Convert R-390 IF deck for use in R-390A, superior sound. R-725/URR like conversion.

The R-390 tuned circuit IF has more pleasing sound than the mechanical filter IF of the R-390A. These are the steps necessary to convert a R-390 IF deck for use in the R-390A, plug and play. 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 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. 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+. See first comment below for the rest of the procedure which was too long for this KZbin description box.

[@awiles79]

Very cool! Just read the article in electric radio magazine

[@kwm380]

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 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. 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.