[R-390] Gain drift

[dog]

OK, new data. I socked 50uV into the antenna balanced input right at 
cold start up. After a few minutes and things settle down, the agc is at 
-5VDC. Carrier meter is 48. Let it run for a while and soon the Carrier 
drops to 25-30 and the AGC goes to -2.4VDC. I've got AM mod going into 
it and the AC output of the diode load is holding steady at about .5VAC 
no matter which carrier reading. Then after a while the Carrier meter 
goes back to 48 with the corresponding AGC of -5VDC. Does this several 
times and I get tired of watching it so one time while it's on the 25 
carrier, I switch it to STBY and back to AGC. Goes right back to Carrier 
of 48 and corresponding AGC. Something is drifting but it's hard to hear 
on the audio, the AGC seems to be working but something is changing the 
gain during warm up.

After letting it sit for over an hour it seems to stabilize on Carrier 
level of 48 and AGC of 4.3. I have a list of uV vs. AGC voltage, I'll 
check all the agc vs uV voltages out after it warms up good.

I'm thinking with no antenna the AGC is down (.6VDC) where it's not 
working so a small change in gain produces a large output on the diode 
load audio output. That's why I'm hearing the difference with no 
antenna. Not so much output difference when there's a fairly large 
signal present and the AGC is working. I'm really not sure how to find 
this other than a shotgun approach. It's very difficult to get to the 
components to shoot freeze spray on them while in operation.

I do have another RF and IF section, but the other RF section doesn't 
appear to work, I tried it once, but I may try it again. The other IF 
section,  the 2KHz mechanical filter has a shorted output and I've got 
it apart for repair so it's disabled at the moment. I may have to fix it 
first before I can proceed.