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.
Ah, that has confirmed one thing, I think. That the signal in question is
some kind of safety shutdown. Now to find out what's triggering it.
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.)
OK...
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
It might just be an To92 package (like a small-signal transsitor). If so,
it migth be a right pain to find.
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
I would use an ohmeter or a good continutiy tester to trace it. If
necessarty, stroke one probe along every pin on the board...
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
Quite possibly. Monitorign the current by the drop acrtoss the 1Ohm resistor.
I think I'd stick a a logic probe on the output of each section of that
339 in turn and see if you get any activity when you unplug or stall a
fan. Of coruse we dont yet know that he genuine shutdowns are caused by a
fan problem. But I guess you could also montiro the outpus fo the 339
when a real shut-down occurs.
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
I wonder what these jumpers are actually connected to...
Why do I wish there were scheamtics of this darn thing available...
-tony