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Help diagnose problem: PSU or mobo (or s.th. else?)


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#1 Alex_B

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Posted 26 July 2011 - 12:11 PM

Hello all. I need your help identifying the faulty part in my computer. It's a desktop PC which suddenly stopped working (between switching it off and switching it on again two hours later). The symptoms are that the mouse and the keyboard get no power and the monitor gets no signal. The interesting thing is that when I plug in the keyboard while the computer is switched on, some LEDs on the keyboard blink to indicate that it has been plugged in, but then they all go out again. Previously, an LED was constantly on when the keyboard was plugged in, even if the computer was switched off. The hard drive goes on when the computer is switched on, I can hear it spin.

I have tried removing the graphics card and the RAM to see if then the mouse and the keyboard get power, but no luck. Unfortunately, the motherboard has no mobo speaker. I ordered one today, but it might take several days to arrive and I need the computer for work. If you could help me narrow down the problem, I will be eternally grateful.
Alex

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#2 cryptodan

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Posted 26 July 2011 - 01:12 PM

reassemble the computer outside the case on a rubber mat or a cardboard box. Power it on and see if it works.

#3 Alex_B

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Posted 26 July 2011 - 01:19 PM

Thanks for the tip, that would have never occurred to me. Never had such a problem before. And if it does work, what would that indicate?

Also, how do I make sure that everything is properly grounded? Or is that not a problem as long as the parts are on something not conducting? And do I need to worry about any of the parts getting too hot?

Edited by Alex_B, 26 July 2011 - 01:25 PM.


#4 cryptodan

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Posted 26 July 2011 - 01:39 PM

If it works outside the case then there is a short between the case and the motherboard.

The ground would be the power supply.

#5 Alex_B

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Posted 26 July 2011 - 02:21 PM

Oh my god, I can't even begin to tell you how grateful I am! It works perfectly outside the case. Are there any standard tricks for finding the short? Or should I buy a new case? The fact that I can just continue working now is just priceless though - thank you so much!

#6 cryptodan

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Posted 26 July 2011 - 02:32 PM

look for any loose screws aka the little brass things called standoffs. Inspect the back of the board and see if you see any wear marks on the screw holes.

#7 Alex_B

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Posted 26 July 2011 - 02:59 PM

Will do. I will report back tomorrow, since it's already 5 am here. Thank you very much for your help!

#8 Alex_B

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Posted 27 July 2011 - 12:57 AM

I haven't found any loose screws and the screw holes on the motherboard look ok to me. However, even outside the case, I sometimes got distorted images on the screen and sometimes just gibberish. This happened even before the boot process started, so it can't be the graphics driver. I have had this problem occasionally before, but now it started happening consistently. If I proceeded with boot when this happened, the system usually locked up after a short while. This also continued happening inside the case.

I have now put everything back into the case, secured the motherboard with as few screws as possible, taken out the external graphics card and am using the mobo graphics card instead, and the problem has disappeared. It looks to me like the graphics card is broken (which is by far my favourite explanation, compared to broken mobo or PSU).

#9 cryptodan

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Posted 27 July 2011 - 05:00 AM

Replace the graphics card.

#10 Alex_B

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Posted 28 July 2011 - 12:06 PM

I guess I might do one day, but since I mainly use the PC for computing, I am happy with the on board card for the time being. Or is there any reason for not using an on board graphics chip for too long?

#11 cryptodan

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Posted 28 July 2011 - 03:59 PM

There is no downfall in using the on board video card. In most cases it should suit regular use like email, web browsing, and general office type work just fine.




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