Author Topic: How to Modify an R-390 IF Deck to Work with a R-390A (Make Your Own R-725).  (Read 47 times)

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Offline W1RC

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This is the procedure to convert a R-390 IF deck to work in a R-390A as was done in the R-725. Tom, N5OFF, wrote this in 2000 I believe.

The main difference between a R-725 and the R-390A is that the R-725 utilizes a custom built IF deck that is very similar in construction to the R-390 IF deck. It has tuned circuit selectivity instead of mechanical filters. The mechanical filters of the R-390A created distortion when that radio was employed for radio direction finding use.

In an internal Collins engineering report published in 1952, Lou Couillard wrote of the improved R-390A, which at that time was called the R-390(XC-3), "Although the superior shape factor of the mechanical filter IF is desirable in most applications, consideration should also be given to an alternate tuned circuit design for use in special applications. The excellent shape factor of the mechanical filters precludes the possibility of linear phase shift across the passband. Where a linear phase characteristic is desired such as in direction finding equipment, a tuned circuit IF is necessary."

Employment of the tuned circuit IF is exactly what was done in the R-725.

Motorola was awarded a contract in 1956 (476-PH-56-91) to prototype the R-725. I know of a couple of these Motorola sets existing today. Packaging of modified sets for quantity DF use were handled by Arvin Industries and Servo Corporation of America.

Approximately three hundred R-390A's were modified to the R-725 configuration. These sets are relatively tough to find today. New IF decks were manufactured by the modification companies (actually salvaging some of the components from the now junker R-390A IF decks) and installed in existing R-390A's. The new IF decks were named "SERIES 500 IF STRIP ASSY." They looked almost just like R-390 IF decks (see the comparison photos), except that the IF connectors were relocated to match the cables and connectors in the R-390A chassis. The circuits were designed to plug and play in the R-390A instead of the R-390. The decks are not interchangeable (until now that is, after performing the modification described herein).

Notwithstanding DF capabilities, a side benefit of the SERIES 500 deck is that it provides a smoother sound than does the stock R-390A IF deck. Mechanical filters are said to "ring" and after a while can be fatiguing to the listener. The purpose of this article shall be to describe how one may "roll his own" SERIES 500 IF deck from a surplus R-390 IF deck. Please note that I don't advocate trashing of a good R-390 to do this mod. The IF deck I started with came from a Motorola junker.

I would urge you to likewise find a junker R-390 as a source of an IF deck for this project . Make sure that the deck is in working condition prior to beginning the modification.

The R-390 IF deck was designed to operate with one 25V filament supply. The R-390A deck was designed to operate with a combination of 6.3V and 25V filament supplies. The task involved in this conversion is to rewire the R-390 filaments to conform to the voltages 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 IF deck must be addressed for full compliance with the voltages available from the R-390A.

Plug P112 of the R-390A shall be plugged-in to the R-390 IF deck at jack J517. There is much commonality here, except for the connections mentioned herein.

In general, you will be converting 25V series connections into 6.3V (herein referred to as 6V) parallel connections for most tubes, and moving the connections of the BFO/PTO/ballast tube series to a different connection point in jack J517. The 6V filament supply shall be provided to the R-390 deck by pin 20 of P112 from the R-390A.

General instructions: Refer to the schematic for the original R-390 as the "before" schematic. Use the best soldering technique you can in this limited access space. Don't insulate or bind any wires until instructed to do so. You will be utilizing some of the new 6V supply connections more than once. Make sure you can recognize your new wires. I used black wire for the 25V supply, red wire for the 6V supplies, and green wire for new grounds.

The first task is to install a B+ dropping resistor to better match the 180V B+ that the R-390 IF deck is expecting. To do this, locate inductor L503 under the IF deck. This will be found snapped into a holder right above pin 2 of J517. Disconnect one end of the coil, and install in series with it (the equivalent of) a 470 ohm 2 watt resistor. This will tame the B+.

The first tube circuit we'll work on is the ballast tube circuit.

V508 (5749) and RT512 (3TF7) These must be supplied by the R-390A 25V filament supply. To do this, sever the connecting wire at pin 8 of jack J517 (underneath the deck) to free this slot up (hint: save access to the connector end of the wire as you will use it to wire supply to V509). Then, sever the connection at pin 2 of RT512 and wire this pin to pin 8 of J517 of the R-390 deck with a long piece of new wire. The filament return connection remains unmodified. This modification will make the BFO/PTO/ballast tube series connections identical to the R-390A 25V filament supply connections. As mentioned above, this 25V supply is insufficient to supply the remainder of the tube filaments, thus the need to employ the 6V supply for this task.

The following 6V tubes shall have filaments wired from the 6V R-390A supply. The filament pins of these tubes are pins 3 and 4. Don't sever any connections unless instructed to do so. The modification will use as much existing R-390 IF deck wiring as possible (and thus may seem a bit screwy to you until finished).

V504 (6BJ6) 6V will come from its existing connection at pin 4. Ground will come in the next step.

V503 (6BJ6) Wire pin 3 of V503 to pin 4 of V504 for 6V supply. Ground V503, pin 4.

V502 (6BJ6) 6V supply will come from an existing connection at V503, pin 3. Ground V502, pin 3.

V501 (6BJ6) Sever ground connection at V501, pin 3 and wire pin 3 to V502, pin 4.

V505 (6AK6) 6V supply will come from existing connection at pin 4. Ground will come in the next step.

V506 (6AK6) Ground pin 4 of V506. Wire pin 3 of V506 to pin 4 of V505 for 6V supply.

V509 (6BJ6) Locate the free wire which was cut from underneath J517, pin 8, and connect it to the 6V filament supply at J517, pin 20.

The following tubes are 12AU7's wired in various series schemes in the R-390. They must be rewired according to their 6V option for use in the R-390A. Note two of the connections require dropping resistors on the 6V source of V507 and V510 to obtain the desired 5.3V filament voltage.

V511 Sever ground connection at pin 5. Connect pins 4 and 5 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 for variable capacitor C525, to pins 4 and 5 through a 3.9 ohm, 1 watt resistor. Ground pin 9.

V 510 Sever connections at pins 5 and 9. Ground will come from existing pin 4 connection. Remove 120 ohm resistor between pins 4 and 9. Wire pin 5 to pin 4. Wire 6V supply from J517, pin 20 to V510 pin 9 through a 3.9 ohm, 1 watt resistor.

There are no changes to any of the other connections in the R-390 deck.

Prior to installing the modified R-390 deck in your R-390A, you must check your work.

Perform continuity checks from J517, pin 8, with RT512, pin 2.

Perform filament supply and ground continuity checks as follows. Note: There will be more than one 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.

Now insulate any bare connections, and use mini-tie wraps to secure the new wires to sturdy nearby points.

To enable final installation of the deck in your R-390A, you'll need to make two adapter cables. These cables shall consist of jumpers (RG-59 is OK, approximately eight inches in length) with BNC's on each end. You'll also need two adapters of the type found on the back of the frame of the R-390A at the IF OUT jack (AMPHENOL 47200). This will provide crossover from MB connection (R-390A standard)to BNC (R-390 standard). Connect P-218 of the R-390A to J-526 of the R-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 other cable. It is a good idea to label these cables. See the photo for reference.

Install the deck in your R-390A. You will notice that the screw holes are the same as for the R-390A IF deck, however the screws of the R-390 deck are of larger diameter. I did not change these screw as they are captive into the deck. The BFO, BANDWIDTH, and power connector of the deck will hold it in place, however I would not install it in a Jeep this way. Changing these screws is optional.

When you turn on the power, make sure your dial lamps light up normally. If they don't, you have a filament supply problem so turn off the set immediately and troubleshoot.

For great sound, instead of using the built in audio deck, I prefer to tap the audio from the diode load jumper at the back of the set. Through a 0.1 uF or larger capacitor, feed this signal into your line audio amp of choice, and enjoy the tuned circuit audio of the new R-725, errrr, R-390A with tuned circuit IF. You'll get the smooth sound of the R-390 and R-725, but have the parts availability and support common to the R-390A for the balance of the set. I've used my modified IF deck in two different R-390A frames, and it worked equally well in both. In my opinion, it makes the long term listening experience much more enjoyable.

Tom Marcotte, N5OFF