@satoslp I’d start of by measuring on the logic board test points.
[image|1484061]
Pin 4 will have to have 12 Volt if your power supply is good. If it does not then you most likely have a bad PSU
Locate diagnostic LEDs in bottom case vents. Plug AC cord into system. Verify if diagnostic LED #1 turns ON. You may alternately check for presence of a 12V DC signal between pin 4 and pin 1 of logic board.
Yes
Standby power voltage being supplied by power supply.
No
Replace power supply.