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So this morning my computer wouldn't turn on. Pressing the power button has no affect. When I turn off the switch at the back and push the power button after turning it back on it seems for about half a second that everything works: fans appear to start moving and the PSU light turns on. This happens only for a brief moment (less than a second) and than everything turns back off. I can repeat this by turning Off and On the switch at the back of the case or by unplugging the power cable out and plugging in again. There is a small led on the motherboard which is always on when power is connected (doesn't matter if the computer is on or off) and it is still on now.

I am trying to diagnose what is the cause of the problem PSU or motherboard, any ideas?

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Is there any "beep code?" – David Nov 8 '10 at 16:01
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Unplug everything you can possibly unplug, internally and externally, and remove all expansion cards and RAM. So, with nothing but the main ATX power header and CPU power header (and front panel and internal speaker connections attached - see comments) attached and no RAM, if the box STILL won't power on, you probably either have a power supply or motherboard problem (the odds of a failed CPU are nil unless you've been overclocking like crazy, and if you're savvy enough to do that you shouldn't be asking this question).

If it beeps at you, which it should if you try to power it on with no RAM, reattach stuff piece by piece (starting with the RAM) to see if you can isolate a problem.

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If you are listening for beep codes, make sure you actually have a case speaker attached. – typoknig Nov 8 '10 at 17:33
@typoknig - Oops! I forget some boards have those disconnectable, haha, sorry. – Shinrai Nov 8 '10 at 17:44
tried it and still the same. I guess I will have to take it to the repairman since I don't have spares to switch and check which one it's actually is. – user54921 Nov 8 '10 at 20:55
@user54921 - I'd give heavy odds it's a bad power supply - if you have another PC around you could swap them. (Obviously this isn't so easy with the motherboard or CPU). Hope you get it resolved soon though - don't forget to come back and pick an answer if someone actually answered your question. ;) – Shinrai Nov 9 '10 at 0:02
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Several possibilities, actually too many possibilities.

I believe this is a job for the repairman.

Nobody can diagnose your problem over this forum, and implementing all the suggestions you will get here will cost you much more than the repairman will.

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+1 - But 'implementing all the suggestions' may cost you more. – CJM Nov 8 '10 at 17:16
There is nothing a "repairman" can do that cannot be described here. This is a place for techies. If it costs more for him to do things than for a "repairman", then that's for him to decide. And he is welcome to be the repair man if he wants. And any replies are not just for him, but for others with the problem, including people that tend to fix things themselves. – barlop Nov 8 '10 at 18:03
@barlop: OK, tell him to change the PSU. If this doesn't work then the BIOS battery, then capacitors, then RAM, followed by disk, then the motherboard. Very technical. Been there, done that. There are a zillion cases like it on SU, and my above advice was the only one that worked so many times that I'm really tired of giving it again and again. I really wish SU had a FAQ. – harrymc Nov 8 '10 at 19:08
@barlop: Oh, and thank you for the down-vote. – harrymc Nov 8 '10 at 19:11
@harrymc - I won't downvote because it's a cogent answer but barlop is right when he says "If it costs more for him to do things than for a "repairman", then that's for him to decide. And he is welcome to be the repair man if he wants. And any replies are not just for him, but for others with the problem, including people that tend to fix things themselves." – Shinrai Nov 9 '10 at 0:03
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It sounds like a failing power supply. You might be able to buy yourself some time with the following trick:

  1. Turn off the switch directly on the power supply.
  2. Wait 30 seconds
  3. Place one hand by the switch on the power supply and the other next to the normal power switch for the computer.
  4. Turn the switch on the power supply back on
  5. After about a 1/4 to 1/2 second pause press the main power switch on the computer. This press should be distinctly separate from activating the switch on the power supply, but before the cpu fan stops moving.

Where I'm at we have a 20 machine computer lab that was last updated about 7 years ago. The lab is (finally!) up for replacement over the Christmas break. There are 4 machines in the lab with ailing power supplies matching the exact symptoms you describe. Rather than replace the power supplies now for machines that are schedule to be swapped out soon I've been able to use this trick to keep them going.

Note that this will only buy you a little time. The trick actually only still works on three of my affected machines, and I don't expect that to continue much longer. My solution in this lab is just to hand an "out of order" sign on them for the rest of the semester when they no longer boot at all. The lab rarely needs the full capacity this late in the semester. It's more for orientation classes early on. But for you, the trick should allow you to use the computer while you wait for a new power supply to ship.

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That's a premature diagnosis - it could be the PSU... or the Mobo or the RAM. I've actually seen other components produce similar results (a dodgy NIC actually). Secondly, if it is the PSU, trying to force the PSU's arm could be counter-productive - the voltage regulation circuitry (on mobo) will do it's best to protect, but nevertheless, 'jump-starting' it might not be the best idea. – CJM Nov 8 '10 at 17:02
What is the name of this process? I haven't heard of it before. Any web links on it? – barlop Nov 9 '10 at 20:27
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It could be one of many components, but it is likely to be the RAM/CPU/PSU/Motherboard (in no particular order).

If you have access to some spare components or a someone else's similar machine, you can possibly isolate the problem yourself. The first task is to test one stick of RAM in another machine. If the RAM is good, leave it (on it's own) in your machine, leave the CPU and the PSU connected, but remove all other RAM/cards/disks/peripherals. If this allows you PC to at least 'turn-over', your RAM may well be the problem.

Likewise, if the problem remains, you need to either test each of the other main components either by proving them in another machine, or by replacing them with a spare one. Be sure to use compatible components for testing - that is, if you try to cannibalise another machine to test this one, make sure that they use similar components. The specification in your user guide should indicate this, or otherwise seek specific information from your OEM.

For your average home or business user, this might prove difficult, which is where your local independent repair shop comes in. Yes, you'll have to pay for labour on top of the replacement parts, but unless you have your own resources, you have no choice.

If you aren't confident under the bonnet of your PC, this might also be the safest option.

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