[R-390] Gain drift

[dog]

I'm beginning to wonder if it isn't the AGC circuit that's messing up. I 
put 455KHz into J513, IF strip, and in AGC mode I get gain drifting 
around. In MGC it seems solid as a rock. Put an extension on V502 and 
with 3mV going into J513 I sometimes get 40 Carrier in AGC, but then it 
drifts down to 10 or so after a while, back and forth. But if I switch 
to MGC I get solid RF readings on the grid and plate of V502. In AGC 
mode the grid and plate of 502 is all over the place with the Carrier. 
Gain of V502 seems to be somewhat less than 30dB on the Boonton mVmeter 
in MGC. Switching in 10/20dB on the sig gen also registers the same on 
V502 plate and grid so that seems real.

I haven't seen anything on checking out the AGC rectifier and amp. Any 
ideas on that? Guess it could be something on the AGC line later that's 
pulling things down too but that's hard to check with the RX off, other 
than the static checks and the -7VDC on the diode load which seem OK. I 
suppose I could pull the RF module to eliminate that much.