Pc powers on then straight off after a second.

Hi all. Thanks in advance for your help! my pc specs are at the bottom of this post.

basically my situation is this. I bought a computer off a friend of mine who had it running stable for 6+ months. i reinstalled windows and all was going well for a few weeks.

then, monday morning, while i was playing battlefield 3, the computer just switched off. no blue screen, just off.

went to reboot and... nothing. fans twitched and that was all that happened. cleaned out dust, checked all connections etc, everything was secure.

tried removing all unneccesary cards/peripherals, still same twitch.

did some reading on the net, and worked out that it is probably either the psu or mobo. so i tested the psu's voltages, everything was within acceptable tolerances. checked for shortages, none found.

so what do you guys think the issue could be? id have to say the mobo, but thought i would ask here first, before i shell out for a new one.

specs are this:
MOBO: DFI lanparty x58 t3eh6
CPU: intel core i7 920
RAM: 6x 2gb corsair ddr3 1600mhz ram
GPU: his 1gb radeon hd4870
PSU: coolermaster 650 watt
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  1. My first thought would be the PSU.

    The following is an expansion of my troubleshooting tips in the breadboarding link in the "Cannot boot" thread.

    I have tested the following beep patterns on Gigabyte, eVGA, and ECS motherboards. Other BIOS' may be different, but they all use a single short beep for a successful POST.

    Breadboard - that will help isolate any kind of case problem you might have.

    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 once you are finished.

    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:

    Check for line power at the PSU input. Extension cords, power strips, and power cords do fail.

    If you have power and no beeps, suspect components in likely order are PSU, motherboard, and CPU.

    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.

    A way that might be easier is to use the main power plug. Working from the back of the plug where the wires come out, use a bare paperclip to short between the green wire and one of the neighboring black wires. That will do the same thing with an installed PSU. It is also an easy way to bypass a questionable case power switch.

    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, long single beeps, or series of short beeps indicate a problem with the memory. If you get short beeps verify that the memory is in the appropriate motherboard slots.

    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.
    At this point, if the system doesn't work, it's either the video card or an inadequate PSU. Or rarely - the motherboard's PCIe interface.

    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.
  2. The first thing to go is usually the PSU, especially when the computer is powered by a generic brand PSU.

    I would get a new one of those first. Tell the friend he owes you help selling you a computer that only worked for a couple weeks and get him to let you borrow his PSU.

    If the one he has now is equally a bad PSU, it is still better than nothing to test with and it would potentially tell you whether or not buying a good one would fix your problems.
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