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

[odyslim at comcast.net]

 I had the same problem, the plates in the antenna trimmer were corroded
and rubbng one another.

 Scott


 -------------- Original message ----------------------
From: shoppa_r390a at trailing-edge.com (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.
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