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?
There are two fans. Both run at what looks like a reasonable speed. Both
have only two wires but they are plugged into three way connectors on the
system board. This led me to suspect that the manual was lying about the
fans being monitored (like it lies about the on-board video and a few other
items which are not fitted in my machine).
However, I found that if I unplugged or manually stalled either fan, the
power supply did indeed shut down about a second or two later, with the above
mentioned line to the power supply pulsing high like it does when the fault
happens. As there are no sensing wires to the fans, I guess the current drawn
by them must be monitored to generate the fault signal.
On many occasions, the power supply has tripped out less than a second after
switching on. In that time, there is a brief flash from the power on LED and
the fans rotate somewhat. This is definately less time than it takes for the
circuit to react to me disabling the fans manually and is not enough time for
the fans to get fully up to speed either. So, I suspect the fans themselves
are ok and either the fan monitoring circuit or temperature monitoring circuit
is more likely to be faulty. (There is good airflow and nothing appears to me
to be getting too hot.)
Overtemperature might be a mechasnical switch, or a semiconductor
sensor (probably the latter).
Nothing stands out as obviously being a temperature sensor but I don't know
what one would look like. Maybe something in a metallic package, mounted
near the top of the vertically mounted system board? I can't see anything that
fits that bill nor anything on flying leads either.
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.
Not really. It's a multilayer board and it is not at all obvious where the
trace goes. There are no likely suspects near the connector and there is lots
of other stuff such as the memory between the connector and any more likely
suspects. There is an LM339 and a largish 1 Ohm resistor over the other side
near where the fan connectors are so perhaps those are responsible for
monitoring the fans? There's nothing near those components that looks like a
likely temperature sensor.
The manual mentions a couple of jumpers which it says can be set to generate
a machine check rather than shutting off the power when there is a fan fault
or temperature fault. This looks worth a try now that I know the fan monitoring
at least could be responsible for the problem although I probably need to
install a different version of VMS first (one from before they messed up the
error log analyser. The version I have at the moment reports I have generated
some errors on a VAX 11/780 in 1996, 15 years before the creation date of the
error log file...)
Regards,
Peter Coghlan.