EDIT: Update 1 - PROBABLY SOLVED: I looked at my spare 1000W power supply, and one of the 5v pins has an extra smaller/thin separate wire on it. My 10 year old 250w PSU doesnt have this extra wire. That extra wire is most definitely the 5v sense wire.

EDIT #2 Update 2 - SOLVED: Apparently I cant just connect the +5 Sense Pin to just the +5 Sense Wire. It needs the actual +5V line too. I also found a resource that says the +5 Sense is not even necessary, it can straight up be connected to a regular +5V: https://www.instructables.com/Revive-a-pre-hi … -IBM-PS2-55SX-/

All IBM PS/2 models uses proprietary power supplies. The Model 55 in particular uses P7 and P14 connectors.

The P7 is pretty easy to make. Only Pin#2 is different from the common AT connectors. I combined an AT P8+P9 adapter, and just grounded pin#2.

P7 Pinout:


However, for P14 connector, I initially thought it needed 3x +5V pins, but it actually needs 2x+5V and 1x +5V Sense.

P14 Pinout


ATX PSUs seems to only have 3.3V Sense, so I cannot directly connect the pin to ATX.

Is there some way to unblock me here, or is it not possible to use an ATX PSU for the model 55 after all?