This is a post for posterity - I wasn't able to find an answer online for this so I'm posting it here in case it helps someone else in the future.

I have an AcerPower 4100 (V66LA motherboard) with a dead PSU. It's an AT style power supply with additional 3-pin connector. Without this extra connection the PC won't turn on.

I discovered that the IBM Model 350 has a similar PSU, which works in the Acer. Disconnected, the 3-pin connector measures +5V on the red and white wires (and black is ground). Since ATX PSUs supply standby voltage I thought it couldn't be hard to get it working. Well, yes... to cut a long story short... the basic ATX to AT power adapters can be used with a simple modification.

1) Get yourself an ATX-AT adapter similar to the one pictured below - make sure the black wire with spade terminal has its own pin in the ATX connector.
2) Remove that black from the ATX connector and move it to pin 9, as illustrated:

3) Remove the 3-pin connector from your old PSU (or find a new matching one). Connect the green turn-on wire from the ATX-AT adapter to the WHITE wire, and the black one you moved to the RED wire. You'll end up with something like this:

Note: I removed the black wire from the 3-pin connector... it's not really needed, but you can connect it to one of the other ground wires in the adapter if you want.

4) Make sure you use an ATX PSU that supplies -5V (usually a white wire). Or you can create that yourself using an LM7905 negative voltage regulator... or buy one of the more expensive adapters that already has that plus a dummy load for the 3.3V rail.

ATX PSU pinout:

NOTE: Interestingly while googling for images to steal for this post, I came across the exact adapter that you need! But it shows as out-of-stock, so I'm not sure if anyone still makes them.
https://www.amazon.co.uk/20-pin-3-Pin-Power-S … 9/dp/B016S7OGVW

Hope this helps someone 😀