I tried a power supply unit from another Alphaserver
1000A and found
this did not cure the problem so I replaced the original. Thinking
that the power supply might be sensing an overload, I disconnected
the power feeds to the disks/cd/floppy and removed all the PCI cards.
The problem was still there.
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.
I have a service guide which mentions the possibilty of the power
supply shutting down due to a fan failure or high temperature but
it doesn't give any details on how this works or even how to find
out if it is happening.
That was my first thought (woithout reading the manuals...)
How many wires do the fans have? If they have 3 or more, it's quite
likely that they are power, ground, and soem kind of monitor line.
Possibly a frequency indicating how fast the fan is turning, possibly a
simple 'I#ve failed' signal. In any case I'd investigate there.
I assuem the fans are all running, and running somewhere near the right
speed?
Overtemperature might be a mechasnical switch, or a semiconductor
sensor (probably the latter).
Can you trace this shutdown sire from the PSU onto the mainboard and see
what it goes to? If you're lucky it'll be a recongisable chip (maybe a
microcontroller). See if that takes inputs from the fane or a temperature
sunsor.
-tony