I know you're trying to fix a II; are you trying to rule out whether the PSU is broken? Have you tried jump starting the computer? If you short two of the pins on the PSU, you can start the machine (I forgot which ones, but you can look it up). This will let you know if the PSU is dead or not. Just make sure to only do it a few times at most, since it may damage the motherboard if you do it too many times.
I've owned several II/IIfx and I've never come across a dead PSU. As such, I'd say it's pretty unlikely the PSU is the culprit. Most likely a corroded trace on the PCB preventing the startup circuit from working.