[R-390] Distortion troubleshooting

[Barry Hauser]

Hi Bill & gang --

This is a pretty dumb idea, but have you tried swapping tubes around -- not
just subbing in new ones?  They say that too hot a tube is problematical for
the PTO -- could that be the case elsewhere, as in the RF deck?

May also be a tube or two with particularly non-linear gain, maybe somewhat
microphonic - stuff that doesn't necessarily show up on the tube tester.  If
you're drawing new ones from the same batch, the problem may apply to all or
most of the run, whereas you have a well behaved, broken in older tube in
the working circuit.  Another dumb question -- did you try tapping the tubes
as you stepped through the signal level testing?

Before you pull the RF deck, you might want to try some voltage measurements
at the tube socket pins using a tube extender or two -- or use the wire-wrap
technique.  Don't rely on the voltages shown in the manual -- I would A-B
them between the two RF and IF decks you have, looking for gross clues at
the various signal levels.

'Nother dumb one:  Did you check to make sure the RF gain pot is the correct
one -- ohms/watts/taper?  Might have been subbed out at some point.

Also, as has often been suggested -- give the tube socket screws a twist to
clear any bad ground tie points that may have some resistance,
capacitance -- or maybe even, uh diode-ence?

My 2, er, 1 1/2 cents worth.  Told you these were dumb. ;-)

Barry
----- Original Message -----
From: "Bill Cotter" <bcotter@pop.uky.edu>
To: <R-390@mailman.qth.net>
Sent: Monday, March 11, 2002 2:07 PM
Subject: [R-390] Distortion troubleshooting


> Thanks for the suggestions, Bill.
>
> The idea of two problems has crossed my mind. Probability suggests
> that two un-related faults occurring at the same time would be
> significantly less likely than one problem causing two symptoms.
> All this is academic until the root cause is found. In the mean
> time, I'm trying to rule out what the problem is NOT. Of all the
> suggestions I've received, two suggestions dominate:
>
> 1) Limiter misbehavior - Tonight I plan to settle the Limiter
> suggestions by breaking the coax connection in the IF deck and
> picking off the audio (diode load) before any coax cable breakdowns
> can occur. The distortion analyzer will provide the numbers for
> comparison, and a signal tracer will allow me to hear the results.
>
> 2) AVC misbehavior - The distortion problem occurs with the
> AVC-OFF, zero volts on the AVC line. For the moment I feel
> confident ruling out any issues in the AVC amp, det, RC, etc
> department. I have also placed this IF deck in another receiver,
> and it worked well. Installing the second IF deck in the bad
> receiver made no difference in the problem.
>
> There still exists a region in the RFG control range that produces
> severe distortion for a given signal. That range shifts downward
> with stronger signals and upward with weak signals. This tells me
> there is a signal level factor in the equation. The IF Gain has
> been set during alignment, but it can be retested for accuracy.
>
> One theory that sounds appealing to explore is the parasitics
> possibility, is sounds so mysterious and ghostly. The second is an
> internmod problem in the mixers. Chasing both of these may cost
> long hours of bench-instrument time, and since the RF deck would
> have to be removed, I think there is a better path to take first.
>
> That's the one you suggested of examining all the resistors and
> capacitors for out of spec condition. Along with examining all
> solder joints, connections and, the grounds of all shields, sockets
> and posts.
>
> To all that replied - Thanks,
>
> bill n4alg
>
>
> > From what you describe, it sounds like the set is operating
> > normally when
> >there is little or no AVC voltage, and it sounds like the set may
> >not be
> >operating normally with a strong signal, but the signal is getting
> >through
> >anyway.  In between you hear distortion, where some stage is
> >affected by AVC but can not operate normally.
> >
> >Sounds like a "fun" problem to troubleshoot.  You'll feel great
> >when you
> >find it!  :-)
> >
> >73 de Bill, AB6MT
> >billsmith@ispwest.com
>
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