[R-390] Strange modulation on carriers

[DJED1 at aol.com]

"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."
 
Tim- Thanks for the information- at least I know I'm not dealing with  
gremlins.  I was going to pull the RF module anyway, so I'll check for  burnt 
resistors.  I've got one spare crystal so I can try that also.
Ed