New build powers on without pressing power button; won't POST

[tycho1974]

So I recently finished a new build, almost an exact copy of another PC I built at the office. I've built almost a hundred machines so far, but I always get nervous the first time I push the power button.

This time, I had everything ready to go, then flipped the main power switch on the PSU. I was extremely surprised when the computer turned on, without me having to actually press the power button on the case. It looked like everything was running, but the machine did not POST, produced no beeps, and displayed no video output. I also noticed that the LED for the power button was not lit — pressing the power button turned the LED on, but produced no other discernible effects. Long-pressing the button (hard power down) did nothing, either.

I powered down by switching off the PSU, then unplugged the front panel connecters from the mobo. I switched the PSU back on, and I got the same result — fans, water cooler, RAM LEDs, everything was "powered on", but no POST, no beeps, no video output.

I actually bought another motherboard (same make and model), and swapped it out for the one I thought was defective. Unfortunately, I got the exact same results. I have been doing this for a long time, and I honestly have no idea what's happening here. Is it a problem with the case? The PSU? Any help would be appreciated, as this is an office computer that we need to get running ASAP.

• 
  Mobo: Asus STRIX B250H
  Case: NZXT H500i
  CPU: Intel Core i5-7600K
  Cooler: NZXT Kraken X52
  RAM: G.SKILL TridentZ RGB Series 16GB (2 x 8GB) 288-Pin DDR4 3000MHz
  PSU: EVGA Supernova 650 G3
  HDD 1: Samsung 850 EVO 500GB
  HDD 2: WD Black 2TB

 

[gingerrankin]

Do you say that if you disconnect the "front pannel connector" from the motherboard it also goes through and starts up as soon as you connect the psu to the wall?

If so, I would rule out problems with the case.

Have you checked that the stands of the mobo are fine and there is no short circuit?

If you have tried with another mobo, surely the problem will be in the psu.

 

[tycho1974]

Do you say that if you disconnect the "front pannel connector" from the motherboard it also goes through and starts up as soon as you connect the psu to the wall?

If so, I would rule out problems with the case.

Have you checked that the stands of the mobo are fine and there is no short circuit?

If you have tried with another mobo, surely the problem will be in the psu.

You are correct in that reading — the PC powers on when I flip the switch on the PSU, regardless of whether or not the front panel connectors (the leads that contain the power switch, power LED, and HD LED) are actually connected to the motherboard.

So you're saying that the likely culprit is the PSU? I can replace it fairly easily; it's just strange to think that a PSU would cause this kind of error. I'll get another unit and see what happens, and report back.

 

[tycho1974]

Big thanks to gingerrankin: I swapped out the power supply for a different model and the computer booted properly. Time to send the defective PSU back for a refund.

Thanks again!

 

[gingerrankin]

Ok. Enjoy it!