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

[Jim Temple]

I had this problem a couple of years ago.  It turned out to be one of the
caps on the rf deck not being properly grounded.  After reworking all the
grounds, I found that one of the ceramic cap grounds that is underneath one
of the cans, was corroded. Tightening the lug on the can restored the
ground, and solved the problem.  So, the moral of the story is that these
old r-390a's, especially the "massacre" ones, need at least a one time
rework of most of the grounding points.

Sincerely,

Jim Temple
Louisville, KY.
 

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Tim Shoppa
Sent: Saturday, December 24, 2005 9:06 AM
To: r-390 at mailman.qth.net
Subject: [R-390] Something in the RF deck breaking into oscillation...

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.
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