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New PSU possibly short-circuited my mobo

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February 5, 2013 11:19:09 AM

Hello everyone!

Here's the deal: a few weeks ago, my computer stopped booting up. I opened up the case and noticed the led on my mobo that's usually always turned on was off, and guessed the problem was probably my PSU (a brandless 420W module). I then went to my local hardware store to have my PSU checked out. Indeed, it turned out to be the problem so I bought a new one (Nano 500W from TopPower) and went home.
Once the component was installed, I promptly booted up the computer and that's when I thought seeing, out of the corner of my eye, a blue flash coming from my motherboard. Since, my computer turns on but doesn't send a signal to any output (mouse, keyboard, screen, etc.) and when I try force closing it by holding the power button, it turns right back on of its own accord 5 seconds after.

Thank you for taking the time to read. Any ideas?
a c 90 ) Power supply
February 5, 2013 12:12:38 PM

Welcome to Tom's Hardware Forums!

Blue flashes are never good and will let the magic smoke out! Yes it does sound like a short circuit but I don't think that the PSU caused it. Some component on the motherboard probably a cap failed and created a short circuit.

I don't understand what you mean by saying "... and when I try force closing it by holding the power button, it turns right back on of its own accord 5 seconds after. "
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February 5, 2013 12:39:21 PM

When you hold down the power button for about 5 seconds, the power is immediately cut from your computer, no? I know it's not recommended, but it's the only way I was able to turn off my computer. Even then, every time after cutting the power, the computer turns back on on its own without me pressing the power button.
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a c 90 ) Power supply
February 5, 2013 1:07:32 PM

Yes, holding down the Power button for about 5 seconds is known as a hard shut-off. After doing that the computer should stay shut off.

If the computer turns ON by itself it means that there is a short in the 'Start' switch. The two leads from the start switch go to two header pins on the motherboard (except in some computers like Dell where it goes into a multi connector).

To isolate this problem, carefully unplug the two tiny connectors that go to the motherboard start (Power) pins, and then use a flat blade screwdriver to briefly short these two headers simulating the closing of the start switch. If by doing so your computer works fine, then replace the start switch with a new one.

Please post the make and model of the computer.
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February 9, 2013 5:26:04 PM

I tried shorting the switch pin and the switch is NOT the problem. It's the same as before: when I start the computer, no output turns on (screen, keyboard, mouse) and if I do a hard shut-off afterwards, my computer turns back on 5 seconds later.
Note: my computer doesn't start this sequence until after I power it on for a first time (another proof it's not the switch).

EDIT: I found the problem! The PSU 4-pin into the mobo wasn't plugged in well.
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a c 90 ) Power supply
February 9, 2013 8:45:15 PM

Glad you solved it!
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!