Server is down for a while!

It seems that my old hardware is showing its age.
I am still troubleshooting but due to the lack of spares, this might take a week or two.
I have narrowed it down to 3 possible suspects: Power-supply, CPU or Motherboard.


Symtoms

When power is pressed the CPU-fan and System-fan spins up for 1-2 seconds and immediately spins down.
It never show POST!

Since I don't have the manual and the server case does not have a speaker this makes it a bit I am down to trail and error.


Previous Tests
1) All external units was removed from the motherboard (SSD, keyboard, Mouse)
2) Battery - Changed Motherboard battery
3) BIOS reset
4) RAM - All 4 RAM modules was pulled out and testet individua