When I took the back of the Zcorp I was a little surprised by how much electronics actually faced me. I guess I forgot that printing is a fairly complicated task made to look easy by custom electronics normally, it also has a lot of gizmos to control and was built in the 90's. I checked the transformer that he had just replaced and it was working and there was power to the PC power supply that runs a standard PC motherboard but it was not turning on. He suggested it might be the power switch but that worked fine too. I ended up taking a large ISA board out of the motherboard so I could get access to the pins that normally connect to the controls on the front of a normal PC. These had a little interface board plugged in which I also removed. Shorting the relevant pins (after finding the manual for the MB online) caused it to spring in to life, the PC part at least.
But why wasn't the switch doing its thing? The little interface board was connected by a ribbon cable to another board that seems to manage the power supplies and connects to the power switch. But the connection to this board was odd, there was one less pin on the board than there was holes on the ribbon connector, you could therefore put it in two possible positions. Some continuity testing made me think it had been put in the wrong position and I was right. Moving it by a hole connected the power switch to the same pins I shorted. Now it booted, did a funny dance with the bins and started beeping. Enough for one night I think!