PSU/Motherboard load

[ononyn0us]

Hello, computer has been non responsive for a few days. Only sign of life is the power led.

Specs as follows :
I7 4770K

Asus Z87-A ATX

Gigabyte GTX 1070 mini

Be Quiet! LI80 730W Gold (7 yrs old)

 

The PSU itself doesn't seem to turn on.

I have unplugged everything from the MB and the PSU except for the ATX, CPU power pins, and the front panel pwr switch , and changed the CMOS battery, without success. When the main AC power is on, unplugged ATX connector delivers stable 5.01V on the 5V purple wire and 2.85V on the green PSON# wire. When the ATX connector is plugged into the motherboard, I read 2.85 V on both the purple wire and the green wire. (This is still normal right ? I don't know anything about electronics but it seems logical that the motherboard would draw some power here ?).

When pressing the power switch on the front panel, the voltage on the green wire doesn't come down, which would be the signal for the PSU to start up. I tested my front panel pwr switch, which seems to be working fine. I've also tried shorting the corresponding pins on the motherboard using a screwdriver. PSU doesn't start up. It does, however, start up on its own when shorting the green PSON# pin to a COM black pin on the ATX connector. (i've only tried that with the ATX connector being unplugged). The fan is on and the voltages seem fine except for the PWR_OK grey pin, which delivers very unstable current oscillating between 2 and 5 V. That could indicate a problem but I've read that could be because nothing draws power from the ATX connector, which prevents the PSU from stabilizing and providing a stable signal on this grey pin.

I've tested the PSU on an older motherboard/CPU/ram with similar results : pressing the ON switch or shorting its pin doesn't turn on the PSU, and doesn't make the voltage drop on the green pin. On this other board however, the 5V purple pin isn't stable but oscillating between 3 and 4.5 V.

Any idea where to go from here ? Some test I haven't performed ?

[Guest]