Computer doesn't start when 12V CPU power plugged in

comp-pls-halp

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Jun 14, 2015
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Configuration (troubleshooting):

Board: Socket AM3+ Gigabyte GA-78LMT-S2PT
CPU: AMD FX4300
PSU: 500W Peak

Other components:

GPU: ASUS Nvidia GTX960
Mem: 12GB (8+4) 1333
HDD: Seagate 1TB SATA

OS and stuff:

Dual boot with Ubuntu linux and Windows 8, but only checked with Ubuntu the couple times it started before dying completely. And I think linux was using the default free drivers.

This machine was working before I moved it to another place.

With only the PSU and board connected (4-pin CPU power connector unplugged), power seems to flow. If I add the GPU, it receives power too and the fans on it work. See: http://i.imgur.com/6IeyKI7.jpg (my phone is ancient, but I think the images are clear enough to convey what's happening). Can't check visual output because CPU not plugged in.

When I plug in the 12V CPU power, the machine doesn't start. The fans momentarily start up for half a second then die. If I add a RAM stick, it's the same deal. And so for any other additional devices. Basically, it's probably due to the CPU power being plugged in.

I've checked the pins on the CPU power connector on the power supply and I get a 12V reading. (See: http://i.imgur.com/z6lXOcT.jpg) So I don't think the connector is busted. Without the connector plugged in, the devices on the board (except the CPU) seem to receive power.

The whole story:

The machine was working before being moved. After it was moved, it started up a couple times and turned off within a few minutes. Later, it seemed to start up with everything but the GPU connected, but I didn't have a VGA cable to check the output. I didn't hear a POST beep though. I think 500W was sufficient for it to work with the GPU plugged in though because it was working well enough before being moved. Then finally the machine died completely and wouldn't start at all.

I think the CPU is most likely the part that is dead. If not that, then the board. The PSU seems fine because it can operate the fans without going through the board and shows the expected readings on the pins.

I'm not really all that experienced when it comes to hardware troubleshooting. I can tinker here and there and put a machine together, but I can't say with certainty what's wrong here. I'm also a bit tight on cash right now, so I need to know exactly which part needs replacement so I may only change that particular one and not buy a whole new machine.

Any help would be appreciated. Thanks.
 
Solution
I'd go for board, and them, if not working, CPU. Or, you can go for both board and CPU and switch to Intel (if budget allows it).
"After it was moved, it started up a couple times and turned off within a few minutes. Later, it seemed to start up with everything but the GPU connected, but I didn't have a VGA cable to check the output."
During transportation, if heavy heatsjnks installed, shocks might damage the board at the CPU socket level.

"When I plug in the 12V CPU power, the machine doesn't start. The fans momentarily start up for half a second then die."
This points to a short at the CPU level. Hard to say if it's only the CPU or motherboard too.
I suggest removing the board from the case, uninstalling all components, including CPU, then reinstalling only CPU and CPU fan, connecting both 24 and CPU power cables and starting the board (still outside the case). If still the CPU fan does not spin, then I'd test firstly a new motherboard.
Check if board and CPU are still under warranty (3 yrs).

 

comp-pls-halp

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Jun 14, 2015
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Board outside case, CPU and CPU fan plugged in, only 24 and CPU connectors plugged in, it dies after starting for half a second as I said before.

Again, if I leave the 4-pin CPU power out, the fan spins. With the 4-pin connector in, it doesn't.

The parts were bought in '12 in another country. Will see if it has international warranty and if it's still under warranty.

If not under warranty, what would make most sense to replace first? Board or CPU?
 

comp-pls-halp

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Jun 14, 2015
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OK, thanks for narrowing it down. I was thinking of making the switch to Intel but wanted to be sure that I'm not getting the source of the problem wrong.
 

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