[R-390] Gain drift

[Larry H]

Dave, you might be on to something there with the agc.  One test you can
make is to disconnect the agc jumper on tb102 3&4, disabling the agc.  With
the mode switch in agc, ground 4 and measure agc volts on 3.  If it
fluctuates, trace it back tube by tube to where it's stable.

Regards, Larry

On Thu, Oct 18, 2018 at 3:28 PM dog <agfa at hughes.net> wrote:

> 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.
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