[ 1. Fan turning in PSU.
  2. Under the load of the motherboard, all power supply voltages/signals as expected. ]


So we know that the power supply is working.

That means that the motherboard is faulty. And because the voltages/signals from the power supply are as expected, we know that the motherboard is not faulty in a way that overloads the power supply.

What Now?

We have eliminated the possibility of shorted capacitors on the motherboard.

Your motherboard could be faulty due to any of the many motherboard components.
However, vintage RAM chips are a relatively high failure rate item, and so we will proceed along that line.

Click here if your computer is an IBM 5150 (PC).

Click here if your computer is an IBM 5155 (Portable PC).

Click here if your computer is an IBM 5160 (XT).