[R-390] Distortion troubleshooting

[Bill Cotter]

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