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My friend's computer wouldn't power on. The fans (and a blue LED near the power switch) come on briefly and then turn off immediately.

I have removed the connections to the drives and the only PCI card, a wireless card. I bought a new power supply and plugged it in, but that didn't change anything.

I am able to turn on the new power supply (or at least its fan) by shorting to ground the green wire in the 24-wire connector that runs from the power supply to the motherboard. This didn't work for the old power supply, so that proves that it was broken.

Is it likely that, when the old power supply died, it ruined something on the motherboard?

  • I noticed that the processor fan turns the wrong way, pulling air towards the processor. The plug isn't reversible. Is this a clue?
  • The case fan and processor fan also run quite slowly. Is this a clue?

I removed the 2x2 connector that runs from the Power Supply to the motherboard, and NOW THE POWER STAYS ON. Well, the 3 fans stay on, but there is no further sign of life. The motherboard doesn't beep.

At this point, I can't turn the power/fans off using the case's power button (even holding it down), so I use the power supply's power button on the back of the case.

Now that I have isolated which connector is the problem, is there any way to fix it? Might it be that a capacitor near the socket is broken and I could replace it?

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"I removed the 2x2 connector that runs from the Power Supply to the motherboard, and NOW THE POWER STAYS ON." Why in the world would you do such a thing?! What motherboard? What CPU? –  David Schwartz Feb 20 '13 at 1:20
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"I noticed that the processor fan turns the wrong way, pulling air towards the processor. The plug isn't reversible. Is this a clue?" The fan blows air on to the heat-sync and through it. It is functioning correctly! Don't try and reverse it. –  Dave Lucre Feb 20 '13 at 1:22
    
David: I unplugged the connector because I was unplugging everything else (devices), and nothing was working. To my surprise, the fans finally did power on and stay on. So I did manage to narrow down the problem a bit. –  user200692 Feb 20 '13 at 5:54
    
DaveL: I am surprised to hear that this is the correct direction, but I suppose both directions would work. –  user200692 Feb 20 '13 at 5:57
    
@user200692: You didn't narrow the problem down at all, and you may have created a new problem. –  David Schwartz Feb 20 '13 at 19:52
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1 Answer

First, disconnect any devices not needed to get to the BIOS: hard-disk drives, CD/DVD, PCI (Express) cards except the graphic card (unless there is an onboard/integrated one)... Then you should check for short circuits on the motherboard, or signs of it - something looking burned. If there aren't any, it's probably the CPU and/or voltage regulator on the motherboard that died. Putting a working CPU instead of the current one is more risky (as the broken voltage regulator could burn it) than trying the current CPU with a working motherboard, so you could continue with the latter step. One of these steps could help you figure out what's wrong and needs replacing.

Depending on how old the system is, second-hand could be feasible.

From About.com:

The ATX 4 pin power supply connector is a standard motherboard power connector used to provide +12 VDC to the processor voltage regulator.

Check the links there for further info on testing the power supply.

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sm4rk0: Thanks for the advice. I did disconnect the devices. To check for short circuits, I checked for lose metal objects (screws) sitting on the motherboard, then looked for burned components, but didn't see any. There may be something underneath the CPU's heat sink. –  user200692 Feb 20 '13 at 5:59
    
Is the voltage regulator a visible component, or is it tiny and also hidden under the heat sink? Is it possible to test the voltage regulator? Can I replace the VR and not the CPU? Why would the computer turn off if its VR isn't working? Does the voltage fluctuate too much? ... I don't have a spare motherboard... About.com did have some good articles... Next, I guess I need to remove the heat sink, and it sounds like I need to get into the messy thermal paste stuff. Heat the sink with a hairdryer and try to twist it off the CPU. –  user200692 Feb 20 '13 at 6:13
    
First: do not remove heat sink. Second: if you decide to remove - don't use hairdryer –  sm4rk0 Feb 20 '13 at 22:38
    
@user200692 (I exceeded 5 minute comment editing time) 3. VRs with their belonging capacitors and coils can be seen here: img.tomshardware.com/us/2004/05/21/… (10 black 3-pin components). Don't try to fix it if you don't have some good experience with the soldering iron, as advanced skill is needed to replace one of them - assuming you have the appropriate replacement part(s). 4. Power supply units (PSU) have automatic shut-down system as a protection from damage due to excess current which usually happen due to a short-circuit. –  sm4rk0 Feb 20 '13 at 22:49
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Wow, I hope this is what it is. I guess I'll clean it first with 90% isopropyl alcohol and then apply a new layer of thermal paste. –  user200692 Feb 25 '13 at 0:33
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