I'm having a persistent problem with an AL-811 and now I'm wondering
if the way my Omni V is keying it might be part of the problem.
Synopsis: When working Ducie Island, I tried to tune on 10 meters and
blew the finals in my AL-811. I replaced them with 572b's, which
worked fine for about ten minutes of testing into a dummy load, at
which time I heard arcing and smelled smoke. Inspection revealed blown
parasitic suppressor resistors in the old-style suppressors. I
replaced the suppressors with Ameritron's newer style, which is a PCB
that sits atop the plate choke and provides a resistor protected from
DC by caps, and an inductor to bypass low frequences around the
resistor. It also provide new tube ceramic caps, etc.
Everything worked fine with no smoke smell for about 30 minutes of 30
or 40% duty cycle testing on all bands.
Then, the amp stopped putting out any power at all. Nothing. The
filaments are still lit, the high voltage reads correctly. When keyed,
the amp shows that it's keyed, and the plate and grid meters both read
about 10% of scale (100 and 20 ma or so, respectively). These readings
remain completely unchanged for any setting of input drive power or
plate and load. Of course, there is no ALC hooked up.
I have checked the bias voltage filter cap on the output of the
anodes, and it checks out okay (.001 uF with my Fluke DMM). I can find
no shorts anywhere, and there is continuity in the entire RF chaing
from input to output where there is supposed to be. The capacitors
that pass RF from the input to modulate the filament biasing voltage
check out okay, too. The input band switch is undamaged and seems to
work fine.
I have spent a lot of time with the T/R relay in the amp, and it's
fine. It's working per spec.
The Omni V reads high reflected power when attempting to excite the
amp, but when I put the amp in bypass, the Omni happily puts out full
drive power into my dummy load. It's almost as if the exciter's output
is not connected to anything when the amp is not in bypass. All this
tells me that it's gotta be on the input side of the tubes. The
behavior is identical on all bands, telling me that it's not because
of the tuned input components.
The input band switch shows appropriate continuity. But if it is the
input bandswitch, I'm wondering if it was damaged as a result of
hot-switching. I'm keying the amp with the Relay output of the Omni V,
and using the Tune button to put out keyed CW. Maybe I should have
been testing with a whistle into the mike, heh, heh.
I'm at a loss. Any ideas of things to check?
And--I'm in the market for a Centaur if anyone has one they want to
sell.
Rick, KR9D
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