[R-390] Something in the RF deck breaking into oscillation...

[Tim Shoppa]

On most bands my R-390A is breaking into oscillation during the
first hour or so of warmup. After an hour, it's rare for it to do this.

An important clue is that as I rotate the "ANT TRIM" I can start/stop/
modulate the oscillation. It sounds sort of like a regenerative
receiver as it breaks into/out of oscillation.

The gears/insulators in the "ANT TRIM" drivetrain are clean.

It's most likely to break into oscillation when "ANT TRIM" is peaked
but sometimes it does it no matter what and other times (especially
after warmed up) it never does it. It's only when it's going from
stable to oscillating that the ANT TRIM has an effect.

Is this a common problem with a known solution, or am I gonna have
to resort to shotgunning everything around the 6DC6 RF Amp?  (I already
subbed out the 6DC6 with no apparent change in symptom.)  I do
not have extender cables/etc for the RF deck so getting underneath
is a rather laborious process and I don't want to do this more than
once.

Looking at the schematic for bypass caps that might go bad and cause
this I see 

  C227 - .047 as the cathode bypass. Is this a brown beauty? But
              being in the cathode bypass it seems unlikely to be
              causing oscillation.

  C229 - 5000mmF as the screen grid bypass. I guess a ceramic
              disc?

I remember seeing three brown beauties in a RF deck - two as
bypasses around the crystal calibrator, and a third somewhere in a
mixer.  So maybe C227 isn't a brown beauty... but then what is it?

Tim.