[R-390] AVC Voltages

[Miles Anderson]

I think we have two separate questions here. (1) The actual voltage on the
AVC line and (2) the behavior of the carrier meter.

With respect to the AVC line, the most common problem (other than failing to
strap the AGC-NOR terminals on the back panel) is a leaky bypass capacitor.
The AVC line is isolated at each controlled stage by a series 22k resistor
and a .005 bypass capacitor. It comes from a very high impedance source.
Leakage in any one of the bypass capacitors will drag down (closer to zero)
the negative bias on the whole line. This also means that you can accurately
measure the AVC voltage with a VTVM or other high impedance  (1 meg or
preferably 10 meg) voltmeter. In checking for leakage, don't forget that the
AVC bus also drives the RF amplifier and is coupled through the bandswitch
and the bandwidth control that switches the IF filters.

The carrier meter is a different story entirely. It doesn't measure the AVC
line directly. Instead, it measures the cathode current drawn by the 4th IF
amplifier (V504). The cathode current of V504 depends on its grid bias.
Since V504 is an AVC-controlled stage, its grid bias is controlled by the
AVC line. In short, V504 has two functions. In its AC mode it is an IF
amplifier boosting the 455 kHz signal to drive the detector. In its DC mode
it is a DC amplifier that drives the carrier meter.

Result: (1) If the AVC line itself is misbehaving as measured by a VTVM,
solve that problem first and look for a leaky bypass capacitor. (2) If the
AVC voltage is OK and only the carrier meter is behaving strangely, confine
your troubleshooting to V504 and, in particular, its cathode circuit.

Miles Anderson, K2CBY
16 Round Pond Lane
Sag Harbor, NY 11963