I am helping a fellow with his dead R-4C. I found the original problem, a puncture in one of the coax cables'
insulation pressing against the 12 volt line, pulling it to ground. Fortunately, this radio had been updated to a 7812 voltage regulator, which self-limited preventing its destruction.
However, someone had made other changes, among them the addition of a potentiometer in place of the 820 Ohm series resistor in the S meter zero circuit. It was poorly done and the pot was dangling in the air. I tried to revert back to schematic original, by putting in the 820 ohm fixed resistor where it belongs, but now I cannot zero the S meter. I determined a few things.
First, there was a 6BE6 in place of V5 (6BA6). Replacing this with a known-good 6BA6 improved the zero but not completely. I then determined that the screen resistor on V2 (6HS6) had been changed from 100K to 10K. This causes this end of the S meter bridge to draw too much current in a no-signal condition and this is the reason for the offset in the S meter (~S8).
Now I could just kludge in the pot again, but do a better job. I know it will zero with this change. I would like to know, first, if there is a good driving reason for there to be a change in the screen resistor value for V2 and also for the use of a pentagram converter tube (6BE6) in place of the pentode amplifier 6BA6 at V5.
Bob probably knows¡
Gary
W0DVN
