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

[Joe Foley]

NUTS!  You beat me to it.

Don't forget to tighten the tube socket
bolts,......... ah, just go through and tighten
everything!

Joe



--- Jim Temple <jetemp at insightbb.com> wrote:

> 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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