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CPU Fan won't spin, GPU fan spins at max, and PC simply won't turn on.

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September 19, 2010 7:38:15 PM

Something extremely weird just happened. I heard a pop inside my desktop, causing the whole thing to shut down. When I try to turn it on, next thing I know the CPU fan is no longer spinning (all is properly connected), while the GPU fan (ASUS 5870) is spinning at max making all the noise it possibly can make. Nothing would show up on the screen, no BIOS, no nothing, just a black monitor waiting to detect something.

I honestly have no clue what to do. I removed the GPU, shut everything down, retry to turn it back on, same thing. Am I supposed to contact a technician? I'm out of ideas. Thanks in advance.
a c 122 B Homebuilt system
a c 172 à CPUs
September 19, 2010 8:05:48 PM

Work through our standard checklist and troubleshooting thread:
http://www.tomshardware.com/forum/261145-31-read-postin...
I mean work through, not just read over it. We spent a lot of time on this. It should find most of the problems.

If not, continue.

I have tested the following beep patterns on Gigabyte, eVGA, and ECS motherboards. Other BIOS' may be different.

Breadboard - that will help isolate any kind of case problem you might have.
http://www.tomshardware.com/forum/262730-31-breadboardi...

Breadboard with just motherboard, CPU & HSF, case speaker, and PSU.

Make sure you plug the CPU power cable in. The system will not boot without it.

I always breadboard a new build. It takes only a few minutes, and you know you are putting good parts in the case.

You can turn on the PC by momentarily shorting the two pins that the case power switch goes to. You should hear a series of long, single beeps indicating memory problems. Silence indicates a problem with (in most likely order) the PSU, motherboard, or CPU. Remember, at this time, you do not have a graphics card installed so the load on your PSU will be reduced.

If no beeps:
Running fans and drives and motherboard LED's do not necessarily indicate a good PSU. In the absence of a single short beep, they also do not indicate that the system is booting.

At this point, you can sort of check the PSU. Try to borrow a known good PSU of around 550 - 600 watts. That will power just about any system with a single GPU. If you cannot do that, use a DMM to measure the voltages. Measure between the colored wires and either chassis ground or the black wires. Yellow wires should be 12 volts. Red wires: +5 volts, orange wires: +3.3 volts, blue wire : -12 volts, violet wire: 5 volts always on. Tolerances are +/- 5% except for the -12 volts which is +/- 10%.

The gray wire is really important. It should go from 0 to +5 volts when you turn the PSU on with the case switch. CPU needs this signal to boot.

You can turn on the PSU by completely disconnecting the PSU and using a paperclip or jumper wire to short the green wire to one of the neighboring black wires.
http://www.youtube.com/watch?v=5FWXgQSokF4&feature=yout...

This checks the PSU under no load conditions, so it is not completely reliable. But if it can not pass this, it is dead. Then repeat the checks with the PSU plugged into the computer to put a load on the PSU.

If the system beeps:
If it looks like the PSU is good, install a memory stick. Boot. Beep pattern should change to one long and several short beeps indicating a missing graphics card.

Silence or long single beeps indicate a problem with the memory.

Insert the video card and connect any necessary PCIe power connectors. Boot. At this point, the system should POST successfully (a single short beep). Notice that you do not need keyboard, mouse, monitor, or drives to successfully POST.

Now start connecting the rest of the devices starting with the monitor, then keyboard and mouse, then the rest of the devices, testing after each step. It's possible that you can pass the POST with a defective video card. The POST routines can only check the video interface. It cannot check the internal parts of the video card.
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September 19, 2010 10:32:23 PM

I tried disconnecting and replugging just about everything. GPU, RAM, etc... The PC's been working fine for months now, however, it's not even a year old.

The very noticeable problem is what I've already said, the CPU fan not spinning even though it's plugged, and the GPU fan spinning at max. I also read through did all I could as indicated in the thread linked. Thanks for the help.

The problem is not the CPU as I also tried a 2nd one, which also refuses to spin...
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a b B Homebuilt system
a b à CPUs
September 20, 2010 12:44:53 AM

The cpu fan not spinning would stop the PC from turning on, but you would see it try - very briefly - and then all would go quiet. When the cpu fan doesn't spin, power should be completely removed by the mobo. That's not the way you are describing this.

The "pop" you heard cannot be a good thing. Look at your mobo and see if any of the capacitors broke open. That would tell you the mobo needs to be replaced, and perhaps the PSU if that caused the problem for the mobo.

If you have a second psu, you could try that. No need to physically uninstall the old one, just unplug it completely, lay the new psu on your PCs case, and plug it in. If that works, replace your psu. If not, it points to mobo.
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a b B Homebuilt system
a b à CPUs
September 20, 2010 3:05:41 AM

I'd vouch for the popped caps, or a shot PSU. I just replaced a junk PSU that was causing the exact same symptoms. New 80+ PSU, problems solved.
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a c 122 B Homebuilt system
a c 172 à CPUs
September 20, 2010 3:29:10 AM

"Breadboard with just motherboard, CPU & HSF, case speaker, and PSU. "

What were the results?
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September 20, 2010 5:33:31 AM


Another, more mundane, thing to do is follow your eyes and nose. It sounds like the "pop" was something physically blowing/shorting. Electrical shorts leave a distinct smell.
A psu is in its own case where you cant see a burn mark, but you will smell it. And a mobo might not show burns , with low volts, but again you should smell it.

But smell will at the least tell you whether something has given up the ghost or not.

Worth a try.


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