[AMPS] 87A Faults #1 and #18

Dick Green dick.green@valley.net
Thu, 26 Oct 2000 01:19:52 -0400


Ah well, Murphy strikes on the eve before the big CQWW SSB contest...

Today, while chasing K5K on 15M RTTY, my 87A tripped with soft fault #1
("Receive PIN back bias voltage not at minimum required level with key down
and no RF drive.") I was able to reset with no problem. It did that a couple
of times in a row on 15M. I switched to 20M and the amp worked OK. Back to
15M, and I get soft fault #1. Back and forth a few times, and I'm getting
soft fault #1 on 15M *and* 20M. Eventually, it happens on all bands. I turn
the amp off, back on, and let it warm up. Still get a soft fault #1 on 15M
right off the bat. But if I cycle the power, it works for a while on 20M
before I get the soft fault. Then it starts getting soft fault #1 on all
bands, any time I transmit. A little more poking around reveals that indeed
the fault occurs simply if the T/R circuit is grounded (i.e, transmit mode
with no RF applied.)

I pull the plug, open the amp, pull the tubes, wipe off the pins, swap them,
and put the cover back on. No change. Still getting soft fault #1 whenever
T/R is activated. But now, when the amp is sitting around in standby, it
spontaneously falls into fault #18 ("Filament current less than 2.0 amp.
Possible open tube filament.") Sometimes I can reset the fault, and
sometimes it won't reset for a while, apparently because it's getting this
status continuously. The #18 fault is somewhat intermittent. Sometimes it
happens repeatedly, and sometimes the amp can sit in standby for quite a
while and won't fall into the fault. It seems as though it's more likely to
get fault #18 just after fault #1 has been driven.

Anyway, since it's late at night and I can't call AlphaPower (or what's left
of it) until morning, any thoughts? There was a post here a while back from
N7TR who got soft fault #1 when his amp was new, and AlphaPower told him
that can happen with new tubes. Maybe that's the new tube arc-over syndrome
they talk about in the manual, although they suggest that will cause a
couple of different soft faults. My tubes are well over 5 years old and have
seen a fair amount of contesting. A quick look at the schematic suggests
that the PIN diode receive bias is derived from the HV supply. So, maybe an
arcing tube could trip that fault. Certainly fault #18 would suggest a tube
problem, but why would it be intermittent? I checked the AC, and it's 247V.
I sure hope it's not a power supply problem...

73, Dick WC1M


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