[R-390] Gain drift

[Larry H]

Dave, When the gain drops and goes back to normal during the 1 hr warm up,
are the transitions from good to bad gradual or quick?  About how long does
it take?  About how long does it stay in each state?  Is there any unusual
noise in either state or during the transition?  Do you need  to let the
390A cool down for some length before it will happen again?  About how
long?  Are you using a 3tf7 osc current regulator?  Have you checked all
the mini bnc connectors?

Regards, Larry

On Sat, Oct 13, 2018 at 6:13 PM dog <agfa at hughes.net> wrote:

> 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.
>
>
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