Before we start, a word of caution. The heater of the 6BH6 comparator is wired in series with a tube in another sub assembly. Thus if you fire up the radio without all sub assemblies being plugged in the voltage regulator will not work and regulated B+ will go to +300v. This is a bad thing. See below.

Probably the most pesky part of the R-390 radio is the audio chassis which also contains an elaborate voltage regulator. This latter is the source of a great deal of heat which in turn leads to the deterioration of adjacent
components. Servicing this can be a bear so these notes are compiled from my recent experience with a dozen or more of these units. Most of the units I have seen have had faulty voltage regulators and a good number have had audio problems due to heat. The VR symptoms are usually no regulation. Regulated B+ reads full B+, around 300 volts. This, if allowed to persist, can lead to other problems like weak or dead 6AJ5 tubes. I have also found the reverse symptom like low voltage around 130 volts or less. The first thing to check is the 6082 regulator tubes.
There have been several instances of them being replaced by 6080 tubes. These have 12v heaters compared with the 28v of the 6082 and surprisingly they will work for a time but failure is inevitable. The next area to check out is the voltage reference tubes.When the unit is operating correctly these two tubes will glow quite brightly for a short period and then become somewhat dimmer as the regulation function starts to work. If they stay bright then check R625 (1k), R626(2.7k) R624 (520k) and especially C608. If this capacitor is leaky the voltage will be low.
Next check the components around V607. C606, R618 and R623 are the critical ones.If none of this works then you will just have to check all the other resistors in the circuit. Be sure the four 47 ohm cathode resistors for V605 and 606 are OK. I had one case where I had to replace all these.