Rebuilding the R-390A Receivers
Brief History - Assessing your Receiver - Disassembly
The Main Frame - The RF Module - The IF Module
The AF Module - The Power Supply Module - The PTO
Restoration - Alignment -
PART 1 - History, Assessing your Receiver, Main Frame, RF Module, IF Module, Audio Module, Power Supply, PTO
PART 2 - Front Panel Restoration, Other Details, Contractors List, Receiver Alignment, Expected Performance, R-390A Diversity Operation, Rebuilds - Dynamotor R-648/ARR-41
PART 3 - Restorations (2) 1967 EAC, Army Security Agency/NSA R-390A, Clark AB/NSA Black Panel R-390A, USMC Olive Drab panel R-390A, Recreation of the Arvin Industries R-725
PART 4 - R-389 Restoration - WARNING! Extreme OCD Detailed Information, R-392 Info, Other R-390 Variants, Security Dial Cover, CV-979 and CV-979A Cabinet
The anodized Black Panel R-390A receivers were used at Clark AB in the Philippines. It's likely they were also used elsewhere by military security agencies under the NSA. These R-390A receivers also have the Microdial installed on the BFO for easier adjustment for the reception of Dual FSK transmissions. More details in the text below.
R-390A - Lots of Restorations and Recreations
1967 Electronic Assistance Corporation R-390A SN: 974 - Restoration Log (2016)
The following write-ups are little different from my usual restoration write-ups. I've written this one in the form of a journal or log that has daily input as the project progresses. I hope this approach gives the reader an idea of the order in which the restoration-rework takes place and the problems encountered along the way as the work progresses. It will also show the reader about how long it takes to complete an R-390A that isn't in terrible condition but certainly was non-operational and incomplete. - H. Rogers, Aug 2016.
|July 16, 2016 - Ham
& Hi-Fi, a vintage audio, vacuum tube and ham radio business in Sparks,
Nevada, had its semi-annual Open House Sale today. Lots of bargains and
"freebies." I was interested in a decent-looking R-390A that was priced
at $70. Not complete by any stretch but I was pretty sure I had all of
the missing parts. I asked owner Ethan, "Is this R-390A seventy
dollars?," just to verify. "Yep!" Hmmm. I paid Ethan and went looking
for a hand truck to move the receiver out to the car. When I returned,
my old friend Mike W7MS, was giving the R-390A a real "going over."
"Well, I see the 3TF7 is still there. That's a surprise. It's missing a
slug rack and RF coil though. I bet it's missing all of the crystals,
too." Mike had flipped the R-390A over when I broke-in, "I already
bought it, Mike." To which we both had a good laugh at the fact that
Mike was critiquing my receiver purchase before I could even get it off
July 17, 2016
- Once I got the R-390 home I was able to inspect it more thoroughly.
Mike was right,...all of the crystals had been taken out of their
sockets. There should have been 17 crystals and all were missing. The
cover had been screwed down so tightly, I thought that nobody had been
in there,...ever. Also missing was the 4-8mc RF-Ant transformer, the
4-8mc slug rack and slugs, all of the tubes in the RF deck, top and
bottom covers, the Utah plate, the front panel bushing for the KC
tuning. Both meters were non-original types that were similar types but
not correct. On the good side,...the dial had a security flip-down dial
display cover, the 3TF7 was good, both 26Z5 tubes were present (and
tested good,) the receiver was a 1967 EAC contact with the correct data
plate and all of the modules were correct EAC with the correct contract
number on each module. None of the sheet metal was "bashed" and,
mechanically, everything looked okay. Luckily, I had a couple of
"parts sets" and extra modules so the missing parts weren't going to be
too much of a problem. The receiver was extremely dirty with loads of
greasy, oily contamination that had very fine black powder mixed in. The
receiver had obviously been stored in a garage or machine shop or some
other location where oil and fine black powder would be everywhere.
July 20, 2016 - Started complete tear-down. All modules out, front panel off, all parts plastic bagged and tagged.
July 21-23, 2016 - Cleaned front panel. I was amazed. I thought the front panel was kind of rough but it was all just the oily dirt and black powder getting into the engraved nomenclature. Careful cleaning first with WD-40 and a soft brush followed by Glass Plus to remove the WD-40 residue resulted in the front panel looking first-rate.
Complete disassembly of the Main Frame was necessary because the oily dirt had worked into the side panel joints due to loose screws. Again, under the dirt everything was in excellent condition.
|July 24 - 28, 2016 - Took RF deck outside for a WD-40 flush of the gear box. The Veeder-Root counter was very dirty and I thought the digits were damaged but, again, it was just the black dirt causing the problem. Luckily, it comes off easy with WD-40. Stripped down the RF deck by removing all 24 RF and VIF transformers, slug racks, return springs. With no load, the KC tuning was checked for "feel" which was normal. Same for MC tuning. Checked cam synchronization by setting tuning to 07+000 and found that the 4-8mc cam was way off. Probably why the 4-8mc RF coil was missing - a former owner was chasing a problem in the wrong area of the receiver (also noted that the mica capacitors had been replaced in remaining two 4-8mc RF transformers.) Mechanically reset the 4-8mc cam to correct position. Replaced the missing 4-8mc transformer and also replaced the remaining two that had been "worked on." All other cams were in alignment. Cleaned and inspected ALL 24 RF & VIF transformers checking for proper rotation of trimmer on each. The replacement 4-8mc RF coil needed to have the trimmers "unstuck" and repaired before it could be used. Cleaned all slugs of dust and any other dirt. Cleaned all slug racks and lubed cam rollers. Reassembled the RF deck, adjusted the fit of all slugs into their respective coil barrels and then checked operation of all of the cams, slug racks, slugs and cam rollers.|
|July 29 - 31, 2016
- Installed a set of crystals in the RF module-Crystal Oscillator (17
crystals required.) Cleaned rotary switches. Reinstalled the RF module
back into the Main Frame. Cleaned and lubed all controls and switches
for front panel. Remounted harness and all controls to the front panel.
Installed two #328 bulbs in the dial cover (originals gone.) Mounted
front panel to the Main Frame temporarily - checking for fit. I only
installed four screws since this panel will have to be "dropped down"
for PTO end-point adjustment and probably for other things, it's best to
just mount it in this manner for now. Original knobs were rough. Since I
had a full set of restored knobs, these restored knobs were mounted to
the control shafts.
August 1, 2016 - Tested the two can electrolytic capacitors and found the triple-30uf to have one defective section. The dual-45uf seemed to reform okay but the values are not very close. Same with the two good sections on the triple-30uf. I will have to rebuild these two units using new electrolytic capacitors for best reliability and performance. I had several spare R-390A can electrolytics and when testing them I found that ALL were defective in some way. Probably time to admit that you can't use and reuse the original, fifty-year-old caps anymore. New electrolytics ordered and on the way.
August 2 - 6, 2016 - While waiting for the replacement electrolytic capacitors, I tested all of the tubes and cleaned the tube sockets on the IF module and re-installed it into the Main Frame. I also had to locate tubes for the RF module since all were missing. The other modules all had W.P.M. heat-reducing tube shields so I also needed to find tube shields for the RF module tubes. All I had was IERC-type, which are very good heat-reducing tube shields, but they don't look like the W.P.M.-types. Since most of the tubes in the RF module are covered by the Utah plate, I went ahead and used the IERC-types.
|August 7, 2016 - I
can't find a 6DC6 (RF Amplifier) anywhere in the R390A spare parts or in
any of the tube boxes. I will have to order a couple. They aren't
expensive tubes but I never seem to have any NOS ones around for some
August 8 - 11, 2016 - Rebuilt the two multi-section electrolytic capacitors. Photographed for the added write-up on this procedure that is now in this web-article in the Audio Module section. Picked up four NOS 6DC6 tubes from Ham & Hi Fi. Installed the remaining modules into the main frame.
Since I didn't set the Veeder-Root counter to XX.000 before removing the RF module, the PTO was not pre-set to 3.445mc. This is a minor inconvenience that requires the PTO be set by powering up the R-390A and measuring the frequency out of the PTO with a digital frequency counter. The procedure I use is above in the PTO section of this web-article.
Since I had already powered up the R-390A, I went ahead and hooked up a 600Z ohm speaker. I had lots of noise but the calibrator seemed pretty weak on 40M. I connected an antenna and tuned around 40M and heard a few SSB stations. Now this R-390A has been completely apart and is certainly quite a bit out of alignment but still it picked up a few signals. This should be a very good sign of things to come.
|August 12-14, 2016 - I've been checking out performance of the R-390A before alignment by listening to various signals on different bands. This gives everything a chance to operate at voltage for awhile to make sure everything is going to function. Since the only "repair/changes" occurred in the 4-8mc RF section with the installation of different RF transformers along with different slugs and slug rack, it was kind of a surprise that the 4mc, 5mc, 6mc and 7mc bands actually would tune in the Calibration oscillator. I was really pleased with the performance on the bands 8mc and up. I had manipulated all of the trimmers on all of the RF and VIF transformers to verify that the trimmers weren't stuck, so I was surprised that 20M and 19M SW performed quite well. I checked for the Calibration oscillator signal on all bands and it was present. I tested the end-point error on the PTO and found it to be 8.0kc. That's the greatest EPE that I've ever encountered, so we'll have to see if the compensation adjustment can correct that much error. I dropped the front panel since that was going to be necessary for the EPE adjustment. I installed all of the correct hardware to mount the correct type meters and soldered the connecting wires up for both meters. After the EPE adjustment I will be able to fully mount the front panel with all screws and washers and proceed with the full IF, VIF and RF alignment.||August 15-17, 2016
- See PTO section on Cosmos PTO. I added my experience with this Cosmos
to that section of this article. The EPE adjustment is virtually
inaccessible from the front through the locking plate and the front and
rear gearbox panels. I had to remove the PTO each time to make the
adjustment and then reinstall to test. Very time consuming. I was able
to adjust the EPE from 8kc down to 0.5kc. Remounted the front panel with
all screws, locking washers, shaft bushings, etc. Checked output on the
Crystal Oscillator section and adjusted all trimmers for peak output.
August 18-20, 2016 - Peaked the mechanical filter inputs and outputs. This requires having the IF module dismounted but still connected to power. Photo and method described in "IF Module" section further above in this article. All mechanical filters were pretty close so, just minor tweaking.
August 21, 2016 - Completed the full alignment. Most adjustments were pretty close but, as expected, the 4-8mc section was quite a bit out of alignment. Installed the Utah plate, top and bottom covers. Connected receiver to the full-size ham antenna. 40M reception is normal now. All other bands are functioning correctly. Adjusted Carrier Level Meter and IF Gain for best performance.
|1967 EAC Performance and
Observations - Here's what I've noticed on this receiver that
is somewhat different than the earlier versions, such as the Collins or
1. Components - many ceramic disk capacitors in RF and IF modules. These modules also have several capacitors that appear to be metalized film capacitors. The AF module appears to have similar capacitors to the old Vitamin-Q types but I think the construction is different with better seals. Certainly the multi-section filter capacitors are of the same construction and questionable reliability. They have the same problems that are found in any electrolytic capacitors that are half-a-century old. Overall, the capacitors seem to be better types than those used in the R-390As built in the 1950s. Cosmos PTO is difficult to adjust the EPE due to the new location of the L701 adjustment behind Z702.
2. Performance - is definitely equal to a rebuilt and recapped earlier version receiver. IF Gain is set at about 60% which is pretty close to where it's adjusted on the recapped earlier versions. With IF Gain set at 60%, most SSB signals demodulate nicely with the RF Gain at about 5 to 7 and the AF Gain at 7. This is using a 135 ft center-fed tuned inverted-vee antenna. SW BC stations usually run about 50 to 60db on the Carrier Level meter depending on the station and the time of day. The coupling capacitors in the AF module were NOT changed to .02uf but the audio sounds very good with noticeable bass response on AM BC and SW BC. Also, strong SSB stations and AM ham signals seem to have good bass response with the original .01uf coupling caps. Overall, a nice performing R-390A that is going to be set up with one of my ART-13A transmitters for awhile.
Time to Complete Project - It took just about one month to complete the rework on the 1967 EAC. This is from a non-operational, incomplete receiver to an entirely functional and totally complete receiver. I didn't work on the receiver everyday so total time actually spent on the project was probably around 20 hours.
Update - Sept. 4, 2016 - I guess I should have cleaned the Antenna Input relay contacts. The procedure is described in the Main Frame section further up this page. It's not difficult to do, even if the receiver is already back together. The symptoms were no (or very little) carrier level indication, a change in the normal position of the ANT TRIM for resonance and relatively weak signals. If STAND BY or BREAK IN were actuated then the signals would return to normal levels. This was the typical indication that the antenna relay contacts were introducing some resistance due to poor contact. In this particular case I don't believe the cause was oxidation because inside the arm, NC and NO were very clean looking. I used just a slight bit of DeOxit and paper to clean the contacts to have them measure zero ohms. Problem might have been some kind of rosin-like coating or something that dissolved with DeOxit. Other than this minor and easy to correct problem, the '67 EAC R-390A has been performing very well.
Update - March 7, 2018 - Read "Creating an Authentic Arvin R-725/URR" further down this page to see what's happened to this '67 EAC R-390A.
Not Another 1967 EAC Restoration Log?
|May 24, 2017 - I
saw this R-390A at yet another "Open House" at Ham and Hi Fi in Sparks,
Nevada last year. It was $100 "as-is." The yellow power cord and the
600Z matching transformer mounted on the back panel must have scared off
any potential buyers, including me. I was tempted though, since the
receiver had both original meters. I "stewed about" this R-390A for
awhile and would usually go into the back storage area at Ham and Hi Fi
just to see if it was still there. Finally, about six months had gone by
and I was again looking at the R-390A that had a 1963 Imperial
Electronics tag on the front panel. I asked Ethan if it was still for
sale since it had been stuck way in the very back of the building for
months. "Sure, I was asking $100 for it at the last open house. Is that
okay?" I replied, "Yeah, the parts on the front panel are worth that to
me." So, into the truck went this newest R-390A.
When I got the R-390A home, I had to investigate that yellow power cable and 600Z ohm transformer. The yellow cable certainly wasn't original but its installation didn't do any damage either. Same with the 600Z transformer that was utilizing an existing stud for mounting. While looking at the back panel I noticed "Electronic Assistance Corporation" with the "FR-36-039-N-6-00189(E)" order number and the "DAAB05-67-C0115" contract number stamped on the back. I then looked at the Crystal Oscillator (attached to the RF deck) and saw the same stamping. I noted that the electrolytics on the Audio Module were date-coded "67." So, I pulled out all of the modules except the RF deck and to my surprise they were all 1967 EAC modules on the FR-36-039-N-6-00189(E) order and DAAB05-67-C0115 contract from 1967. This receiver was a 1967 EAC R-390A that for some reason had an Imperial Electronics tag installed. Further inspection revealed that the receiver was nearly complete and original. The only missing parts were the correct ID tag and the "Utah plate."
Although I had originally thought when purchasing this R-390A to use it as a parts source for my 1961 Capehart (with OD front panel) this one is just too nice and original for that purpose (the Capehart will just have to wait.) I'll get started on this '67 EAC during the summer and write a restoration log as I proceed along with the rebuild.
June 12, 2017 - Started on this EAC. I had already obtained an original Utah plate and a repro '67 EAC data tag. All the modules except the RF module had been pulled and were setting with the receiver. I started with the IF module, the AF module and the Power Supply. Each module was cleaned and the tubes tested. Any weak tubes found were replaced. The two electrolytic filter capacitors were reformed. These capacitors were date coded 1967 and both checked okay. Reforming seemed normal with the two 40uf caps drawing about 10uA after reforming at 280vdc and the three 30uf caps drawing about 15uA to 25uA at 280vdc after reforming. The final test will be to see how the filter caps perform in the receiver. I didn't have very good luck with the last EAC receiver's filter caps which had to be rebuilt. These filter caps seemed to form nicely but performance is the final test.
June 13, 2017 -
Completed cleaning and tube testing on the above modules. I had to
double-check the wiring on this EAC power supply as the wiring didn't
look like most PS modules. Though the wires were not "tucked" under the
chassis, as is normally done, the wiring was correct. Just an anomaly of
that particular PS assembler and final inspector. I had to replace all
of the 5749 tubes in the IF module since they all tested weak. They were
all RCA-JAN brand and date coded 1965. Also, one 5814A in the IF deck
needed to be replaced. On the AF module I replaced one 5814A. Both 26Z5
tubes checked okay in the PS module.
New for 2022 !
Recreating a U.S. Army Security Agency Motorola R-390A
Recreating a Clark AB "Black Panel" aka NSA R-390A
Refurbishing the 1961 Capehart USMC OD panel R-390A
I've always been intrigued by the clandestine, crypto, signal security types of reception use that the military and government agencies like the NSA had for the R-390A. I knew about the Clark Air Base use of "black panel" R-390As and I had seen some of the BFO Microdial installations on other R-390A receivers. I even have a "parts set" Motorola R-390A that has the remains of a Microdial and also U.S. Army Security Agency (ASA) stickers on the rear panel. The opportunity to build up a black panel Microdial R-390A presented itself with obtaining an original "grained and anodized" black R-390A front panel. Where the panel originally came from was an unknown but it appeared to have been stored for quite a period of time. At one point someone had installed some incorrect switches and pots probably to have a place to mount some of the knobs. The panel also came with both of the correct meters. I had several R-390A "parts sets" and a good supply of various modules. However, for the Clark AB Black Panel I decided to use the 1967 EAC receiver that I had gotten from Ham & Hi Fi in 2017 that still had a few problems after its refurbishment (described above in "Not Another 1967 EAC Restoration Log.") The receiver was out in the shop installed in a rack with two other R-390A receivers. I had really forgotten exactly what the final reworking had consisted of. I knew I had robbed some parts and exchanged some others. So, first I had to test this R-390A and correct whatever I had done to it. >>>
|>>> July 6, 2022 - In reading what I wrote in 2017, I really didn't know what the status of this receiver was now in 2022. A few years ago I installed this receiver into a CRAIG relay rack that was out in the shop. Then the R-725 project came along and that required the other EAC R-390A receiver I had. The R-725 works great, but I read what I wrote about that project and it doesn't mention what parts I robbed or swapped from this R-390A. I must have left the good PTO in this receiver as mentioned in the prior write-up. I wouldn't have needed it since the R-725 uses a special PTO that I had for that project. Next was the data plate. I mentioned in the prior write-up that I installed a repro data plate but upon inspection this receiver now has an original EAC data plate installed (SN: 974) from the first EAC receiver (that was converted into the R-725.) Since the R-725 has a special data plate, which I had, I must have removed the repro data plate and installed this original one in its place.|
|July 7, 2022 -
Functional Test Results - I performed a functional test which did verify that the
"good" PTO was still installed in this EAC receiver since the tracking was
excellent and the EPE was around 1kc or less.
I performed a reception test on WWV 10mc and got a CL reading of about 50db. The CAL would drive the CL meter to 40-50db. All bands functioned.
The test on 2-4mc found that 2mc CAL signal was about 40db and 3mc was about 20db. This indicates that I really didn't do anything about this problem other than the temporary substitution of the 2-4mc RF transformer to confirm the problem. I must have put the good RF transformer back into the other EAC receiver that became the R-725. So, the low signal performance on 3mc is still a problem that needs to be corrected. I have many spare 2-4mc RF transformers so hopefully I have at least one good one. NOTE: These test results changed when the test was carefully set up. The test results really changed when I went through all of the tubes finding that many were marginal, one was shorted and one was erratic acting. NOS tubes corrected this "problem" along with several other minor issues.
July 8, 2022 - I ordered a Servo Instruments 1309 Counting Dial. This is the type of Microdial that was used on the various security agency receivers and the military crypto, signal security, intercept receivers. Some receivers had a turns-lock that limited the rotation of the BFO shaft. It incorporated the lock into a replacement panel bushing. The 1309 Counting Dial unfortunately is just the Microdial and no turns-lock is included. The receivers that did have a turns-lock installed will have a hole for the mechanical rotation stop tab on the backside of the panel.
July 9, 2022 - Checking What R-390A Parts I Had - I inventoried all of the R-390A parts that I have. I was curious about the 1961 Capehart with the Olive Drab front panel. I had disassembled that receiver over ten years ago and never put it back together because I didn't have a set of meters for it. The spare black panel came with proper meters that can be used to complete the OD Capehart. As it turned out, I found all of the Capehart modules but unfortunately I had robbed Z-503 off of the IF module (as described in the proceeding write-up.) I had several IF modules that probably can provide another Z-503 but that will be rework necessary when I get to the OD Capehart part of this project. At least I still have all of the original modules and that receiver did work at the time of disassembly but that's no guarantee on the electrolytic filter capacitors. Also, I seem to remember that the Capehart AF module had a bad transformer on one of the audio outputs.
All total I have eight "parts sets" in various states of completeness (or lack of completeness.) The best condition, other than the OD Capehart was an Imperial that looked pretty good, certainly restorable but was, of course, minus the meters. There was a Motorola with US Army Security Agency stickers on it and the base of a Microdial on the BFO but the overall condition was very rough with only the RF deck and AF module present. There were a couple of rough receivers that were missing their data plates. Lots of RF decks, PS modules, Audio modules, PTOs, Main Frames and front panels. I had four IF modules but I don't think any of them are in great shape. One is a Collins IF module with the fixed-tuned mechanical filters (1956 MWO never incorporated.) So, lots of parts available and I should be able to do the Black Panel receiver and the OD Capehart without too much trouble. It's also a possibility, if I can purchase another 1309 Microdial, I could create a US Army Security Agency receiver from my fully restored 1956 Motorola R-390A. Also, the ASA stickers would have to be salvaged or reproduced using the originals on the "parts set" ASA Motorola.
Recreating an "Army Security Agency" Motorola R-390A
July 11, 2022 - Microdial - Received the Model 1309-100 Microdial today. Original mounting had a turns-locking bushing that only allowed a specific number of rotations, three being standard. I didn't have one of these special bushing so I'd have to use the regular BFO panel bushing to mount the base. The depth of the dial knob mounting on the shaft controls the mesh of the internal gears. Works well. But, I checked the "parts set" Motorola that still had the Microdial base mounted and that base was glued to the panel. With close examination and I could see slight traces of the glue which looked like the old (really old) brown "contact" cement. When I get to the point of actually mounting the Microdial, I'll find out if the panel bushing mounting will work,...it should. But,...if not, I guess that might be why the Motorola Microdial was glued to the panel. At any rate, I ordered another 1309 Microdial today (one for the Motorola ASA R-390A project and one for the Clark AB Black Panel.)
July 12, 2022 - Testing the Motorola R-390A - I did a functional test on the Motorola R-390A today. It had been probably a year or so since it had been powered up but that wasn't a problem. After all, I completely restored this R-390A in 2011 and that included a replacement of all capacitors in all of the modules. In fact, most of the photographs in Part 1 of this R-390A write-up are of the modules from this receiver. Back to testing,... I connected up a 600Z loudspeaker and the two half-waves in phase antenna out in the shop. Lots of signals on all bands. SW BC signals on 12mc pushed the CL meter up to +60db. I tuned in XSQ on 16.8mc (about 1500hrs PDT) and it was quite strong. XSQ is one of the coastal stations in mainland China. They send out a CW ID after a frequency warbling that is sent four times. This is continuously repeated. These coastal beacons are located in many countries but the China and South Korea beacon stations are particularly strong here in the West. The Motorola also received a number of 20M ham signals so I think the Motorola is working quite well.
July 13, 2022 - Removing the ASA MWO Labels - I was able to remove the three U.S. Army Security Agency MWO labels off of the back of the "parts set" Motorola R-390A. One label was in rough condition after the removal. The second label came off fine but was worn through in the center where something had rubbed against it in the past. The third, which had actually been under the first one removed, came off in good condition. I had tried to remove the first label by wetting it but that tended to damage the already fragile paper. The other two labels were removed using a razor blade and this worked quite well with no damage occurring during the removal process. The third label, being that it had been underneath another label, retained its gloss and looked very nice. Also, the MWO number wasn't "scratched out" as the MWO numbers on the other labels had been. This glossy label will be the one I'll install on the new creation of an ASA Motorola R-390A. The label is shown in the scan above right for a close-up and the photo below right showing the label installed. Also shown is the metal military AC plug mentioned a few paragraphs down.
Mounting the Microdial Details - Since changing the Motorola R-390A into an ASA-type isn't very involved, I started that project first. Installing the Model 1309 Microdial requires first removing the IF module. Then slightly enlarging the BFO panel hole by about .060" (out to .50") to allow clearance of the embossed centering ring on the 1309 base. The enlargement of the hole diameter shouldn't be through the panel. Only enlarge the diameter about halfway through the panel (it only needs clearance for the base centering boss.) I didn't have the special turns-lock bushing but I was able to use the standard panel bushing for mounting the base. Best results would be to use an original turns-lock or to use a slightly longer panel bushing, maybe about .125" longer to allow using all of the washers. For now, I just mounted the 1309 base with the bushing, a flat washer and the mounting nut and that seemed to work fine.
|Microdial Operational Details - The 1309 Microdial has to be taken apart to mount it. The base is mounted first. The embossed ring will assure that it's centered. Insert the 1/4" BFO shaft and clamp from the rear. Insert the counter knob and then install the outer mount/turns counter. The outer mount is held in place on the base with three screws around the perimeter at the rear. I set the Microdial to 20.0 since the desired BFO adjustment was +/- 13kc and that happens with about 1.25 turns of the BFO shaft each side of zero beat. So, with my set up 20.0 is 455kc on the BFO and 32.0 would be about 468kc and 08.0 would be 442kc. Now slide the counter knob slightly forward and tighten the two set screws. Try the dial, it should adjust smoothly (it isn't coupled to the BFO shaft yet.) Next, reinstall the IF module and connect the bandwidth shaft and BFO shaft to the IF module shafts. Tighten the shaft clamp on the Bandwidth. Now, connect the IF module to the R-390A via its cables and power up the receiver. With the BFO turned off, tune in a CAL signal and adjust the frequency dial for exact center frequency. Turn the BFO on and adjust the BFO shaft from the IF module with your fingers for exact zero beat and then install the coupling shaft and tighten the shaft clamp. Try adjusting the BFO with the Microdial. If the mating of the counter knob and the turns counter/outer mount is correct, the tuning will be smooth and have a nice "feel" to the adjustment. Test by adjusting the receiver tuning to +13kc above the CAL signal, then adjust the Microdial approximately 32.0 and you should hear (barely) the BFO zero beat with the tuned frequency. Repeat for -13kc from the CAL signal and you should hear the zero beat with the BFO at about 08.0. Don't expect this to be "dead on accurate" but it should be close.|
IMPORTANT NOTE: Remember that the Microdial is a turns-counter
and +/-13.0 on the dial is actually about 1.3 turns of the BFO shaft. It's a coincidence that
it also happens to be close to a +/- 13kc excursion either way from the BFO center
frequency. I originally set the BFO Microdial zero beat at 13.0 but that
ends up confusing the operator into thinking the Microdial is a frequency
when it's actually shaft turns. In fact, for typical SSB demodulation
the Microdial only has to be +/-00.6 of zero.
Mounting the ASA Label - To complete this transformation, the U.S. Army Security Agency MWO tag was glued in place on the back panel. Although there was still some of the original adhesive on the back of the label it certainly wouldn't be sufficient. I used white carpenters glue applied to the back of the label with a Q-tip. This should be a thin and even coating. The label was placed on the back panel with the receiver resting on its grab handles. This allowed using a plastic pad and a weight to press down on the label to keep it perfectly flat while the glue dried. After several hours the weight was removed and the label installation looked quite original.
AC Plug - The receiver had a civilian-type black rubber AC plug installed and that didn't look quite right. I installed a military metal AC power plug to complete the change of this Motorola R-390A into an U.S. Army Security Agency receiver.
Operational Details - Tested the operation and the receiver performed well. The adjustment of the BFO is not as "fine" of an adjustment as one would think. Most SSB and CW will demodulate with the BFO set to +/- 00.6 turns from 20.0 on the dial or 19.4 on the dial for USB (I know it's hard to remember that the Microdial is reading relative "turns" not frequency.) The wide range of adjustment was for RTTY and DFSK signals. It's a nice addition though. Also, of some interest, I checked the serial numbers on this Motorola R-390A and the real ASA "parts set" Motorola R-390A. They are fairly close with the newly transformed ASA receiver being serial number 1873 and the "parts set" receiver being serial number 973 (exactly 900 units apart,) both receivers on the same contract from 1956. So, it's about as close as I can get to an original ASA R-390A receiver.
Recreating a Clark Air Base/NSA "Black Panel" R-390A
Front Panel Condition "as received" - Shown to the right is the black anodized front panel I'm going to use for the Clark AB/NSA R-390A. The panel has many minor blemishes and defects (perhaps the defects will add to the authenticity and character of this recreation.) It appears that the "graining" that was done to the panel before anodizing wasn't carefully done and there are several areas where the "graining" is counter to the horizontal direction it should have. The pin-point "spotting" might be surface problems that weren't addressed before anodizing. In fact, in some places it looks like the anodize and dying process was performed and the panel wasn't entirely stripped! The most likely reason for the defects would be that this was a somewhat recent anodize job that was done by an incompetent metal treatment shop. Who knows? Maybe it was. The second possibility is that this really is a vintage anodizing of a panel that was worked on to correct some damage problems that resulted in some of the sanding and grinding ultimately showing through the finished product with the final disposition being that the panel was rejected. That's why it was never installed on a receiver. The engraving fill paint is mostly present but it's a dark gray color. Some residual red and white paint can been seen in some places in the engraving and that seems weird since part of the anodizing process uses dilute sulfuric acid as an electrolytic. Physically, the panel is in good shape with only two small mars at the bottom right (and that's an easy touch-up) and the top right where the corner isn't square.
So, the first step will be to work with this panel to see if any improvement is possible. The panel was coated with oil or wax (maybe Armor All) that really enhanced the condition problems. With a thorough cleaning, touch-ups and with properly filled engraved nomenclature, the panel should look in believable condition and maybe it might even look like a vintage piece (and maybe it is,...it's real history is an unknown.)
|July 14, 2022 - Front Panel Restoration - Looking at the photo of the panel the engraving looks like it's filled with white paint but it's actually dark gray. To have a more "believable" fill paint I used a "manila" color that is a mixture of white, light brown and just a touch of black. Just using white paint or white lacquer sticks will end up with the nomenclature looking way too bright. Manila looks aged but in good condition. Unfortunately, all of the engraving had some issues. Most of the nomenclature had to be "gone over" with a special tool I made to "dig out" the old gray paint. However, on the right side of the panel, some of the engraving had some anodizing and black dye in the engraved areas that had to also be removed. Looking at the "before" photo, CARRIER LEVEL is very obvious that there's a lot of black in the engraving. It's a laborious process to remove but all of the engraving had to be cleaned in this manner so that the new "manila" fill would look correct. Also, on the panel I'm going to do the red fill that I've seen on a couple of black panel receivers. The CAUTION area, DIAL LOCK, ZERO ADJ., STAND BY and CAL will have the red fill while the remaining engraving will be manila color.||July 15, 2022 - Front Panel
Restoration cont. -
Continued on with the "manila" paint fill for the panel nomenclature.
Most of the engraving actually requires two coats of paint for the
proper fill amount. Although I've mentioned it in many other places in
these write-ups here it is again,...I use Artist's Acrylic to mix the
proper color. The paint is applied using a saturated Q-tip just to one
area of nomenclature at a time. Once the paint is applied, I allow about
two minutes for it to set up. Then I use a small piece of paper towel
that's dampened with Glass Plus and folded as flat as possible. I use
these types of paper towels to remove the excess paint. I only use a
paper towel piece one time, then it's discarded and a new one used to complete
the clean up. Sometimes a second coat is needed for a good fill. Once
the paint has dried for about 10 minutes it's okay to go over the area
with a slightly dampened paper towel to further clean up the residual fill paint.
This has to be done for each engraved area. Don't try to do a large area
with several areas of engraving - it won't work. Just do one set of
engraving at a time. It will take about an hour to do all of the panel.
Cut up the paper towel pieces in advance to help speed the process.
The five engraved nomenclatures that are going to be filled with red paint will be next.
|July 16, 2022 - Front
- I mixed red with just a small dab of black to dull down the color a
bit. I had to do this part of the fill carefully to be sure to not get
the residual red paint into the manila nomenclature. Only the STAND BY and CAL
are really close. As expected, the panel's appearance has improved
substantially with cleaning, touch ups and the engraving fill. I'm
pretty sure that once all of the components, knobs, hardware, etc., are
mounted, the panel blemishes should be much less noticeable.
I had to do the slight modification to the BFO PITCH control hole, that is, increasing the diameter of the hole up to 0.5" to allow for the embossed centering ring for the base of the Microdial. I also used a longer panel bushing for this installation and that allowed using all of the mounting hardware. Too bad I don't have the turns-lock assembly but it's not necessary and wasn't installed on all Microdial equipped receivers.
Further prep on the back side of the anodized panel was required. Aluminum oxide is the ultra-hard coating that the anodizing process creates and it is non-conductive. But, so was the original paint. That's why there were two strips of bare metal where the back of the front panel mounts to the main frame. The bare metal assures that the front panel is grounded so it provides the shielding and ground returns necessary. Additionally, one of the AVC time-constant capacitors is grounded to the back side of the panel by the CL meter. This area also has to be bare metal. I'm not sure if the original anodized black panels were masked or just ground off as needed during installation. In this case, this anodized panel had not been properly prepared on the backside. I had to mask the two long strips for the main frame contact. Once masked I used a Dremel tool to remove the aluminum oxide in only the areas needed. Same for the small area next to the CL meter.
|July 17, 2022 - Front Panel
Completion - I
had to use the Dremel drum shaped Al-Ox tool about 0.5" diameter. It worked
really well and fast to remove the anodize just in the area that I
masked (and I also "masked up" with a N95 due to the fine powdery oxides
that became airborne in this removal process.) When the tape was removed the bare metal areas looked almost
I had broken one of the mounting screws that are located behind the data plate (mounts the resistor PC board.) I had to dismount the bracket and then drill out the broken screw. These are "undercut" FL 6-32 screws so I ended up robbing an original off one of parts set to replace the broken screw.
The meters have to be mounted before the panel is mounted to the main frame. Then the panel is placed face down in front of the main frame with two 16ga. wires mechanically supporting the panel running from the top hole in the panel to the top hole in the main frame on each side. This provides a solid support for the front panel as the parts are installed. All of the components were placed in their proper holes and the mounting hardware installed "finger tight" only. The three screws, lock washers and nuts that support the harness clamps to the front panel have to be installed. Now is the time to solder the wires that connect to the CL meter and the LINE LEVEL meter. At this point the panel can be put in place but I had to be sure to install the KC and MC inner shaft bushings and the bushings for Bandwidth and ANT TRIM. With the panel in place now all of the screws can be tightened, all of the control mounting hardware can be tightened and the knobs installed. I had to play around with the MC and KC bushing positions to achieve a decent feel to the tuning. At this point the dial bezel wires can be soldered and the dial bezel mounted to the panel. Finally, the data plate can be installed.
I installed the Microdial on the BFO. Then I installed the PS module and the AF module. I connected a 600Z loudspeaker and an antenna then AC power was applied. The receiver worked like it had at the original testing so there's a bit of work left to do for top performance. I did receive XSG 16.898mc about 569 at about 1500hrs. XSG is a coastal beacon in China.
|July 18, 2022 - Testing
- I calibrated the CL meter and then used the CAL oscillator as a signal
source. When checking 2.000mc the CL read 60db. When checking 3.000mc
the CL meter read 55db. When checking 4.000mc the CL meter read 55db.
This looked pretty normal. As a further test I checked 7.000mc and the
CL meter read 65db. Again, pretty normal. I tuned in WWV 15.0mc with a
tuned antenna and it read 65db on the CL meter. I tuned in a couple of
really strong utility signals in the 16.0mc range and most read between
50db and 70db on the CL meter. All of this looked pretty normal for a
typical R-390A that had been aligned a few years ago.
July 19, 2022 - Performance Analysis - I've been listening for a couple of days now and it seems this R-390A is working pretty well. I've copied most of the Chinese Coastal Beacons on 16.8mc. Copied KPH on 16.625mc on CW on a Saturday early afternoon. 20M hams and SW BC up in the 15mc region are present. There are a couple of weird minor issues though. First, when listening to SW-BC stations the audio seems low compared to the carrier. Second is the limited range of the RF Gain control. All of the RF Gain action is between 8 and 10. At 10, all signals seem normal on the CL but disappear if the RF Gain is reduced below 8.*
I wondered why the dial lamps were so dim. Well, #345 lamps is why. I thought that I had replaced these lamps but obviously I didn't,...weird. The #345s might be 6 volt lamps but they draw a lot less current than the correct #328 lamps. The high filament resistance of the #345 bulbs in combination with the series 2.7 ohms resistor results in the dim illumination. A pair of new #328 lamps installed and the dial lamps were up to the normal illumination.
Wrap-up - As I had hoped, once all of the components were assembled onto the front panel, along with good condition nomenclature, the blemishes are hardly noticeable. I guess if it looked perfect everyone would know it's a recreation. With the minor blemishes, it could almost pass for an original,...well,...maybe. Still,...it's very cool.
|*Follow-up - July 25, 2022 -
Performance Problems Solved
- I got to thinking back about when I went through this particular
R-390A in 2017 and I suspected that I either hadn't tested the tubes or
maybe I used a different tube tester (I had just gotten a really neat
TV-2 about that time.) I read though what I wrote in 2017
and apparently I did test the tubes and there were a lot of bad ones
that I replaced at that time, apparently from my stock of "used tested
good" tubes. I must have had a very low criteria for acceptance back then.
The new test (five years later and virtually no hours on the
receiver) used the TV-7B I've used for years and involved all 26 tubes. I found five tubes that were at or below minimum acceptable, one tube
that acted erratically and one tube
that had high leakage (shows as a short.) The test results were as
follows,...all tubes in the IF module
were in "tested as new" condition. The PTO tube tested "as new." The PS
rectifiers tested good (above 40 on the TV-7.) The AF module had both
6AK6 tubes testing at minimum acceptable, the two 5814A tubes tested "as
new." The RF deck had two weak 6C4 tubes (the R-390A circuit pushes
tubes and they always seem to be weak in receivers that are in "as
found" condition.) Also,
the 5654 (6AK5) for the crystal oscillator was the tube that was showing
a short. The RF amplifier 6DC6 tested erratically and was marginal when
stable. It was replaced with a NOS 6DC6 tube. The OA2 and the 3TF7
tested good. I was surprised at these
test results because I thought that I had been very thorough back in
2017 - but, obviously NOT! All of the suspect tubes were replaced with
either NOS tubes or "tests as new" tubes. With ALL good
tubes, the R-390A pushes the CL meter up to +75db when tuned to WWV
15mc. RF gain range is now 5 to 10 using the CAL+BFO on 6.000mc. The
Chinese Coastal Beacons on 16.8mc can be received with the RF gain at 6
and are still easily Q5 (AF gain at 6.) When listening to SW-BC stations
now, the signals have full modulation and the audio quality is very
good. With all "new" tubes I was able to reduce the IF Gain control to
about 75% advanced and WWV 15mc still reads +65db on the CL meter (no
signal CL is about +5db.)
IMPORTANT NOTE: This "problem" was a very good illustration as to why ALL of the tubes in the R-390A need to be in NOS or in "tests as new" condition. Although any R-390A will seem to function adequately with some marginal tubes installed, top performance requires that ALL tubes are in excellent condition and "test as new."
After the "bad" tube discoveries in this receiver, I thought I better retest the performance of 3.800mc compared to any frequency above 4mc to see if there actually is a loss in the 2-4mc range. Using the CAL and the CL meter I tested several frequencies and found that the CAL would push the CL meter up to +40db at the lowest and up to +60db at the highest. Listening to various bands and frequencies there didn't seem to be any obvious reduction in sensitivity. I'm going to assume that the original observation of low levels on the 2-4mc range was probably due to all of the marginal and defective tubes. Performance of the receiver is now just as expected for a recently aligned and serviced R-390A.
USMC 1961 Capehart R-390A with Olive Drab Panel
Several years ago my old friend Mike W7MS told me about a Capehart R-390A he had seen a few years earlier that had an olive drab front panel. Mike was told then that the Marine Corps was responsible for the repaint. In 2010, I was refurbishing a 1961 Capehart and another friend, KØDWC, was doing some OD painting over at his QTH. He had automotive quality OD paint that had been custom mixed and he was spraying using a compressor and siphon spray gun. On a lark, I brought over the "stripped" Capehart front panel and sprayed it OD. It turned out too dark for my taste, so I rubbed and scrubbed it with various abrasives (the OD paint had "hardeners" added) with the intention of just ultimately stripping it but that aggressive treatment actually lightened the OD color into something I thought looked nice and vintage. At that time, W7MS had just recently told me about the USMC OD panel Capehart R-390A he had seen, so with that information in mind, I decided to refurbish my Capehart into a recreation of a USMC version of the receiver. Quite recently, July 2022, I reconfirmed with Mike that the receiver he had seen years earlier had been a Capehart with an OD front panel. The double-check with Mike was to justify what I was about to do next to this old recreated OD USMC R-390A.
Just before the closing of the Western Historic Radio Museum in 2012, I had disassembled the Capehart. All of the modules were taken out of the main frame. I think I was planning to rebuild the receiver with new capacitors or something (not necessary since most of the caps are original polyfilms.) The receiver was working but not to the capabilities of a typical R-390A. >>>
|>>> Through the move to Dayton, I kept the Capehart parts together. Sometime later,
I acquired a large collection of R-390A "stuff" that included several parts set R-390A receivers and many
extra modules of all types. I stored all R-390A parts on the shelves of one large 48"
wide, 24" deep by 72" tall shelving unit. For some reason, while rebuilding one of
the EAC receivers, I robbed Z503 off of the Capehart IF deck. I also
robbed the CL and Line Level meters for one of the other R-390A projects
several years ago. Actually, for the past ten years just the Main Frame
and Front Panel were bolted together. All of the other modules and parts
were scattered in with all of the R-390A parts.
When I purchased the black anodized front panel for the NSA recreation it came with both the CL meter and the Line Level meter. Those two parts had been missing from the Capehart for quite a while. So, here it is July 2022, and now I'm thinking about bringing this OD panel recreation of a USMC R-390A back to life.
Locating Modules - I've gotten most of the correct modules together. I didn't remember which PTO was originally in the Capehart. Years ago, I was going to install a Raytheon rebuilt PTO into this receiver but I couldn't find any of the PTOs that had Raytheon labels. I did find two really nice Cosmos PTOs, so I'll start with the best one and see how it goes (if it's a dud, I have lots of other 70H-12 PTOs in the parts collection.)
I'll have to repair the Capehart IF module which hopefully won't be too "hacked" (I don't really remember how careful I was in removing Z503, hopefully I didn't do too much damage.) I have a couple of good "donor" IF modules to provide a Z503. This Capehart IF module was in very good cosmetic condition and did function, otherwise I'd replace it with the fairly nice Amelco IF module I have.
As I remember, the original Capehart AF module had a bad transformer on one of the audio lines. I tested the AF transformers and T601 (LOCAL AUDIO) has an open primary. I have several AF modules so replacing the bad transformer shouldn't be too much of a problem. NOTE: Unfortunately, Capehart put their name on the two audio transformers so a "non-Capehart" transformer will be noticeable - but only to OCD restoration enthusiasts (like me.) In the depot, they would have just replaced the module, maybe that's what I should do.
I got down the Capehart RF deck and spent about ten minutes on it (straightening two bent brackets) before I noticed that the cams weren't sync'd at +7.000mc and the Crystal Oscillator module was gone. This wasn't even the right RF deck! Looking back on the top shelf there was another Capehart RF deck but this one didn't have any RF transformers and the slug racks were just setting on top of it but it had the Crystal Oscillator module and the cams were sync'd correctly. I didn't remember pulling all of the RF transformers but I do have a large plastic bag full of RF transformers so I guess that's where they are. The other "bent" Capehart RF deck did have all of its RF and vari-IF transformers installed along with all of the slugs and racks so I could rob those if I need to (probably an easier solution in the long run.)
I found two Capehart PS modules in the pile and I'm pretty sure about which one is original to this receiver since the other one has bashed-in tube socket collars.
All of the knobs had been removed (why'd I do that? No doubt to replace beat-up knobs on one of the other projects.) I may end up having to restore a set since most of the spare knob sets I have are in rough condition. All of the tubes are missing in the IF, AF and RF modules. 3TF7 might have to be ordered but I have the rest.
Rebuilding Some of the Modules - I definitely have to work on the RF deck, the AF module and the IF
module so those three units were brought in from the shop first. The RF
deck is the most complicated and is missing the most components so it
will be first on the bench. The RF deck reconditioning will use the
"junk" Capehart RF deck for parts since what is missing on the "rebuildable"
deck is present the "junk" deck.
Aug 1, 2022 - RF Deck - I took a look at the gearbox and I must have never given the Capehart RF module the "WD-40 Gearbox Flush" treatment. It's described in the Rebuilding the RF Module section in Part 1. It takes almost an entire can of WD-40 and a long handle paint brush to really flush-out all of the excess grease. I don't know why so many of the earlier R-390A gearboxes seem to have been "greased." The manual specifically indicates NOT to ever use grease (but who reads the manuals?) Anyway, the last two R-390As I refurbished were 1967 EACs and in both receivers the gearboxes were very clean and didn't need the flush. However, this Capehart was full of dirt, dust, dirty dried-up grease and even some spider webs. I did the flush outside and then brought the RF module into the shop and put it on some rags on the floor to drain for awhile.
I also brought into the upstairs rebuilding shop the main frame, the AF module, the PS module and the PTO. I already had the spare IF modules stored upstairs. Since the RF module was still out in the shop drying out from the WD-40 flush, I cleaned the Main Frame thoroughly along with the front panel using Glass Plus. Once the RF module is clean I can put it on the bench and begin reassembly.
More to come as this project progresses,...
Creating an Authentic Arvin Industries R-725/URR
I wasn't really looking for another project but when nearly all of the parts turned up in a trade, well,...I couldn't help myself.
Finding the Parts - I received an e-mail from an audiophile-collector friend of mine asking if I'd be interested in purchasing all of his R-390A parts. There was a main frame with most of the modules, another RF deck, an Audio deck, PS deck, PTO and a front panel, all for $100. It sounded like a good deal so I went over and picked them up. When I got the parts home and closely inspected them I discovered that the main frame was a '67 EAC that had the R-725 mods installed. The main frame still had the Arvin Series 500 IF module installed. The Series 500 modules were built by Arvin specifically for the R-725/URR.
Essentially, the Series 500 IF deck is just like the IF deck used in the R-390. Six stages of IF amplification and no mechanical filters. The original R-390 IF deck used BNC connectors for input and output but the R-390A used BNC Junior connectors. The Series 500 uses BNC Junior connectors to match the R-390A and also the new versions performed any other changes necessary to make the Series 500 just a "drop in" conversion for the R-390A.
Among the other R-390A parts was a Cosmos PTO that had a ferrous metal shield installed around the outer shield-can. There was also a mod to the PTO that had an extra wire exiting from the PTO tube socket area. Another part that was included (but wasn't installed in the main frame) was a small chassis with a 25vac transformer mounted on top and a couple of resistors underneath.
Unfortunately, someone had severely damaged the R-725 main frame. One side looked like it had been hit with an axe. Some of the harnesses had been "chopped" to remove their Amphenol connectors. The front panel was missing. The Veeder-Root counter was missing. Luckily, the special added harness for the addition of the small 25vac transformer chassis was still present although it had been cut for some reason. At least the harness was all there but bifurcated.
I was missing the correct data plate since the original
front panel was missing from the junk R-725 main frame. In early
February 2018, I received a data plate for an Arvin R-725 from Moe
Sellali CN8HD/W9, in Chicago, who is an ardent R-725 enthusiast. Moe
told me that my Series 500 IF module should have a serial number
ink-stamped on the rear of the chassis. According to Moe, when Arvin
completed the R-725 mods to each '67 EAC R-390A, this was the serial
number that was stamped on the front panel data plate. My Series 500 was
stamped "074" so Moe sent me the R-725 data plate with "74" as the
serial number. >>>
photo left: The Arivn R-725/URR built from the 1967 EAC R-390A (originally SN: 974) with the installation of an Arvin Series 500 IF deck, the hum bucker chassis, the special PTO, IF output conx and the Arvin SN: 74 data plate. From the top, the most apparent R-725 addition is the Series 500 IF module. Note how the input and output coaxial cables connect to the mounting bracket for the Meter and IF Gain potentiometers. Also, note that the rear panel IF output requires a special right-angle coaxial fitting with the cable routed to J14 on the rear left corner. Also, the Amphenol power connector is turned 90 degrees from the standard R-390A IF deck.
Purpose of the R-725 Modifications - For Adcock Direction Finders - or - Was that just a Cover Story? - The usual purpose that is given for the R-725 mods was for compatibility with military portable direction finders that used four vertical antennae per installation along with three receivers. The DF system used went back to the Bellini-Tosi type of DF set-up that used two crossed loop antennae with a rotating loop inside to create a radio-goniometer. Bellini and Tosi had discovered that crossed loop antennae would "re-radiate" the signal they were receiving within the small field inside the antenna's space. The "re-radiated" signal retained all of the directional properties of the original signal and could be measured for varying signal intensity dependent on direction. The crossed loop antenna size didn't affect it frequency of operation allowing for reduction in the size of DF loops on LW. Of course, the original Bellini-Tosi system dated from around 1900 and the system was sold to the Marconi Company around 1907. By the early twenties, vacuum tube amplifiers were being added to increase performance capabilities of the DF antennae systems. The most common B-T DF systems used the crossed loops but some larger systems used the four-square vertical antenna system and a rotational loop (the goniometer) within the square. This system was developed by Adcock during WWI and because the connections to and from the four square verticals were underground it didn't respond to skywave propagation and allowed ground wave DFing over long distances. The B-T DF and Adcock systems continued to evolve and improve and the systems were used throughout WWII. During WWII, oscilloscope displays began to be used for direction indications. After WWII, larger DF systems continued to be developed up to the mammoth "elephant cage" antennae ("Wullenweber" was the actual name) that were over a thousand feet in diameter and consisted of several "rings" of circular antennae all working to provide accurate DFing over great distances and wide frequency spans. By the 1990s, most of these large arrays were becoming obsolete and nowadays most have been dismantled.
The mechanical filters used in the R-390A resulted in signal path phase shifts that caused errors to show up in the DFing electronics. When used with the four square antennas, the low frequency modulation added via the radio-goniometer interacted with the mechanical filters creating the error. Early versions of this DF set-up had used R-390 receivers and the radio-goniometer was located quite a distance from the receivers to reduce any interference. In the 1960s, the USAF wanted to reduce the size of the entire DF system so it could be towed around on a trailered hut. This meant the radio-goniometer had to be in the same room as the receivers. This was going to require some protection to certain receiver circuits. The R-390 had been out of production for several years, so the solution was to design the new portable system to use modified R-390A receivers that could be easily purchased. Arvin Industries was the main contractor with Servo also doing some rework. The modified receivers would have the Series 500 IF module, essentially a R-390 IF module that was slightly updated to not require any rework to the R-390A receiver it was installed into. That eliminated the mechanical filter phase shift problem. Additionally, with the close proximity to the radio-goniometer, a 60hz hum appeared on the PTO tube filament and that also interfered with the LF modulation of the DF system. A special "hum bucker" chassis was added to the receiver that essentially operated the VFO tube, the BFO tube and the 3TF7 Ballast tube on +25vdc. Also, a grounded ferrous metal shield was added to the PTO housing to prevent hum "pick up." Arvin bought new R-390A receivers in 1967 from Electronic Assistance Corporation and the modifications were installed and, when complete, the receiver was tagged as "R-725/URR." The tags will generally show Arvin Industries as the contractor but sometimes Servo will be encountered. The quantity of R-725/URR receivers needed by the USAF was fairly small (less than 300, according to Moe) and thus today the R-725 is seldom encountered. Contact number on the R-725/URR was DAAB05-67-C-2338.
However, was there another purpose that was the "real" reason that the R-725 was created? According to an article that appeared in Electric Radio in January 2006 by Chuck Teeters, there was a "top secret" purpose for the R-725 and the receiver "mods" were initially created for that "secret" project. The R-725 was a product resulting from the Cold War jamming that was common between the USA and the USSR. In the mid-to-late 1960s, there was a new system that was being developed called "Tropicom" that was an upgrade to the antennas and transmitters to improve HF communications for the military. The upgrades also included the incorporation of the "F9c" anti-jamming/crypto system. The F9c system used a spread spectrum transmission of digital noise and signal that ran through a digital encrypo-key generator that had 144 stages of looped-feedback that also fed through phase modulators to maintain proper phase relationships of the signal and noise. When used with a R-390A on the receive end, the phase changes in the mechanical filters interfered with the recombination process and the system didn't work. When used with R-390s with a standard IF amplifier circuit, the F9c system worked fine. Since the R-390 dated from the early-1950s, there was only a limited supply of those receivers still available and those that were available needed constant maintenance. The ultimate solution was to have new R-390A receivers built with new-build R-390 IF modules installed.
In order to keep the F9c project "secret," the actual use of the R-725 couldn't be known to those outside the project. Since there really was the Adcock DF system upgrades that really did need a non-mechanical filter type R-390A, the R-725 was directed to be built for the DF purpose only. However, those running the F9c project had the R-725 order quantity doubled and half of the R-725 receivers were procured for F9c use while the other half went to the DF systems. The secret classification stayed on with the F9c system and it was used for quite a long period with many upgrades over the years. So, even though half of the R-725 receivers were used in direct finders, the other half had a "secret life" used in the anti-jamming/crypto communications world of the NSA, USAF and the Signal Corps.
Testing the R-390A with a Series 500 IF Module - With the donation of the Arvin R-725 data plate it looked like I had all of the parts to build-up a R-725 if I could supply a complete 1967 EAC R-390A. According to Moe, when Arvin built-up the R-725 receivers they purchased new '67 EAC R-390As direct from EAC to fulfill the contract, thus all Arvin R-725s are converted '67 EAC R-390A receivers. I decided to use my '67 EAC SN: 974 R-390A because this receiver had recently been partially "cannibalized" to complete another EAC R-390A. I needed to replace a defective RF transformer on the 2-4mc antenna stage and do some minor alignments. Luckily, the "junk" R-725 RF deck supplied a good RF transformer. The first step was to check out and test the Series 500 IF module. One of the IF transformer cans was severely dented and needed "body work" to correct. All of the tubes were missing. I checked over the underneath and all components appeared to be in good shape. I gave the Band Width switch a DeOxit treatment. I needed tubes and tube shields. I found all of the tubes in my tube storage. The shields were "borrowed" from the EAC IF deck as was the 3TF7. The Series 500 is a "tight fit" but it does fit (see above photo.) The chassis is somewhat longer so the captive screws are located on the chassis rather than on the flange. The Band Width and BFO shafts are shorter than on the standard IF deck. The input and output coax connectors are in a different location but the cables reach easily. There is no clearance for the rear IF output cable as it is directly behind one of the 12AU7 tubes. The junk R-725 main frame even had the rear IF output connector totally removed. A special connector is required for the IF output on the R-725 conversion. The Amphenol connector has to be turned 90 degrees but everything lines up and there is ample flexibility to allow for this connection.
With power applied, everything came up as expected. The first thing noticed was that the IF Gain must have been at "maximum" - it was. After some testing and listening, I reduced the IF gain by about 50 percent. This provided ample IF gain and much lower noise levels. Carrier Level was adjusted on 15mc to zero with the antenna disconnected. BFO was zeroed. I didn't do a 455kc IF alignment since this was just a "check out" but the IF deck already seemed to be performing better than expected.
Mounting the Hum Bucker Chassis - Mechanically, the "hum bucker" is mounted in front of the R-390A power supply. This requires a bracket with pem-nut on the PTO side plate and two holes on the outer side panel to mount the "hum bucker" chassis. I removed the PTO side plate from the "junker" R-725 main frame because it had the original bracket already mounted. I removed the original PTO side plate from the R-390A and installed the R-725 side plate in its place. I carefully measured the original "junker" R-725 main frame side panel for the correct location of the two mounting holes. I then marked and drilled the R-390A side panel in the original manner. These modifications allowed the "hum bucker" chassis to mount exactly as it did in the original R-725. Now the "hum bucker" installation was electronically and mechanically complete. On to the PTO next.
Testing and Calibration the PTO - I'm using the original Cosmos PTO from the junk R-725 main frame. This PTO already had the mod installed that lifted pin 3 of the VFO tube from chassis. Then a wire was connected to pin 3 and it was routed back to the PTO connector where it is connected to the unused pin C. Also, a .01uf ceramic disk was installed from tube socket pin 3 to chassis. The PTOs that were used in the R-725 had a ferrous metal shield installed over the can of the PTO and this PTO did have that shield installed. I have a R-390 PTO test fixture that was given to me by W6MIT. The test fixture allows powering the PTO and employs a digital turns-counting dial to accurately set the end-point error to <0.5kc. I had to supply +195vdc B+, Regulated +150vdc, 6.3vac and chassis ground. Output was measured from the coaxial cable of the PTO using a digital frequency counter. I used a Lambda 25 for the B+ and 6.3vac and a regulated +150vdc supply. With the PTO on the fixture and powered up, the first step was to adjust the PTO output to 2455kc, then set the counter to 00.0 and tighten the coupler. The fixture counter works the opposite to how the PTO functions in the receiver. Since it's a mechanical readout on the drive rotation it doesn't really matter and our actual check was to verify that the PTO output changes from 2455kc to 3455kc in exactly ten turns. A quick check revealed that the end-point error was close to 1.0kc. I ran thru each turn to check linearity and this PTO was "right on." If it had been necessary to adjust the PTO end-point I would have followed the procedure as detailed in the PTO section further up this web page. To install the PTO only requires that it be set to 3455kc output with the R-390A having xx.000 on Veeder-Root counter. When the R-390A Veeder-Root counter is set to xx.000 then the Oldham coupler aligns correctly. The power connector is installed and the output coax connected to the RF module. This completed the PTO modifications and, in fact, completed all of the R-725 mods necessary.
Installing the Special Right-angle Coax to BNC fitting for IF Output - If it's attempted to fit the original IF output coax cable onto the original coax box BNC Jr to BNC output fitting, it will become obvious that there isn't enough clearance due to the 12AU7 tube directly in front of the connector. For the R-725, Arvin replaced the rear panel BNC Jr to BNC connector with a special mini coax input at a right angle to BNC output connector. This "low profile" fitting provided enough clearance to then connect the exiting cable to J-14 which is the IF Output on the Series 500 IF module.
To install the right-angle fitting requires a slight enlarging of the mounting hole which Arvin apparently did by filing the hole until the connector fit (I did check the junk main frame and it showed evidence of filing.) The coaxial cable should be installed onto the connector first. The center conductor of the coax is routed through the right-angle tube and the shield is placed over the outside of the tube. The center conductor is soldered to the center pin making sure the teflon spacer is installed afterward. Then the crimping barrel is placed over the shield and right-angle tube and crimped in place. Then the back cover nut is installed. The BNC right-angle fitting with coax attached can then be mounted to the rear panel of the receiver with a locking washer and nut. Then the BNC Jr. end connector can be attached to J-14. This completes the installation. Thanks to Moe CN8HD/W9 for supplying the correct coaxial right-angle fitting (as mentioned, my coaxial fitting was missing from my "junk" R-725 main frame.)
Testing the R-725 - The R-725 mods were for a DF set-up (or for Tropicom) so the changes to the PTO tube, BFO tube and 3TF7 tube filament supply are very subtle and not noticeable by just listening. However, the "big change," that is, adding the Series 500 IF module and thus eliminating the mechanical filters and adding more IF stages,...that is very noticeable. In fact, it's impressive! The gain is amazing. I have the Series 500 IF gain set to 50% and the strong signals will still send the Carrier Level meter to 80 or 90 db. If I tune off of the signal, the meter drops to 10db. The selectivity is still very good. Just about as good as mechanical filters. I've used the R-725 several times as a station receiver (set up with the T-368 and using two half-waves in-phase antenna) and it can always be counted on the "pull in" the signals and is easily able to cope with any QRM. Audio quality is good and sounds pretty close to a typical R-390 receiver. Probably one could sum up the R-725 as an "easier to work on R-390" with all of the benefits of the R390 without as many headaches.
Wrap-up - Well,...what is it? A restoration or a recreation? I was extremely careful to use authentic R-725 parts harvested from a "destroyed, incomplete" R-725. I was very careful to exactly duplicate how the wiring harness was integrated into the R-390A harness. I even used the original hum bucker harness for authenticity. Original R-725 sheet metal was used where needed. Even the receiver used for the conversion was a 1967 EAC R-390A. The data plate used was an exact copy, etched tag - not a silk screened tag but one made just like the originals. Even the serial number stamped in the tag matches the serial number ink stamped (in 1967) on the back of the Series 500 IF module. And, all of the R-725 parts came from the same "destroyed, incomplete" R-725 which must have been the original SN: 74. So,...when looking over this R-725/URR,...I consider it an authentic restoration of SN:74. Just that the original R-725 SN:74 was modified in 1967 and my modification/restoration was performed in 2018. Close enough,...right?
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