So the power supply fan is turning (continuously).

All that proves is that part of the power supply is working.
Therefore, both the power supply and motherboard are still under suspicion.

What Now?

You will need a multimeter, and the knowledge of how to use it to make DC voltage measurements.

Whilst plugs P8 and P9 are still plugged into the motherboard, make the following voltage measurements:

P9	4/5/6 3	Red White	+5 volts -5 volts	Expected to be something over + 4.8 volts (source: IBM manual) Expected to be something over - 4.5 volts (source: IBM manual)
P8	4 3 1	Blue Yellow Orange	-12 volts +12 volts POWER GOOD	Expected to be something over - 10.8 volts (source: IBM manual) Expected to be something over + 11.5 volts (source: IBM manual) Expected to be something over + 2.4 volts (source: IBM manual)

Click here if all 5 lines (POWER GOOD, +5 volts, -5 volts, +12 volts, -12 volts) measured as expected.

Click here if any of the 5 lines did not measure as expected.