tinted lacquer.) In
the original MFP application, the knobs were also sprayed with MFP. I
cleaned the knobs with Glass Plus and a soft bristle brush. Whatever MFP
was on the knobs was left on but I didn't spray any additional MFP. If
you can't find yellow-tinted lacquer or can't find someone to mix you a
batch, another MFP substitute is Amber Shellac. This is thinned with
denatured alcohol and sprayed. You can also lighten the Amber Shellac by
mixing in a little Clear Shellac to get the correct MFP color. The only
slight disadvantage of using shellac is that it isn't very durable.
However, for our "non-military, not it constant use" application it
seems to work fine.
Photo right shows the RAO-7 SN: J444 chassis and the appearance of a chassis that has had the MFP treatment. Note that obviously the tuning condenser wasn't coated. Most of this MFP is original. What I've done is to coat the cover on the antenna input-1st RF amplifier box which was not coated since it was an original-part replacement cover. Also, the AC power input receptacle was a NOS replacement and had to be MFP coated.
|The Plastic Dial Cover - All of the originals dial covers were made out of an early type of plastic that warps, discolors and becomes opaque with age and exposure to water or light. The original material was cellulose acetate, a type of celluloid that is very sensitive to water. If water is used to clean the dial cover it will actually begin to dissolve the surface and "fog up" the material. Original cellulose acetate dial covers should be cleaned only using clean mineral oil. Modern Lexan in a 14" x 11" sheet is available in .090" thickness from Home Depot. Lexan is very easy to cut and very resistant to cracking. I cut the semi-round cut-out using a hole saw first. Then the full dial cover is cut out using a jig-saw leaving the round hole now being a semi-circle. I added the small notch for the logging dial pointer using a Dremel tool and a round file. The original dial covers had a thin boarder of masking tape but this was probably because the old plastic had a tendency to crack. The masking tape can be added for authenticity but it isn't really necessary with Lexan. The dial cover sets on the two lower tabs and the notch provides relief for the logging dial pointer. That's all that holds the dial cover in place. When the dial escutcheon is mounted to the front panel, the plastic dial cover is then held in place securely.
|photo left: The new Lexan dial cover showing how
the dial cover should just fit below the middle screw hole of the
escutcheon and between the debossed panels of the control plates. Note
the the bottom of the dial cover rests on the two tabs on each debossed
panel. Although the original dial covers had a border of masking tape it
isn't necessary with Lexan material.
photo right: Close-up of the rounded cut-out for the tuning shaft and the notch that provides clearance for the logging dial pointer. Note the the pointer is bent to allow it to clear the dial cover yet lay flat against the back of the plastic. In both photos the protective plastic sheeting hasn't been removed from the Lexan.
|AC Power Input - The RAO-7 originally uses a twist-lock, three pin male, welled receptacle for the AC input. The power cable originally was two conductor with a shield. At the time, many Navy AC sources had "floating" AC lines, that is, the neutral wasn't grounded. Since the circuit uses a power transformer, it doesn't matter that nowadays neutral is grounded. Also, most RAO-7 receivers don't have their original shielded power cables anymore. It's okay to use a three conductor cable and use the ground pin of the twist-lock as the chassis ground, it's tied to chassis anyway. This RAO-7 had been modified to use a modern test-equipment type of rectangular receptacle and a molded AC test equipment power cable. Luckily, I had the correct, original type of welled, AC twist-lock receptacle which was installed so an original type RAO-7 power cable can be used with the receiver. I applied MFP-color lacquer so the new socket would match the other components nearby.
|Retaining Screw - The front panel has a 6-32 hex head screw mounted with an off-center hole washer and a spacer washer to the backside behind the dial escutcheon on the AC/B+ switch side. This screw end is normally deformed so it can only be loosened, not removed. The purpose of the screw and washer is to provide a firm clamp for the sheet metal bracket the holds the AC/B+ switch and the AVC-MVC-CW switch. The original screw was broken off of the panel. I drilled out the remaining piece and chased the threads of the hole. I found a hex head 6-32 and made a off-center washer. This was threaded into the panel from the backside. During reassembly of the receiver, this screw was tightened against the AC/B+ switch bracket. I had to make sure the screw wasn't too long since the dial escutcheon has to mount over it. I then loosened the screw and deformed the end of screw so it couldn't be totally removed. I then retightened.
|Other Hardware and
Miscellaneous Stuff - The grab handles were badly nicked up
with a lot of missing paint. I sanded these to even out the surface and
then painted them with black nitro-lacquer. On top of the crystal filter
assembly two solder "globs" were indications of some sort of past
hamster work. I removed the solder and then "touched up" the area with MFP to match the rest of the crystal filter box.
The RF input box cover wasn't MFP'd. I thought it was replaced sometime in the past and it should have been coated when the receiver chassis was sprayed, so I went ahead and sprayed the cover with MFP. Now, I've seen some RAO-7 receiver chassis (both in person and in photos) where this cover is NOT MFP-coated but the rest of the chassis is coated. However, that would require temporarily covering the RF input box opening while the MFP is sprayed onto the chassis. Then installing the non-MFP'd cover. It doesn't seem likely,...but anything is possible.
photo right: MFP coated data plate
|Anti-Backlash Adjustment - When tuning the RAO-7 it was obvious that there was significant backlash in the gear box. Since the receiver had the front panel dismounted and the tuning dial assembly had also been removed, the anti-backlash would be an easy adjustment. With the gear box cover removed and with unobstructed access to the front of the gear box, the front bearing hub mounting screws have to be removed. Then the hub can be slightly pulled forward. This disengages the drive gear from the idler gears and allows adjustment of the spring-loaded gear on the left side. By holding the right side gear, the left gear is rotated clockwise about a quarter of a turn and held in place. Then the hub and front tuning shaft are slid back into the gear box. The tuning shaft gear keeps the anti-backlash gear in place and the spring-loading eliminates any backlash in tuning. Also, you have to watch the idler gear below the tuning shaft gear. This idler gear has an embossed projection that interacts with another projection on the tuning shaft gear. The difference in the ratio of the two gears results in these two projections acting as "stops" once every ten turns. The easiest way to set the stops is to insert the tuning shaft into the gears with the tuning condenser fully meshed. Have the idler gear stop on the left side of the tuning gear stop. Insert the tuning shaft while holding the drive gears with the proper backlash. Test the gear set by tuning the gear box from the low end to the high end of the range. At the high end, the stops should again hit. When set correctly, the tuning stops will be slightly below the beginning of the dial scales on the low end and rotate to quite a bit beyond the dial scale at the high end. Be sure to check the tuning condenser drive gear set screws. They should be tight to prevent any movement in the tuning condenser shaft other than that imparted by the gear drive. When everything is set correctly, the tuning shouldn't bind and no backlash should be felt when the tuning and the stops should happen as described. Normally, this adjustment was done at National and unless the gear box has been tampered with it shouldn't ever need readjustment (guess this one had been tampered with.) I also cleaned and lubed the gear box before reassembly.
|Tuning Shaft Reassembly - The tuning shaft should have a special spacer on the tuning shaft that fits slightly over the tapered split-bearing on the gear box hub on one side and presents a conical surface on the opposite side. This conical surface mates with the concave side bearing surface on the backside of the hub of the logging dial. When the logging dial is positioned as far back as it will go on the tuning shaft and the tuning shaft itself pulled all the way forward and then the logging dial set screw tightened, there will be the correct spacing for the main tuning pointer behind the logging dial and clearance for the backside of the tuning dial cover in front of the logging dial. When the dial bezel is mounted, there should be sufficient clearance between the logging dial pointer and the logging dial. Before mounting the dial bezel, install a thin brass washer on the tuning shaft and position it back against the hub of the logging dial. When the dial bezel is mounted to the panel then the tuning shaft fork will bear against the brass washer in front of the hub of the logging dial and ground the tuning shaft and bezel. Install thin hard-fiber washer and then the cone compressible spring washer and finally the tuning knob. When all together, the RAO tuning should feel somewhat firm and have a slight resistance to the action of tuning. Although the earlier NC-100A gear boxes with their weighted tuning knob allowed for "spinning the knob" type tuning, the RAO-7 has a somewhat different gear box with another idler gear inside along with a non-weighted knob. Apparently the USN wanted the RAO-7 tuning to not be "velvet light weight" but rather the receiver tuning action should be firm and deliberate (and it wouldn't move under vibration from ship operations.)
|IF Alignment -
The most difficult part of the alignment procedure is that the RAO-7 has
to be on its side to access the catacomb for front end tracking
adjustments. There's also one crystal filter adjustment that is accessed
under the chassis. The IF is 455kc but actually the exact crystal
frequency should be used for best crystal filter performance. The signal
generator is coupled to the Mixer grid thru a .1uf capacitor. The output
of the receiver is used for monitoring the gain by using an Audio Output
Meter. This will require the RF generator to supply a modulated signal.
AVC should be turned off. Only the minimum amount of signal level should
be used in any receiver alignment.
Crystal Filter Problem - When I attempted to find the crystal "peak" frequency for the filter I couldn't hear the usual abrupt signal increase that happens as the generator frequency is swept across 455kc. I suspected that the crystal was inactive for some reason. The left side panel has to be removed from the crystal filter assembly to access the components that are all located inside the metal box. The screws are all self-tapping hex head sheet metal screws. There is one stud mounting the panel to the chassis so that nut has to be removed from underneath the chassis. When I had access to the inside of the crystal filter I checked all of the connections to see if any were broken or were disconnected but everything looked good. I unsoldered the crystal which is the type that is entirely contained within a bakelite housing with terminals on the sides. I tested the crystal by "ringing" it. That is, connecting a RF signal generator, the crystal and an oscilloscope in series with the return being the 'scope ground to signal generator ground. At the crystal's active frequency, there should be a pronounced sharp increase in the signal shown on the 'scope. There was no "peak" at all. This indicated that the crystal was inactive for some reason. >>>
|>>> Since I had a "junk set" NC-200 that still had the crystal
filter assembly and since the NC-200 filter uses exactly the same
crystal filter as the RAO, I had a parts source for a replacement
crystal. The crystal was removed from the NC-200 and tested. It showed a
very pronounced activity at 455kc, indicating it was a good crystal.
Installation into the RAO was followed by a quick test to make sure the
crystal filter did function. Then the side panel was reinstalled. Since
the filter had been apart and now had a different crystal installed, a
new IF alignment had to be performed. Many hams don't use crystal
filters but I find them to be one of the most important features on
vintage receivers. Even on AM, a functional crystal filter will always
be a valuable tool in combating QRM.
RF Alignment - Tracking was very close on bands E, D, C and A. However, Range B was aligned to the image frequency and this required quite a bit of readjusting to get everything back to the correct tuned frequency. Alignment to an image is most likely to happen on Range A but the high end of Range B is over 14mc and with a sufficiently strong signal input from the RF generator it would be possible to hear the image and mistake it of the correct alignment frequency. When aligned to an image, the tuning range will not track properly (error at 10mc was about 500kc) and the gain across the range will not be equal. It's important, especially on Range A, to keep the signal generator level as low as possible. Also, check the location of the image,...it should be 910kc below the actual signal frequency input. To test the image location will require a fairly high signal input level to actually hear the image. If the image is at 910kc below, then that particular LO is adjusted correctly. It's also possible to adjust the Mixer to an image. As before, keep the signal generator level as low as possible to avoid mistaking the image for the correct frequency. A proper load should be used for the RF sections. Use a .1uf capacitor for Range E and a 100 ohm resistor for the other ranges.
|RAO-7 Performance and Panoramic
Adaptors - When the RF tracking alignment was completed, the
accuracy of the tuning dial was impressive for a WWII vintage receiver.
Of course, the resolution is limited but for WWV checks and other known
transmitted signal frequencies, the accuracy was excellent. As mentioned
above, the tuning action is somewhat firm. If the anti-backlash is
adjusted correctly, then this doesn't prevent tuning in CW or SSB
signals easily. The BFO can be slightly adjusted to aid tuning in a SSB
signal. The BFO adjustment rapidly changes the frequency so any "fine
tuning" in CW or SSB will require a "light touch."
Sensitivity was good, especially in the CW mode. The AVC is very strong and cuts back the sensitivity quite a bit in the AVC mode which is how most AM listening is done. Selectivity is about 8kc bandwidth without the crystal filter and can be narrowed as needed with the filter. Audio is pretty good considering the restrictions that the USN wanted in the receiver. With a large, matched speaker, AM and SW BC stations sound very good.
Since the RAO-7 was specifically for use with a panoramic adaptor, most of the RAO-7s were operated in MVC or BFO. If the receiver were in AVC then the center peak signal on the panoramic adaptor would be attenuated due to the AVC action cutting back the sensitivity of the receiver. The panoramic adaptor circuitry does increase the gain at the edges of the bandwidth sweep and that gives the impression that signals 20kc or 30kc off center are much stronger than the "tuned signal." Using MVC on the RAO-7 results the panoramic display showing relative signal strengths more accurately. Or, if you know why the tuned signal appears reduced in amplitude, then just use AVC - especially if you're operating in Net-type communications - and enjoy the panoramic display of the adjacent frequency activity - aka: QRM!
If the RAO-7 was used for communications, then that most likely would have been in the CW mode. In its WWII use, most RAO-7 receivers probably were very seldom used with the AVC turned on. However, today most RAO-7 receivers will probably be used in the AM mode with AVC on. Expect the sensitivity to be somewhat restricted by the "strong" AVC-action. Typically, for AM, RF gain will be at 9+ and the AF gain will be set to about 5 for loudspeaker listening. If the AM sounds a bit rough, just reduce the RF Gain a little. Phones are connected directly to the output transformer so the AF gain would have to be reduced significantly for headset listening. You can also connect the 600Z ohm loudspeaker to the Phones jack if you don't want to use the rear chassis terminals.
The RAO-7 doesn't have remote standby capabilities due to its intended use during WWII. In fact, none of the RAO receivers have remote standby. It's easy to just use the front panel B+ ON-OFF switch for the standby function. Receiver antenna input isolation must be considered and this can be accomplished with either an external DowKey relay or by using the Sending relay internal to most commonly used vintage military transmitters. Separate transmit and receive antennas could also be used.
|RAO-7 Wrap-up - I'm not
really sure if many of the RAO-7 receivers were ever used for
communications. Surveillance seems
to have been the primary role for the RAO-7. Still, it's a great
receiver for the vintage military radio station.
I had my two as a "pair" and "paired with" an AC operated ART-13 transmitter. Certainly not protocol during WWII but I did see in a 1948 Radio News magazine where a ham station in Hawaii was so equipped. Nice receivers that, although they weigh a lot, also provide a lot of performance not to mention their incredible bench presence.
Nowadays, that is 2021, the RAO-7 H720 has been paired with the Navy RCX Panoramic Adaptor SN:46 and operated with the Navy-Collins T-47/ART-13 transmitter. RAO-7 J444 is set up in the shop and can operate with the ART-13A located there.
In looking at the photo to the left, how did I ever get H720 on top of J444? In moving the RAO-7, it's very easy to pull the receiver out of the cabinet. This reduces the weight by about 20 pounds,...down to about 55 pounds for the chassis. I put the cabinet on top of J444 first and then installed the receiver into the cabinet. Most "heavy lifting" can be avoided by some form of disassembly and, in this case, disassembly was also necessary when I moved H720 off of J444.
Shown to the right is that photo from Radio News - May 1948 showing KH6DA Lt.Cmdr. H.E. Warren ret. (in the middle) at his station in Honolulu in 1948. The ham station consisted of the RAO-7 receiver and to the right is the ART-13 transmitter. Actually, the article in Radio News was about KH6DA using his ham station for a net operation for a meeting of a "camera club" that he belonged to.
- The fact that the RAO receiver is so durable that most examples will
operate on all original parts and, that most vintage military
radio collectors do operate their RAO receivers that way, has probably
given those hams the impression that the RAO is just another average
performing WWII receiver. Certainly, the oil-filled paper-dielectric
filter capacitors and absence of any electrolytic capacitors in the
circuit are responsible for the RAO's
long-term operational survivability. However, all
examples of the RAO are now over 75 years old. And, like any
three-quarters of a century old entity, there are going to be some
underlying issues that do affect performance somewhat.
I've operated two RAO receivers that have been rebuilt, my just recently completed RAO-3 and a RAO-7 that had been rebuilt by a fellow ham back in the 1990s. As a comparison, I've also operated my "all original" RAO-7 sn: H720 quite a lot. While H720 is usable and functions fine, the two rebuilt RAO receivers do have better audio and much better RF tracking. Could H720 be improved with just an alignment? Probably. One has to consider the rarity of the "all original" aspect of H720 and accept that its performance "is what it is." But an alignment is non-invasive and generally improves performance (especially if the last alignment was over 75 years ago.)
Why aren't the RAO receivers more popular with hams operating vintage military radio stations? I only know of two other hams that use or have used RAOs as their station receiver. N6MKC used a RAO-6 paired with a TKC-7 transmitter and NU6F uses a rebuilt RAO-7 (the one that was rebuilt in the 1990s) once in a while as his station receiver. I've used both my RAO-3 sn:1064 and RAO-7 sn:H720 as station receivers. All vintage military radio operations are on 75M and that doesn't present any challenges for the RAO. Adjacent frequency activity is about the only issue on 75M and the RAO crystal filter can handle that with ease. So, performance probably isn't the reason for lack of popularity. History might be a factor. Almost all RAO receivers were used for intercept, surveillance or relay communications. A few were on ships and submarines but the romance of WWII shipboard operation probably can't be attributed to most RAO receivers. Weight versus performance is probably the primary complaint. At 75 pounds, the RAO is a challenge to move around. Communications grade audio with the lack of robust bass response usually isn't a concern for vintage military radio collectors but it might be factor when combined with the hefty weight of the RAO.
Maybe I have a "soft spot" for the RAO but I think it's a great receiver and an accurately rebuilt RAO is a pleasure to operate and listen to.
Hardcopy - Original Manuals, RAO-3 NAVSHIPS 900,359-B, RAO-7 NAVSHIPS 900,554, RAO-7/9 NAVSHIPS 900,356 - hard to beat original manuals,... BAMA edebris has some RAO manuals online
U.S. Navy "Catalogue of Electronic Equipment" Volume 2 (NAVSHIPS 900,116) - Detailed descriptions of all RAO models.
Radio News magazines from 1946 up to 1950 have lots of advertising from military surplus dealers showing what was available and the prices then
Fair Radio Sales catalogs from the 1960s, a little bit later but also show what military surplus items were available and the price then
QST magazine ads 1930s to 1950s for NC-100 history and coil catacomb information some info on WWII RAO in photos in wartime issues
Online - www.navy-radio.com Nick England's fantastic compilation of Navy radio history and photos. Many photographs of different RAO receiver versions. Lots of information.
Bama Edebris - Boatanchor Manual Archive online manuals allows reading and checking data without having to buy a copy of a manual. Of course, you can download and print if you need a hardcopy.
|Henry Rogers - Radio Boulevard-Western Historic Radio Museum © 2021, corrections and updates May 2021, added RAO-1 B&W artwork from original manual Apr 2022,
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