The power supply voltages all check out ok under modest load (operation = idling due to lack of boot up), at the connectors on the far end (such as the back of the plug into the MB). An exception is the supply cable to the DVD drive and the HD, where I had to check at the rear of the connector @ the power supply, but the HD and DVD drive were still connected to that supply cable. The DVD will eject and load, and the HD appears to be spinning (both after power on, obviously).
One other oddity about this machine is that it has a SINGLE small round multipin connector color coded purple that appears to serve both the keyboard and mouse. (Usually with those older connectors, one is purple and the other a light green, for the keyboard and mouse to be separately plugged in to.)
Here I plugged my keyboard’s purple plug into the purple connector and used a (confirmed working on my laptop) USB wireless mouse. Surely that would not prevent the initial screen display and/or a successful POST beep upon power-up??
It was just a shot in the dark. You have other problems.