R-390A Audio Modifications
Chuck Rippel, WA4HHG
Some of you may know that I restore R390A receivers as a hobby. Here is an easy "soft " (reversable) modification which I came up with that really improves the R390A audio by mellowing it out a bit.
The audio tubes, V602A and V603 are coupled with 300V, .01mfd "Vitamin Q" type capacitors. These have been found to leak causing distorted audio. The values are also tend to make the audio amplifier somewhat restrictive. Changing these to .022 (or even .047 or .05 mfd) 400V (min) Sprague Orangedrops improves bass response and the increased voltage rating coupled with modern construction helps to keep the leaking problem from re-occuring. Do NOT use disc ceramic capacitors!
2- .022mfd Sprague Orangdrop Capacitors
Unplug the receiver and remove the bottom cover. Locate the audio subchassis (it's on the bottom of the radio and has five tubes on it) . Remove the audio chassis by pulling off the 2 modular plugs and loosening the 4 green captive phillips screws. Lift the audio chassis onto an appropriate workspace and turn it upside down. There is a 2" wide curcuit board which runs 2/3 the long length of the audio chassis. Locate C-604, C-605 and R-614 which are mounted on that pc-board. Remove the two screws holding the board to the audio chassis then fold it with the cables off to the side in order to expose the bottom of the board.
Remove C-604, 0.01 mfd 300v Vitamin Q.
Remove C-605 0.01 mfd 300v Vitamin Q.
Remove R614 560 ohm 1 watt resistor.
Replace C-604 and 605 with the orangedrop capacitors. Be sure to first make sure the capacitors do not take up too much vertical space and not allow the audio deck to properly seat in the main chassis. If space is a problem, try mounting the capacitors on the opposite side of the pc board from which they were removed. This is especially true if you use 0.047's or 0.05's or use 600V capacitors.
Replace R614 with the 560 ohm 2 watt resistor.
Replace the PCB, put the audio chassis back in the receiver and re-assemble the radio.
During operation, you will immediately notice that there is a more mellow sound to the receiver. This is due to the increased bass response allowed by the larger value capacitors. If you check the 560 ohm resistor, R614, you will probably see that it has changed value going higher in resistance. I have seen them go to 700-800 ohms.The receiver can be put totally back to "stock" in a matter of minutes by simply reversing the process.