[R-390] R-390 NON A Problem

Sat, 12 Oct 2002 15:29:15 -0700

Temporarily remove T207 and watch pin 5 of that transformer.  Also watch the
B+.  If pin 5 fluxuates with the static, or if there is any voltage drop
between B+ and that pin (across R220), then you have verified a defective
C325.  If the B+ and pin 5 fluxuate together, then you need to look
elsewhere.  In fact, C325 should smooth any fluxuation if the problem is
elsewhere.

Depending on the quality of the caps in the RF deck, you may only need to
replace the one capacitor.

Are you having problems on all bands?  I have had to replace mica caps, even
very small values, in the R-390 grid circuits in mine.  They were leaky and
readily detected using a Simpson 260.

73 de Bill, AB6MT, NNN0BIO
[email protected]

----- Original Message -----
From: "David Faria" <[email protected]>
To: "r-390" <[email protected]>
Sent: Saturday, October 12, 2002 5:13 PM
Subject: [R-390] R-390 NON A Problem

> GE List.  I'm playing with one of the nicest 390's I've seen in a long
time
> and its got a rather irritating problem.  It has what I would call a
static
> crash ever so often and I was curious if anyone else has had this problem.
> I think its a leaky cap. on the RF Deck.  I have by-passed the tube stages
> one at a time on the deck using a tube extender with a filament resistor
to
> to maintain the remaining tubes on the deck.  The filaments r in series.
I
> seem to have narrowed it down to the last mixer in the series(V205) using
a
> scope and bypassing.  I am seeing fluxuations in both the grid and plate
> voltage when the crash occurs(Pins 1 and 6).  My guess is c-325 off the
> plate circuit of that tube.  Any speculation would be appreciated.  Is it
> typically the plate cap. leaking to ground that would do this?  Would a
> capacitor checker be able to determine if this cap is bad?  What is the
> reason the cap will just not fail?
>
> I've checked all the tubes on a TV2 and thumped em trying to create the
> problem.  I have replaced all the modules except the RF deck and the
problem
> is still there.  Sometimes it will run for 30 minutes without a problem
and
> then start.  This is a very nice unit and I don't want to recap. unless
its
> necessary.
>
> Thanks for ur responses
> Dave Faria
> _______________________________________________
> R-390 mailing list
> [email protected]
> http://mailman.qth.net/mailman/listinfo/r-390