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I have a fairly standard 2-year old desktop computer (quad-core intel, single hard drive, decent video card, 300W power supply) which recently started acting up.

I'm not sure what the cause is, so hopefully you can help.

Sometimes (once a week-ish) I press the power button and nothing happens. No blinking, no sounds, no nothing. If I remove the power cord (or flip the switch on the power supply) I hear a capacitor discharge. If I leave it in the "no power at all" state for about 5 minutes then I can put the plug back in and the computer works perfectly.

What is the issue? What do you think I have to replace?

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migrated from serverfault.com Jan 2 '11 at 11:13

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3 Answers 3

up vote 2 down vote accepted

I would test using another power source. This worked for me a few times.

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I've seen this many times before and usually it's the capacitors on the mainboard or in the power supply that no longer work. The mainboard's capacitors are easily checked by opening up the computer case and checking all the capacitors. Failing capacitors have a slightly rounded (or blown up) top surface, while they should be perfectly flat. You might also see some of them have started leaking. (See image, the top 4 are faulty, the bottom 3 are still ok.)

alt text

I used to replace these capacitors myself by taking out the mainboard and soldering some new ones on the board. If you don't have the time/skills/materials to do so, ask your local tech guy or order a replacement mainboard.

The same could be happening to your power supply, I would not recommend opening this up yourself though. There's a risk of electric shocks and when opening a power supply, warranty is void immediately when breaking the case seal.

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But in cases where capacitors are leaking, often it is required to press power button several times, so that capacitors would charge enough for computer to turn on. It seems that the problem described is inverse of typical capacitor plague description. –  AndrejaKo Jan 2 '11 at 11:31
    
@Andrejako That depends on which capacitors are affected. I've come across this problem dozens of times and everytime it was a capacitor issue. –  BloodPhilia Jan 2 '11 at 11:32
    
I never heard of something like that. Well, I guess I learn something new every day. –  AndrejaKo Jan 2 '11 at 11:49
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+1, the old capacitor plague... en.wikipedia.org/wiki/Capacitor_plague –  Moab Jan 2 '11 at 16:11

My guess would go towards a BIOS problem, like the battery on the motherboard being unable to provide enough energy to keep BIOS settings safe. The settings would be in an unstable state, along with the devices and the motherboard - probably stuck in this unstable state thanks to the capacitor you mentioned in the question.

By removing the power cable long enough, the capacitor discharges and all the BIOS settings and devices would be completely reset.

My first suggestion is to replace the motherboard battery (see this Wikipedia article).
Those batteries CR2016 usually are cheap, and easy to replace.

For the time being, in order to accelerate the discharge, you may want to try to remove the power cable, and, then, press the button ON of the computer.

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This could be easily confirmed by the date in the bios being reset when the battery can no longer power the RTC. Easy enough to check. I have seen cmos batteries cause a myriad of strange issues without losing the clock settings though. Dell had a slew of defective cmos batteries a few years back, caused all kinds of weird startup issues. It had the Dell engineers stumped until I suggested to replace the cmos battery, that cured it. I never did get any feedback from them on what the battery was doing to cause this. –  Moab Jan 2 '11 at 16:17
    
The date is not lost when I power back on, nor is any of the BIOS settings. The computer is 100% healthy when it finally turns on after the "no power" wait. –  Paul Tarjan Jan 2 '11 at 18:11
    
The MB battery maybe be low enough to cause unstable results, while still able to keep the clock running. Honestly I'm not sure, but a low battery can have various effects... Two years is usually the period from which the battery may die. It is very cheap - I wouldn't bother to change it and try. –  ring0 Jan 3 '11 at 2:25

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