[R-390] Distortion troubleshooting

Mon, 11 Mar 2002 21:48:38 -0500

On the Collins list someone recently told the story of a 6U8 that tested
very good on a tester, but was actually TOO sensitive.  The elements had
changed shape or spacing in some manner giving the tube excessive gain. 
Of course it tested real good on a tester, but behaved poorly in the
circuit.

Another thing to consider is bad plate, screen or cathode resistors in
the tube circuits.  In my case I had both an IF and RF deck where
resistors that carried power/current (i.e. plate or cathode, sometimes
screen) had aged due to over heating and drifted way out of tolerance. 
Could a bad cathode resistor cause a tube's grid AGC action to not
perform properly?

Barry Hauser wrote:
> 
> 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" <[email protected]>
> To: <[email protected]>
> 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
> > >[email protected]
> >
> > _______________________________________________
> > R-390 mailing list
> > [email protected]
> > http://mailman.qth.net/mailman/listinfo/r-390
> >
> >
> 
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