Here is the text for the conversion.
73 Tom
Conversion of the R-390 IFDeck for Use in R-390A For Improved Audio,OrMake Your Own R-725 Thomas F. Marcotte, N5OFF Asmany of the readers of Electric Radio know, the US Army created a modifiedR-390A version for direction finding known as the R-725/URR. I wrote about this modified radio in thispublication a few years ago.The main difference between a R-725 and the R-390Ais that the R-725 utilizes a custom built IF deck that is very similar inconstruction to the R-390 IF deck. Ithas tuned circuit selectivity instead of mechanical filters. The mechanicalfilters of the R-390A created distortion when that radio was employed for radiodirection finding use. In an internal Collins engineering report published in1952, Lou Couillard wrote of the improved R-390A, which at that time was calledthe R-390(XC-3), "Although the superior shape factor of the mechanicalfilter IF is desirable in most applications, consideration should also be givento an alternate tuned circuit design for use in special applications. The excellent shape factor of the mechanicalfilters precludes the possibility of linear phase shift across thepassband. Where a linear phasecharacteristic is desired such as in direction finding equipment, a tunedcircuit IF is necessary." Employment of the tuned circuit IF is exactly what was done in the R-725. In addition to the new IF deck, the R-725was given a new filament transformer and a minor modification for the PTO toprovide improved stability. Motorolawas awarded a contract in 1956 (476-PH-56-91) to prototype theR-725. I know of a couple of theseMotorola sets existing today. Packagingof modified sets for quantity DF use were handled by Arvin Industries and ServoCorporation of America. Approximatelythree hundred R-390A's were modified to the R-725 configuration. These sets are relatively tough to findtoday. New IF decks were manufacturedby the modification companies (actually salvaging some of the components fromthe now junker R-390A IF decks) and installed in existing R-390A's. The new IF decks were named "SERIES 500IF STRIP ASSY." They looked almostjust like R-390 IF decks (see the comparison photos), except that the IFconnectors were relocated to match the cables and connectors in the R-390Achassis. The circuits were designed toplug and play in the R-390A instead of the R-390. The decks are not interchangeable (until now that is, after performingthe modification described herein). NotwithstandingDF capabilities, a side benefit of the SERIES 500 deck is that it provides asmoother sound than does the stock R-390A IF deck. Mechanical filters are said to "ring" and after a whilecan be fatiguing to the listener. Thepurpose of this article shall be to describe how one may "roll hisown" SERIES 500 IF deck from a surplus R-390 IF deck. Please note that I don't advocate trashingof a good R-390 to do this mod. The IFdeck I started with came from a Motorola junker. I would urge you to likewise find a junker R-390 as a source ofan IF deck for this project . Make surethat the deck is in working condition prior to beginning the modification.TheR-390 IF deck was designed to operate with one 25V filament supply(unfortunately the 25V supply available in the R-390A is insufficient to powerall of the filaments in the R-390 IF deck). The R-390A deck was designed to operate with a combination of 6.3V and25V filament supplies. The taskinvolved in this conversion is to rewire the R-390 filaments to comply with thevoltages available in the R-390A and provided at the main IF deck connector,plug and play, without the addition of any new power transformers. Each of the twelve tubes in the R-390 IFdeck must be addressed for full compliance with the voltages available from theR-390A. Refer to the schematic in Fig 1as to the final filament wiring configuration. We will also drop the B+ a bit.PlugP112 of the R-390A shall be plugged-in to the R-390 IF deck at jack J517. There is much commonality here, except forthe connections mentioned herein.Ingeneral, you will be converting 25V series connections into 6.3V (hereinreferred to as 6V) parallel connections for most tubes, and moving theconnections of the BFO/PTO/ballast tube series to a different connection pointin jack J517. The 6V filament supply shallbe provided to the R-390 deck by pin 20 of P112 from the R-390A. Generalinstructions: Refer to the schematicfor the original R-390 as the "before" schematic, and Figure 1 as the"after" schematic for filaments. Use the best soldering technique you can in this limited accessspace. Don't insulate or bind any wires until instructed to do so. You will be utilizing some of the new 6Vsupply connections more than once. Makesure you can recognize your new wires. I used black wire for the 25V supply, red wire for the 6V supplies, andgreen wire for new grounds.Thefirst task is to install a B+ dropping resistor to better match the 180V B+that the R-390 IF deck is expecting. Todo this, locate inductor L503 under the IF deck. This will be found snapped into a holder right above pin 2 ofJ517. Disconnect one end of the coil, and install in serieswith it (the equivalent of) a 470 ohm 2 watt resistor. This will tame the B+.Thefirst tube circuit we'll work on is the ballast tube circuit. V508 (5749) and RT512(3TF7) These must be supplied by theR-390A 25V filament supply. To do this,sever the connecting wire at pin 8 of jack J517 (underneath the deck) to freethis slot up (hint: save access to the connector end of the wire as you willuse it to wire supply to V509). Then, sever the connection at pin 2 of RT512and wire this pin to pin 8 of J517 of the R-390 deck with a long piece of newwire. The filament return connectionremains unmodified. This modification will make the BFO/PTO/ballast tube seriesconnections identical to the R-390A 25V filament supply connections. As mentioned above, this 25V supply isinsufficient to supply the remainder of the tube filaments, thus the need toemploy the 6V supply for this task. Thefollowing 6V tubes shall have filaments wired from the 6V R-390A supply. The filament pins of these tubes are pins 3and 4. Don't sever any connectionsunless instructed to do so. Themodification will use as much existing R-390 IF deck wiring as possible (andthus may seem a bit screwy to you until finished). V504 (6BJ6) 6V will comefrom its existing connection at pin 4. Ground will come in the next step. V503 (6BJ6) Wire pin 3 ofV503 to pin 4 of V504 for 6V supply. Ground V503, pin 4.V502 (6BJ6) 6V supply willcome from an existing connection at V503, pin 3. Ground V502, pin 3. V501 (6BJ6) Sever groundconnection at V501, pin 3 and wire pin3 to V502, pin 4. V505 (6AK6) 6V supply willcome from existing connection at pin 4. Ground will come in the next step. V506 (6AK6) Ground pin 4of V506. Wire pin 3 of V506 to pin 4 of V505 for 6V supply.V509 (6BJ6) Locate thefree wire which was cut from underneath J517, pin 8, and connect it to the 6Vfilament supply at J517, pin 20. Thefollowing tubes are 12AU7's wired in various series schemes in the R-390. They must be rewired according to their 6Voption for use in the R-390A. Note twoof the connections require dropping resistors on the 6V source of V507 and V510to obtain the desired 5.3V filament voltage. V511 Sever groundconnection at pin 5. Connect pins 4 and5 together for 6V supply, ground pin 9. V507 Sever connections at pins 4, 5 and 9,including the two resistors (one 120 ohm and one 22 ohm). Wire 6V supply from your previous work at V505, through the deck opening forvariable capacitor C525, to pins 4 and 5 through a 3.9 ohm, 1 watt resistor. Ground pin 9. V 510 Sever connections atpins 5 and 9. Ground will come fromexisting pin 4 connection. Remove 120ohm resistor between pins 4 and 9. Wirepin 5 to pin 4. Wire 6V supply fromJ517, pin 20 to V510 pin 9 through a 3.9 ohm, 1 watt resistor. Thereare no changes to any of the other connections in the R-390 deck.Priorto installing the modified R-390 deck in your R-390A, you must check your work. Perform continuity checksfrom J517, pin 8, with RT512, pin 2. Perform filament supplyand ground continuity checks as follows. Note: There will be more thanone ground connection at various tube sockets, but the filaments should have continuity exactly as shown. Filament supply is checked from J517, pin 20. V501, filament, pin 3; ground, pin 4.V502, filament, pin 4; ground, pin 3.V503, filament, pin 3; ground, pin 4.V504, filament, pin 4; ground, pin 3.V505, filament, pin 4; ground, pin 3.V506, filament, pin 3; ground, pin 4.V507, filament, pins 4 and 5; ground, pin 9.V509, filament, pin 4; ground, pin 3.V510, filament, pin 9; ground, pins 4 and 5.V511, filament, pins 4 and 5; ground, pin 9. Nowinsulate any bare connections, and use mini-tie wraps to secure the new wiresto sturdy nearby points. Toenable final installation of the deck in your R-390A, you'll need to make twoadapter cables. These cables shallconsist of jumpers (RG-59 is OK, approximately eight inches in length) withBNC's on each end. You'll also need twoadapters of the type found on the back of the frame of the R-390A at the IF OUTjack (AMPHENOL 47200). This willprovide crossover from MB connection (R-390A standard)to BNC (R-390 standard). Connect P-218 of the R-390A to J-526 of theR-390 IF deck with one of the cables. Connect P-213 of the R-390A to J-525 of the R-390 IF deck with the othercable. It is a good idea to label thesecables. See the photo for reference. Installthe deck in your R-390A. You willnotice that the screw holes are the same as for the R-390A IF deck, however thescrews of the R-390 deck are of larger diameter. I did not change these screw as they are captive into thedeck. The BFO, BANDWIDTH, and powerconnector of the deck will hold it in place, however I would not install it ina Jeep this way. Changing these screwsis optional. When you turn on the power, make sure yourdial lamps light up normally. If theydon't, you have a filament supply problem so turn off the set immediately andtroubleshoot. Forgreat sound, instead of using the built in audio deck, I prefer to tap theaudio from the diode load jumper at the back of the set. Through a 0.1 uF or larger capacitor, feedthis signal into your line audio amp of choice, and enjoy the tuned circuitaudio of the new R-725, errrr, R-390Awith tuned circuit IF. You'll get thesmooth sound of the R-390 and R-725, but have the parts availability andsupport common to the R-390A for the balance of the set. I've used my modified IF deck in twodifferent R-390A frames, and it worked equally well in both. In my opinion, it makes the long termlistening experience much more enjoyable. If you have any questions about the mod,please feel free to write. 73 DE N5OFFQRT Thomas Marcotte is a registered professionalengineer working in Lafayette, LA. References: Cost Reduction Program for RadioReceiver, R-390/391( )/URR, L.W.Couillard, Collins Radio, 1952. TM 11-856/TO31R1-2URR-154, RADIO RECEIVER R-390/URR, January, 1955. Servo Corporation ofAmerica drawings for R-725 series 500 IF deck, Order No. 36-039-N-5-00093(E),September, 1965. TM 11-5825-231-24,DIRECTION FINDER SETS AN/TRD-23, AN/TRD-23A AND DIRECTION FINDER SETS AN/TRD-15AND AN TRD-15A, August, 1973. TM 11-5820-358-35, FIELDAND DEPOT MAINTENANCE MANUAL, RADIO RECEIVER R-390A/URR, May, 1980. Special thanks to WallyChambers and George Rancourt.
From: Charles Steinmetz <csteinmetz at yandex.com>
To: r-390 at mailman.qth.net
Sent: Thursday, December 8, 2016 9:13 AM
Subject: Re: [R-390] DIY R-725 Conversion documents?
Bill wrote:
> Can someone post the source of the R-390A to R-725 conversion document?
> Or, send me a copy off-line? I have been contemplating this conversion
> and have accumulated the parts.
I believe Tom posted the text of the article on this list some time ago.
You can find it by searching the archives. It is also reprinted in
the later revisions of the Y2K manual.
The original article, from Electric Radio #97 (May 1997), also contained
at least one circuit diagram, which you probably also want. A reprint
of the issue is available from ER:
<https://www.ermag.com/index.cfm?v_link=product_detail&v_key=521>
Best regards,
Charles
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