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Help - Where's my problem: PSU, CPU, Mobo?

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  • New Build
  • CPUs
  • Systems
Last response: in Systems
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July 19, 2009 4:21:50 AM


My system was running fine for 2.5 years, and then I come hope to a BSOD (an IRQ error). Rebooting it, it would get to where I could hit F8 and select Safe Mode, but it would just immediately reboot over and over at that point, not actually booting.

I removed many components hoping to isolate the problem, and after removing the video cards and HDDs, when turned on, it would just beep one long unending beep, and the system wouldn't post.

Not knowing what that error was, I have now completely disassembled the system, outside of the case, just the PSU, CPU, motherboard, CPU fan, one stick of RAM. Now when I turn it on, the fans turn for about half a second and then turn off with nothing happening unless I cycle the power switch on the PSU.

This made me think I had a short on my motherboard, but it's sitting on cardboard on a wooden table, and I've visually inspected the board for any problems or blown caps - nothing. I also tested the PSU by itself using a paper clip over green and black pins - it turns on and runs just fine. Still the system does nothing more than spin the CPU fan for half a second.

At this point, I'm not sure where the problem is, I have no other spares or compatible replacements for these parts.

So where's my problem? PSU, CPU, or motherboard?

ThermalTake Toughpower 850W
Intel X6800
ASUS Striker Extreme (the original, not II)

Thanks!

More about : problem psu cpu mobo

July 19, 2009 8:06:57 AM

Just tried four sticks of RAM (two of which have never been used), in different slots, same result: fans spin for half a second, then they stop and nothing, no beeping, anything. The LEDs on the motherboard stay on, but nothing will happen if I hit the power button until I cycle the PSU switch. Feels to me like the PSU is detecting a short?

I've tried all the steps in that post (this problem's so basic most of them aren't relevant), any other suggestions or previous experiences with something like this?
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July 19, 2009 11:48:20 AM

Vol said:

I also tested the PSU by itself using a paper clip over green and black pins - it turns on and runs just fine. Still the system does nothing more than spin the CPU fan for half a second.


That really does not mean that much. It doesn't check the 3.3 or 5 volt outputs. It doesn't test the "PowerOK" circuit. All it really proves is that the PSU can produce 1/2 amp or so at somewhere around 12 volts.

If you have a multimeter, you can check all the outputs. Yellow wires should be 12 volts, red 5 volts, orange 3.3 volts, blue wire -12 volts, purple wire is the 5 volt standby. The gray wire is really important. It sends a control signal called something like "PowerOK" from the PSU to the motherboard. It should go from 0 volts to about 5 volts within a half second of pressing the case power switch. If you do not have this signal, your computer will not boot. The tolerances should be +/- 5%. If not, the PSU is bad.

Unfortunately, once you are down to the motherboard, CPU, and PSU; all you can do is test by substituting known good parts.
July 19, 2009 5:38:44 PM

I'll pick up a multimeter this afternoon and perform those tests.

This problem - half a second of power to the fans then nothing - could really be a CPU or PSU issue?
July 19, 2009 7:32:45 PM

Does the board have integraded graphips? You should have some graphics in the system. Some systems do boot without any graphics but some might not, I don't know.

Also if there is a shortcircuit somewhere where low current flows, it might not be able to actually burn anything so you could have a short and not being able to see or smell it.

You should be careful with alternative hardware you test. Anything or everything could be defective and could ruin the rest. I once tested a motherboard which turned out to gone bad, it managed to burn my alternative CPU from other computer, on top of everything else the damn thing had managed to burn so far. Originally it was the power which shorted the motherboard, the power fried because of fuse blowing up in the apartment.

So with a cheap CPU you can partially test if your CPU is OK and so on. If the system doesn't boot with alternative CPU for example, you can't know the CPU is OK, but at least you can know the CPU isn't the only problem.

I would try with a known working power first, because a faulty power might have damaged the board and thus putting anything functional in the board, it might damage it too.
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