Motherboard problems

[New_tech123]

Hello, I'm working on a client's computer and it has a power problem. He has a 750 watt corsair psu that doesn't seem to want to boot the pc. I checked with a tester and the psu is working. I then tried a 500 watt psu and the pc started right up. Then I tried a 1200 watt psu that I was using to test another computer and nothing (it too was tested). Then I tried a 850 watt and it booted right up. I took out the gpu to see if maybe it was pulling too much power and nothing changed. So basically it booted with 2 out of the 4 good psus I had. Is it safe to assume it's a problem with the mobo or cpu? The client had also bought a new psu that didn't help. 

PS. when I say it's not booting, even the psu wont turn on. No weird beeps also. The mobo is an Asrock.

 

thanks

[Saieshan Dadi]

I'd say you could try to flash the bios with one of the psu's that boot. Do it at your own risk though, BIOS are funky and changing things could brick the working setups. I'd suggest having a rollback plan for contingency