Most people here experienced with the IBM 5150, hearing
only that symptom, will say to themselves, something like, "
This is 'classic'. The fan in the power supply is DC powered, most likely connected to the +12V DC line. The power supply actually started, and the fan (getting DC voltage) started to turn, but within a very short period of time, the power supply then shut itself down due to a problem (internal or external). "
That is the odd bit. The hard drive spindle is powered from the +12V from the power supply. Backed up by the fact that you have measured +12V (reaching the motherboard). So why does the power supply fan start and then stop?
If the fan was AC powered, it would continue to run if the power supply shut itself down. It must be DC powered. Even if we assume a 5V powered fan, you have measured the presence of +5V (and -5V) being generated by the power supply.
A possibility is multiple problems, one being that within the power supply, the fan is powered by some circuitry that is dedicated to generating a DC voltage for the fan only, and at turn-on time, that circuitry is starting then stopping. Seems unlikely, but cannot be discounted yet.
The fan problem certainly needs to be looked into first, otherwise the power supply could overheat, damaging something.
I think the first thing to do is to confirm that you are not changing variables between the observations. The IBM 5150's stock power supply, if good, will start-then-stop if it is either inadequately loaded, or is overloaded. Confirm for us that you didn't change anything between observing the following three:
* "
the fan is not spinning at all though it twitches once when I turn it on"
* The hard drive's spindle always turns, even when the power supply's fan no longer turns.
* All voltages (including POWER GOOD) are as expected, as shown at [
here].