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728 posts

Ultimate Geek
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Topic # 138473 6-Jan-2014 09:11 Send private message

We came home from a few days holiday to find that my wife's PC wouldn't start.  After a lengthy diagnosis by elimination, it seems to come down to a faulty 8 pin motherboard power cable.  It's modular, so I swapped it for an identical cable, and everything works fine.  But I'm left puzzled by why it broke in the first place.  My sister-in-law, who was house sitting, had started it up earlier that day without problem.  It seems to have just stopped working all of a sudden.  Can anyone suggest something plausible?

There are a couple of other "faults" which may be completely unrelated, but which I'll mention here in case they help the diagnosis:
1. The motherboard's software (Asus) occasionally reports absurd temperature and voltage alerts - things like -250 degrees, or 0.1v.  We looked into this, and it seems to relate to the Asus software rather than being genuine temperature or voltage faults.  If the temperatures or voltages had fluctuated as wildly as they have been reported, then there would have been other problems, and in every other respect the computer has been rock solid.

2. The front panel power switch has been playing up the last few months.  Most of the time it works fine, but occasionally it won't power on when you push the button - you have to wait anything up to 10 seconds before it turns on.  We put this down to being an intermittent fault in the microswitch.

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Uber Geek
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M2 Group

  Reply # 961759 6-Jan-2014 09:53 Send private message

Cables and connectors fail... not sure what you're asking exactly... also the power switch not always working may well be related.  See if that fault goes away now.

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Uber Geek
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  Reply # 961767 6-Jan-2014 10:00 One person supports this post Send private message

I'd suggest they're all related, and I'd put it down to the bad connection. Dodgy connections will give you all sorts of weird issues.

With voltages as low as 3.3 - 12v it doesn't take much to affect or stop them. A change in temperature or humidity can sometimes have a drastic effect. Also, bad connections often generate significant heat, which will eventually ruin the connection.

Temperature sensors generally output a variable voltage and the software calculates the temperature. -250 is probably the bottom of the scale, and means the sensor was not being supplied enough/any power.
The accompanying voltage reading backs that up.

The power switch issue was probably a symptom of the actual fault.

728 posts

Ultimate Geek
+1 received by user: 31

  Reply # 961899 6-Jan-2014 14:22 Send private message

So far the front panel switch is working, though that was an intermittent fault so time will tell.

Andrew - the voltage errors were ridiculously all over the place. I think we might have even seen negative voltages. Basically, they bore no resemblance to reality, and if they were I would have expected to see accompanying stability problems or crashes, which we never did. Is it possible for there to be actual voltage fluctuations of that nature without seeing other evidence of a power problem?

728 posts

Ultimate Geek
+1 received by user: 31

  Reply # 962652 7-Jan-2014 19:22 Send private message

So - the problem is not solved.

We went to turn on the PC this evening, and it wouldn't start. I took off the side panel and tried to turn it on by shorting the power switch pins on the motherboard, and that didn't work. So, to me, that eliminates the front panel power switch as being faulty.

However, it's unclear whether this is a fault in the motherboard, the PSU, or the cable. I'm disinclined to suspect the 8 pin power cable as I originally thought, but it doesn't rule out the 24 pin cable. It could be the PSU, but the only way to troubleshoot is to swap out the PSU from my PC again, and that's a nuisance I'd like to avoid. There could be a fault in the motherboard, but that's far beyond my knowledge, so I wouldn't know where to start troubleshooting a motherboard fault.

Anyone have any thoughts or suggestions? Does it sound like the PSU or a cable? I do have another 24 pin power cable which I can swap out, and see if that makes a difference, but if there's a better way to proceed, I'm all ears (eyes).

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Uber Geek
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  Reply # 962735 7-Jan-2014 20:50 Send private message

Dry joints on MB connector(s)......

Even with the new problem it still fits andrewNZ post

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Uber Geek
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  Reply # 962766 7-Jan-2014 21:30 Send private message

You're pretty much down to trying things.

I'd get another power supply and try it with that, you don't necessarily need to mount it properly, just connect it instead. Keep in mind that if the power supply is bad, it could have taken the motherboard out too.

You need to be a bit careful from here on, check your data backups.

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Ultimate Geek
+1 received by user: 31

  Reply # 962973 8-Jan-2014 08:49 Send private message

So I swapped out the 24 pin power cable, and it still didn't work. So I then took out the PSU, including the power cord, the 24 pin cable, and the 8 pin power cable, and plugged them into my own computer. Using a screwdriver to short the power pins on the motherboard, I could only get the PSU to power up once, through I tried about a dozen times. When I plugged my own PSU back into my PC, it worked fine, so I think that confirms that it's a PSU/cable problem, rather than a motherboard or switch problem.

I've used two different sets of 24 pin and 8 pin cables, so it's probably reasonable to assume that the cables are not faulty (unless both sets are coincidentally broken). Which only leaves the PSU itself. Time for RMA? It's less than 12 months old. The problem for me is that it's intermittent, which always makes me nervous about sending something back to the retailer. There's every chance they will just power it up, assume it works, and send it back...

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