[R-390] Gain drift

[dog]

Larry,

I have to let the rx cool off before it acts like this again. Upon 
startup, it has what I would call normal gain. Then after about 5 mins 
it drops, not all of a sudden but a gradual, but over maybe 5 seconds to 
the low gain point. Then after about another 5 minutes it drifts back to 
normal the same way. It seems to go through this cycle until after about 
an hour or so it stops and the gain is very near what it was at startup. 
I have seen the gain drop all of a sudden, but that was just once. When 
the rx is interrupted if it's on low gain cycle, the gain jumps back to 
normal, usually but not always. Yes, using the 3tf7. I have played with 
the mini bncs when the gain is low to no avail.

I was thinking about pumping in a signal to the balanced input from my 
transceiver, it puts out a +7dBm port that I can pad down if necessary, 
and watching various spots through the RF/IF signal path with the 
spectrum analyzer. But first I need to get some baseline measurements. 
Was thinking about doing this also in MGC mode to stop the AGC effect. 
Maybe I could isolate at least a stage that's causing it that way 
instead of having to shotgun it?

I also have a Fluke 189 recording voltmeter that I can use if necessary 
but I'm thinking the SA will be the easiest method.

I should check it below 9MHz first.

Dave


On 10/13/2018 23:55, Larry H wrote:
> 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
>