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A friend of mine told me that its PC suffer strange behavior, sometimes doesn't reach the Windows welcome screen, or it switch off suddenly during it working or produce weird graphics on its monitor.

Yesterday I got it and the first thing I try is connecting an external USB HDD. It switched off immediately. The same does not happen for example with a Wireless USB dongle.

The computer is more or less 5 years old. My idea is that the PSU has something wrong.

Do you suggest buy a new one, or could be also the motherboard the problem?

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At the end I substituted the PSU with a new one, and the problems seems vanished. –  Drake Jul 27 '09 at 13:39
    
Glad you fixed it! –  jjnguy Jul 27 '09 at 14:32
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3 Answers

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It sounds to me like your PSU may be going. If your computer puts more load on the PSU than it can handle it will shut itself off.

(This happened to me once. I had a bad PSU, and my computer would shut off when I tried to print or burn a DVD/CD. Getting a new PSU solved my problem.)

Invest in a good one now and you can move it into future computers.

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yeah, it was the PSU –  Drake Jul 27 '09 at 13:40
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Sudden switch-offs are often caused by high temperatures or broken memory chips. Try the torture test of Prime95 and watch what happens to the core temperatures with something like SpeedFan as a first step.

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thank you balpha I would try it, but the problem happens also when the computer is switch off from "cold", this is the reason why I excluded the problem of temperature at first look –  Drake Jul 15 '09 at 10:22
    
...switched ON from "cold" –  Drake Jul 15 '09 at 10:23
    
What about the memory issue? Prime95 cann also torture your memory, so it might still be worth a try (if the problem is highly reproducable, you can also just try switching/removing memory chips). –  balpha Jul 15 '09 at 10:42
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USB ports are supposed to have current limiters on them, but many do not. So if you plug something in that is pulling to much power, it could cause your computer to lock-up.

I've actually have seen a USB connector for a camera that can easily be put in backwards, which has caused the computer to stop working. The strange thing is that the screen didn't change at all. I assume that the graphics card derived it's power from the 12V rail.

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