----- Original Message -----
From: Paul Christensen <w9ac@arrl.net>
To: <amps@contesting.com>
Sent: 06 October 2002 19:47
Subject: [Amps] Failed 8877
> I narrowed the root cause to a broken 8877 filament....although it was
> difficult to find beacuse it is intermittent. At the onset of filament
> current, it opens and after powering down the amp, and checking with a
DVM,
> the filamant resistance reads fine. I finally got it to read an open
> resistance right at the pins with the transformer Molex connectors
> unplugged. I can repeat the sequence and by putting the DVM in series
with
> the filament, I can see the DVM read AC current for a brief instant, then
> it goes to zero. When this occurs, I can then see the filament read open
> resistance right at the pins. Once I pull the tube and measure the
filament
> resistance, 75% of the time, I can then see good resistance. The other
25%
> of the time, it will read open. I do not have a spare 8877 at the moment.
I have a pair of 8877s that do just the same - they come from the 'quality
problem' period and came from industrial generator pulls, but I know nothing
else of their history. In these, turning them upside down would cause an
open, and back the right way would remake the contact, sometimes with an
extra tap needed. I experimented with one and found that winding the heater
up very slowly avoided it going open circuit at switch on, and after a 24hr
heater burn it seems to have a reliable contact again and runs the correct
current, although I haven't tried allowing inrush current and it hasn't been
tried at high power either.
Steve
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