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CPU fault or motherboard ?


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11 replies to this topic

#1 wuzzo

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Posted 30 January 2013 - 01:37 PM

Hello,
One of my PCs failed to post. All lights are up, fans working but refuses to post. Nothing on screen at all.
I've swapped out the PSU so it's not that. I've removed the memory and disconnected everything. No post.
Left with just the CPU and the motherboard I don't know which is at fault. It's an AM2 system so both are difficult to replace and so I'll very likely replace both with FM2. However, I'd like to know which has failed.

I'm no technician so I'll outline a behaviour in the hope that somebody will twig it. When the CD is connected to the mobo its light comes up but its tray won't open. When I disconnect the CD from the mobo - leaving it powered- its light come up and its tray works. When I leave the CD powered but disconnected from the mobo and I connect the HDD to the mobo the CD tray doesn't work. I don't know what's going on but it feels like a mobo power circuitry thing to me. Any ideas appreciated.
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#2 Nanobyte

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Posted 30 January 2013 - 02:46 PM

Are you sure that the display is working OK? Have you tried booting with the CD drive completely disconnected?

#3 Pretorian

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Posted 30 January 2013 - 03:21 PM

I quote Nanobyte: if a basic system ( mobo + ram + cpu, no hd, no cd\dvd\bd, no peripheral ) fail to work and the monitor works fine we have two ways, as you said.
Try the basic system and tell us what happens.

An hint: on some systems it happens that mouse make conflict with the system, so try another mouse also.

#4 dpunisher

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Posted 30 January 2013 - 04:00 PM

Not going to post without memory.

If it were mine, I would disconnect everything except videocard, 1 stick of RAM, CPU and cooler. No drives, no case fans, no USB, keyboaRD, mouse or other peripherals at all. Retest. Ck for bulged/leaking motherboard capacitors.

You might get a boot and hold for "keyboard error" message, and it that case you can start plugging in stuff and troubleshoot.

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#5 wuzzo

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Posted 30 January 2013 - 04:13 PM

I've tried all of those with the exception of disconnecting the case fans. You really think that's a possibility ? They're running fine.

Display, graphics card, memory, Cds, PSU, floppy, mouse and keyboard are all good.

Why would attaching the HDD cause the CD tray to quit working ?

Edited by wuzzo, 30 January 2013 - 04:16 PM.

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#6 Nanobyte

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Posted 30 January 2013 - 05:01 PM

Why would attaching the HDD cause the CD tray to quit working ?

I don't suppose many people have experience of the tests you did with the CD drive when the PC has failed POST. You happened to notice because you were fiddling around. As Dpunisher noted, the normal procedure is to disconnect everything to try and achieve POST.

The behaviour could be caused by a floating voltage that's neutral. It could be that when the HDD is plug in, it changes that voltage to force the door closed. I don't think you can read anything into what you found. Once the PC POSTs OK, then adding the hardware one at a time may identify issues.

Edit: I know that you have swopped the PSU but I thought I'd mention that under certain conditions such as voltage levels or CPU temperature, the PC will shut off and cannot be rebooted until AC power is totally disconnected from the PC. That lockout is to stop a PC cycling on and off. I don't know if that is present on all PCs.

Edited by Nanobyte, 30 January 2013 - 05:39 PM.


#7 wuzzo

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Posted 31 January 2013 - 06:30 AM

OK, I've now tried everything suggested and the PC does not post. Lights, fans etc power up but no post. Motherboard ? CPU ?
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#8 Nanobyte

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Posted 01 February 2013 - 03:16 PM

Is there a way to tell where the fault is without some highly specialized equipment? There is a limit to how much you can test. A repair shop may have some discarded boards around so they could swop the two components. That's how many problems are narrowed down.

#9 dpunisher

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Posted 01 February 2013 - 04:25 PM

Is there a way to tell where the fault is without some highly specialized equipment? There is a limit to how much you can test. A repair shop may have some discarded boards around so they could swop the two components. That's how many problems are narrowed down.


Pretty much. The OP is at the "replace with known good component" stage now.

I am a retired Ford tech. Next to Fords, any computer is a piece of cake. (The cake, its not a lie)

3770K @4.5, Corsair H100, GTX780, 16gig Samsung, Obsidian 700 (yes there is a 700)


#10 wuzzo

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Posted 02 February 2013 - 03:40 AM

Here's what I'm going to do. I have another machine with the same AM2 motherboard and CPU . Rather than try to replace the faulty machine's components with AM2 kit I'm going to swap out both the Motherboard and CPU for FM2 kit. Although I still won't know if it's the AM2 motherboard or CPU which has failed I'll put them both away towards the day when the good AM2 box fails- and may that day be far, far away. That's when I'll find out.
Thanks for your advice, chaps, much appreciated.

Edited by wuzzo, 02 February 2013 - 03:40 AM.

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#11 Wooshie

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Posted 03 February 2013 - 12:34 PM

Look at the motherboard for obvious popped capacitors on the motherboard. A lot of cheap oem AM2 motherboards have this problem.

#12 wuzzo

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Posted 03 February 2013 - 03:06 PM

There's no obvious damage and it's a retail Gigabyte board. No worries, I've replaced both the board and the CPU. First failure in ten years across dozens of machines. Can't complain too much.
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