[R-390] Strange modulation on carriers

[Tim Shoppa]

WB2LHI writes:
> I finished tweaking all the adjustments on my newly
> recapped IF stage and started checking on the PTO
> runout.  When I would tune through the  calibrator 
> signal with BFO off I get a modulation, which varies
> somewhat with  where in the passband I tune.
> It's there for all bandwidths, and I'm not  sure if it was 
> there before I took the IF module out.  Strangely, it
> occurs  on all the bands below 8 MHz, but
> not above.  I assume it's modulation of  the 17 MHz 
> oscillator, and will check it out when I remove the
> RF module for  recap.  Has anyone seen a symptom
> like this?  Any suggestions for  troubleshooting
> it?

FM'ing of the 17 Mc oscillator, with the deviation
strongly correlated to the AGC action (thus you noticing
it as you tune through the calibrator), is commonly
noted. In my experience it is a few hundred Hz
in the "bad" cases but maybe it's more or less for you.

It is partly a design flaw but for example a flaky
17Mc crystal will be more pulled than a good strong
17Mc crystal. This is probably at least one of the
reasons why surplus 17Mc crystals are hard to find!

Some blame this on B+ regulation (or lack thereof) and
this is possible too. You can determine whether it's
B+ regulation or AGC action by turning off AGC.

In one of my particular 390As, replacing R211 (which was
high by several hundred percent) and R209 (which was
charred black) made the problem
much less evident. Yeah, I still hear some whoop-whoop
on strong CW with AGC on, but it's a reminder that I don't like
AGC on CW anyway!

Unfortunately I did not independently replace R211 and
R209 to see which one was responsible for the fix. Yeah,
guilty of shotgunning, but resistors that are high by
several hundred percent or already burnt to a crisp
deserve a lot more than a shotgun!

Tim.