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I searched all over google for the answers for this and most of them read that the PSU is at fault but i did the paper clip test where you connect the green wire to one of the black wires (ref: http://aphnetworks.com/lounge/turn%5Fon%5Fpsu%5Fwithout%5Fmotherboard%5Fthe%5Fpaperclip%5Ftrick) and the fan spins.

So i thought my mobo was having problems so i returned it, got a new one and i get the exact same problem. So i dont think it's mobo at fault.

Update

I placed the motherboard over an anti-static grey clear bag (the one that you get for free when you buy a motherboard) just incase the case was shorting the motherboard, unplugged everything except for the RAM and CPU and the samething happens - it turns on for a second (I can see the fan spin) and then shuts down immediately. keyboard or screen is not even attached.

Update 2

I found the solution my problem. I posted it below. Thanks for your help guys!

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Can you add more details, about other components added to the system? Including quantities, and the model and watt capacity of your PSU. It may be as simple as there's not enough power capacity for what you're trying to run. –  Nick Josevski Sep 25 '09 at 1:57
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5 Answers

A bit hard to diagnose without seeing, however if someone came to me with this problem I would do the following.

Just to rule out any other sort of problem with the PSU, can you try turning on your PC with only the power cable, screen and keyboard connected and see if you can get into the BIOS, if you can, it means that either your PSU has insufficient output for the amount of hardware you have, or a piece of hardware is bad.

If it is the insufficient power in the PSU, try buying a different higher rated one.

If a piece of hardware is bad, try plugging in components one at a time until it breaks again.

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It could also be a short but the handling would be much the same as above. –  emgee Sep 25 '09 at 2:12
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Two ideas come to mind:

  1. This is a feature of the motherboard. It won't power up when AC is applied until you hit the power button. Sometimes when a motherboard is set to stay off when AC power is applied, it blinks on for a split second then shuts down. Only after you press the power button (or short the power button header pins together) does it actually power up to boot.
  2. The motherboard has a short on it which causes the PSU's overcurrent sense to trip. This would be unlikely to have on two new motherboards in a row.
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The first time I built my own computer, I installed the CPU heat-sync and fan incorrectly. The computer would turn on briefly, but then shut off since the motherboard sensed that the CPU got too hot. Are you sure you have installed the CPU/RAM correctly?

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That could be my problem as well but I'm not sure how i could've installed it incorrectly. What mistake did you do when installing it? –  burnt1ce Sep 26 '09 at 14:09
    
I don't recall exactly, but I think I neglected to snap the fan into place. There were two plastic levers on the fan that needed to be snapped down, to push the heat-sync onto the processor. –  Peter Di Cecco Oct 3 '09 at 20:36
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I know this is an old post, but it's a common problem. One of the best things is to put new heat compound on the fan/CPU. Take off the old compound first (the OEM compound usually isn't enough) with a few alcohol pads (until it's completely clean) and then put a good bit of the new stuff (not enough to over flow over the sides) to cover the entire processor. Put the fan back on and snap it down. That usually does the trick.

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up vote 1 down vote accepted

Thanks for your help guys. I found what the problem was and it was my power supply.

Even though it was able to give the right amount of voltage and it passed basic power supply tests with Tiger Direct (computer retail store) with some sort of volt meter, they diagnosed the problem as a incompatibility issue with my motherboard and power supply. This diagnosis was wrong and the power supply was defective. I replaced the power supply with a brand new power supply with the same model and it works.

The specs are: Power supply: Corsair CMPSU-550VX 500W ATX 12v Motherboard: Gigabyte MA785GM-US2H motherboard

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