NC-173

[Steve Harrison]

At 09:22 PM 1999-06-15 -0400, David Franklin wrote:
>All of you "old salts" are going to get a chuckle out of this, I next
>decided to substitute the tubes.  The 6AC5 AVC Amp was the culprit!
>Replacing it made the hum totally inaudible.  The tube tests good with
>no shorts in my tube tester.

I don't know whether tube checkers routinely check for continuity between
the cathode and filament; but I imagine that most likely, the filament may
have deformed sufficiently to actually contact the cathode, injecting 60 Hz
hum directly into the AVC. This problem could probably be easily diagnosed
by identifying the frequency of the hum: 120 Hz indicates a power supply
filter problem whereas 60 Hz almost always can only occur from the filament
circuit. A shorted power supply rectifier may also cause 60 Hz hum; but in
that case, I would think that something else would have blown long before
now, like the filter cap or transformer!

I had something like this happen a few years ago. I had a Galaxy V
transceiver with the stock plug-in calibrator. I could not shut off the
calibrator. It turned out the tube had a filament to cathode short; the
calibrator was turned on and off by grounding the cathode, which was
grounded through the short to the filament. There was also some 60 Hz hum,
which, after I realized that the calibrator switch was OK but the cathode
remained grounded, had to be caused by a short to the filament. The trash
can ate that tube right quick.

Old-time TV servicemen quickly learned how to identify scads of tube
problems by carefully examining the picture and/or listening to the audio.
Jack Darr's column in one of the major electronics mags of the time (can't
remember it's title now) was my absolute favorite as it was ALWAYS very
interesting to read and understand the "war stories" of tracking down
various problems, often by reverse-engineering a particularly-troublesome
circuit itself. Darr always emphasized the efficiency of using the
serviceman's greatest and cheapest tool: the gray matter between the ears.

73, Steve K0XP

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