Some of you might remember my post about the
Alpha amplifier with the oddball tune-up problem.
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Bill reports back in with the answer and the
problem found.
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[cut and paste]
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To: <amps@contesting.com>
Date: Wed, 11 Jul 2001 15:13:40 -0400
From: Bob Sullivan
Subject: RE: Alpha 77
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Skipp,
This was the A77 that had very low power out on all bands with a very
shallow plate dip. So, for example, at 30w drive with 150ma grid current
the
plate might dip to 750ma with power out around 100-150w. All bands tuned
right on the money per the manual (tune and load) in a dummy load. There
were LOTS of suggestions - bad relays, bad metering circuits, bad input
circuit, bad bias, bad tuning network (miswired), bad torroids, bad
coupling
caps, etc.
Many suggestions didn't make sense based on the "correct" tuning
points. I carefully checked bias voltage and the output network and
bandswitch for wiring errors, bypassed the output vacuum relay, replaced
coupling caps, monitored bias during operation - all this checked good. I
never suspected the vacuum variable tuning cap since the amp "tuned"
correctly!
So the capacitance must have been correct. Finally - in desperation
I replaced the vac variable (no small task) and, voila!, everything
works. I
have NO idea what is wrong with the capacitor except to note that
normally
you can "see" through them but this one is completely opaque with a color
you see when a tube getter is flashed.
Regards and 73,
Bob W0YVA
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