Question PC doesn't turn on, but USB ports have power ?

[Poccio]

Hi everyone,

I am having some issue with my 6-year-old PC I can't seem to pinpoint.

As per title, since the other day, my computer has decided it could stop turning on. When I try to turn it on, literally nothing happens. And even when plugged in with the switch on, I get no life signs from the mobo. I thought initially it could be an issue with either the PSU or the mobo, but the strange - at least within my knowledge - thing is that I have power from the USB ports both on the case and on the mobo.

Before posting here, I did some troubleshooting:

• Tried a different socket and power cable
• Paperclip test on the PSU (and the PSU seems to work)
• Removed the CMOS battery, waited a bit and re-inserted it
• Tried booting without the GPU
• Disassembled and re-assembled most of the PC, checking for contacts and loosened cables

But the PC is still not giving any life sign...

For what it's worth, no power surge or pretty much anything else took place, and the computer wasn't even moved. Here are the major components of my configuration:

• Intel i7 7700k
• NZXT Kraken x53 AIO Liquid Cooling
• Aorus GA-Z270X-Gaming 5
• Vengeance 16GB RAM
• Geforce MSI 1080
• EVGA 750 80 plus gold

I would really appreciate any help I can get with this. Thanks!

 

[Aeacus]

thing is that I have power from the USB ports both on the case and on the mobo.

USB is powered by +5V rail. CPU/GPU are powered by +12V rail. So, if your PSU fails to provide +12V rail, PC won't turn on, but USB ports still get power.

EVGA 750 80 plus gold

PSU model (or part number) is? Also, how old the PSU is, and was the PSU bought new or used/refurbished?

 

[Poccio]

Thanks for the fast response!

USB is powered by +5V rail. CPU/GPU are powered by +12V rail. So, if your PSU fails to provide +12V rail, PC won't turn on, but USB ports still get power.

This makes perfect sense. I used this USB thing and the successfull paperclip test to prioritize faults hypotheses elsewhere, but you're right, neither is actually proving the +12V rail is not faulty. Thanks for the insight!

PSU model (or part number) is? Also, how old the PSU is, and was the PSU bought new or used/refurbished?

It's a EVGA Supernova 750 G2, part number 220-G2-0750. It should actually be still under warranty, so I should be able to get a replacement.

I happen to have another working PSU around in the next few days, so I think I'll try swapping them and see if it solves the issue.

Thanks a lot! I'll let you know the result.

 

[Aeacus]

EVGA SuperNova G2 is one of the flaghship models of EVGA and is good quality. Still, there can be lemons, even among the best and this is the sole reason why PSU has a warranty. So, RMA it and get a replacement.

I happen to have another working PSU around in the next few days, so I think I'll try swapping them and see if it solves the issue.

If you test with 2nd PSU, make sure to use power cables that come with that 2nd PSU as well. Don't "reuse" power cables of your EVGA unit, since often, the pinout on PSU side is different and you can fry (short circuit) your hardware.

 

[Poccio]

If you test with 2nd PSU, make sure to use power cables that come with that 2nd PSU as well. Don't "reuse" power cables of your EVGA unit, since often, the pinout on PSU side is different and you can fry (short circuit) your hardware.

Oh, thank you, you are a life saver! I honestly didn't know about this, I would have 100% re-used those already in place. As soon as I'll have a chance to try it, I'll let you know how it goes.

Thank you!

 

[Poccio]

I am finally about to try it.

Just a question to be sure (and safe before further damage), considering I could have ended up frying the hardware with the wrong cables. The second PSU I have at disposal is a Corsair HX1500i. Am I safe temporarily replacing the old PSU with this model?

 

[Poccio]

I double-checked searching a bit online and didn't find any reasons it shouldn't be safe. So I did try the switch.

The problem is that nothing changed, and I am sure the other PSU does work (it was powering the other PC right until 1 hour ago). Once plugged in, literally nothing happens (no leds, nothing, just usb ports that work) and, if I try to turn it on, it's as if I never pressed the button.

I am starting to go back to the dead mobo hypothesis. Do you happen to have any other ideas/tests?

Thanks again!

 

[Aeacus]

I am starting to go back to the dead mobo hypothesis. Do you happen to have any other ideas/tests?

If PSU is sound, then next step in power delivery chain is MoBo VRMs (voltage regulating module). MoBo replacement is only fix for that.