|Darn shame you changed the motherboard.|
Be sure you know the difference between 115VAC (or your line voltage) and 5 - 12 volts dc. Some old computers use line voltage on the switch. I doubt yours does but be careful.
You need to diag how the power supply turns on. Most have a switch on the front of the computer. It is a momentary push on button. It has two wires normally. They should short between the two when you push the button. Use digital volt ohm meter to test. Then if OK move on to the paper clip to start psu. See web pages on how to do that. If you can't paper clip the psu then remove all but a known tested good hard drive on the psu and test it with all other connectors off. If fails then psu.
Why did it take me over a year to phone in a problem to ATT?