I was considering that route with AmigaKit ATX adaptor, but which ATX power supply size wise will fit the Amiga? Mini ATX? and what about the power switch?
IIRC, the A3000D PS has an extra signal not common to other power supplies, even most of the other Amigas don't have it. It is a 60 Hz AC signal (probably low voltage) that I believe was used for timing of the native video on the motherboard (maybe the FF?). I can't remember.
Naturally, the Amigas date to a time before the 3.3 VDC power rail.
I imagine Amigakit is on top of all of this with their adapter.
