[R-390] Gain drift

[Alan Victor]

You might want to monitor your local oscillator injection levels into all
mixers depending on the band chosen. Sounds like it is not band related so
just check those. I have seen LO injection levels drop off with increased
temperature rise over time. Alan

On Sun, Oct 14, 2018 at 7:06 AM dog <agfa at hughes.net> wrote:

> 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
> >
>
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