I think it could be the Audio circuit - I replaced the PSU of one of my S900's with an ATX power supply and I have issues with the sound, the S900 with the stock PSU works fine. I could try swapping them and check the audio. PCI is working fine.
I got two S900s a few months ago and also had issues with the PSU of one unit.
Swapped them around, but one PSU was dead.
So I've did this ATX conversion and the colors of the three-pin connector didn't match too, but with a little bit fiddling around (and find out, that maybe not the PSU...
That's the pinout I've used, but my S900 has a slightly different pinout - my J26 connector don't have a purple wire, just a red one for example. I think I try to get an old-style ATX PSU on eBay and try it with it, because I've used a modern one.
SCSI doesn't work properly, I've tried it. I get a Happy Mac but it's not going any further. I get the chime and the screen comes up and then it sticks on the Happy Mac.
I can't use the power button on the Umax, I have to touch the Ground- and the Power On-Cable to get the computer started...
I've got two Umax S900 a few weeks ago, one with a faulty power supply. Tried to it, but it still doesn't power on.
I've found a website with ATX conversion for the Umax S900, but the new ATX style PSU's don't have the -5v rail and there are no adapters with the third...