At 03:42 PM 7/17/2011, you wrote:
I have an Alphaserver 1000A which began intermittently
halting, same
snip
When it was failing regularly, I found a line going to
the power
supply which normally sat at 0V and pulsed high just as the power
supply shut down. This line did not pulse the same way when the
power was turned off normally. I suspect it is a signal from the
system board to the power supply requesting it to shut down, or
less likely, a signal from the power supply to the system board
signalling it has a problem and is about to shut down.
That sounds like a crowbar circuit. If these are
linear power supplies, it's more likely to have a
crowbar, but I've seen some switchmode power
supply units with it. There's a simple circuit
somewhere that senses a failure that's in some
way dangerous to the equipment operated by that
power supply, maybe just a zener, resistor, and
maybe a capacitor too, to sense overvoltage.
Trips the crowbar by firing an SCR to short the
output so it will shut down abruptly. Called a
crowbar because it's like dropping a very large
conductor, like a crowbar, across the output terminals.
Since over voltage can fry a lot of expensive
kit, they use a crowbar to pop a fuse or breaker,
rather than risk more damage.
The crowbar is possibly adjustable, but I'd start
by checking the specs on output voltage and make
sure it's adjusted correctly on all output busses.
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