This is the second one I have done and I have the install process done pat. This is the 80 watt PicoPSU with 80 watt 12 volt brick.
I retained the original power switch and installed the 2.5mm dc power jack in place where the original AC cord plug was. This Pico plugs into an ATX 20 pin male connector that is then wired to the original 6 pin ST motherboard power connector.
Reason I did this was because this machine is a UK PAL machine with the original 220v power supply and since I am in the US it wasn't going to do me much good. I have always thought the internal power supplies on STs were there weak point.