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
PART 3 - Misc Info on R-390A, Restorations 1967 EAC, another 1967 EAC, Arvin R-725, R-390A Diversity Operation
PART 4 - Dynamotor R-648/ARR-41, R-389 Restoration, R-392 Info, Other R-390 Variants, Dial Cover, CV-979 Cabinet
Miscellaneous Information on the R-390A - Restorations, Variants and Accessories
1967 Electronic Assistance Corporation R-390A SN: 974 - Restoration Log (2016)
This is a 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 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.
Creating an Authentic Arvin 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 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 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 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 restoration was performed in 2018. Close enough,...right?
Dual Space Diversity Operation with the R-390A Receivers
|If you're lucky enough to own two R-390A receivers and
have room for widely separated antennas, you can easily set up the pair
to operate in Dual Space Diversity. Good separation of the antennas
would be at least one wavelength at the frequency of operation but
usable diversity effect can usually be obtained with closer spacing if
necessary. Space Diversity assumes you will be using two similarly
polarized antenna and are relying only on the phase differences of the
radio wave based on the spacing of the antennas. You can also try "Polar
Diversity" which relies on a vertical antenna for one receiver and a
horizontal antenna for the second receiver. Polar Diversity doesn't
require that the two antennas be separated by great distances and
assumes that there will be a benefit from the reception of two different
polarizations of the incoming radio wave. This assumes that some
splitting and rotation of the radio wave will occur as it propagates
through the ionosphere and is returned to earth. Generally, space
diversity helps with fading signals and polar diversity helps with phase
distortion due to wave rotation.
With either method of Dual Diversity reception, the receiver set-up is the same. You will be connecting the DIODE LOAD from each receiver together. The receiver that you plan on operating as the "master" will have to have the DIODE LOAD terminals jumped while the "slave" receiver doesn't have the terminals jumped. The "slave" receiver is only operating to the detector stage and its audio output is not used. You can connect 500 ohm resistors across the LINE AUDIO and the LOCAL AUDIO on the "slave" receiver. You will also have to install the jumps to connect AGC DIV terminals together on each receiver. You will also have a wire connecting the AGC DIV from each receiver together. A speaker on the LOCAL AUDIO is only required on the "master" receiver. To listen to just the "slave" receiver, turn the RF GAIN on the "master" receiver to 0 and what you hear thru its speaker is the "slave" receiver. Also, if you want to listen to just the "master" receiver, turn the "slave" receiver's RF GAIN to 0 and what you hear thru the speaker is the "master" receiver only. With both receivers operating and connected to their respective antennas, tune in a strong shortwave broadcast signal. Have both receivers' RF GAIN set to about 8. Don't set the RF GAIN on either receiver to "full on" (10) or each receiver will "fight" the other one for control of the AGC line. By alternately reducing the RF GAIN of each receiver to 0 you should be able to end up with both receivers tuned exactly to the signal. Once the SW BC signal is tuned in on each receiver you will need to "balance" the RF GAINs. Slowly increase the RF GAIN on the each receiver alternately to the point where you see the CARRIER LEVEL meter showing some response. Adjust the RF GAIN on each receiver until you have the highest CARRIER LEVEL readings on each receiver without one receiver or the other "overloading" the AGC line. When "overloading" occurs the CARRIER LEVEL meter on one receiver will drop much lower in its reading and with a reduction in the RF GAIN of the other receiver you'll see the meter reading jump back up. By "balancing the receivers" you get the best diversity response and the best sensitivity. You will note that the two receiver's CARRIER LEVEL meters will react differently since each receiver is responding to a phase difference in the radio wave based on the separation of the antennas. You should see deep fades that cause one CL meter dip while the other receiver's meter remains steady. You should also see a reduction in phase distortion if you are using the polar diversity set up.
Remember, you can only use AM reception on this type of Dual Diversity. That's because CW or SSB reception requires the BFO to be in operation and the BFO dominates the detectors and spoils the diversity effect. For RTTY reception special TUs were used like the CV-116 that was designed for diversity RTTY. Diversity CW reception required Tone Keyers.
So, give Dual Diversity reception a try if you can. It's interesting and sometimes beneficial to copy.
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