Okay. I removed the motherboard now. Have everything layed out.
Motherboard has only the cpu, cpu fan, 24 pin power, 8 pin power, and the case speaker plugged in. Nothing else. No power at all.
However, if I plug in the little 2 pin case power button plug, it powers on. But again, just for a few seconds.
Trying to work backwards to see what it is, and getting nowhere.
Double checked the cpu, and no pins bent. Thermal paste is applied evenly with no spillage.
Motherboard looks fine.
The cpu and psu fans both spin when there IS power.