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PSU probably dead -- need expert opinion

By richardpianka
Aug 13, 2011
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  1. I'm pretty sure my PSU is dead, but I just wanted to check with people who have more expertise before I go buy a new one (I'm a software guy).

    My PSU is an XFX 750B; I've had it for maybe 8 months. Over the past 2-3 months, if my case was standing upright it, it would randomly shut off. I could only get it to post, boot etc. if I laid it on its side (which is exactly what I did). Last night (on its side) it shut off entirely and still will not turn back on.

    So, I've been using a multimeter to diagnose the problem. My power cord in has the appropriate ~115V before it gets to the power supply. Having the power supply switched on and plugged in, I then went to the 24 pin ATX connector. With the black prong on a ground, I checked every other pin. All of them were 0V except for the green PS_ON (3V) and the purple VSB (5V).

    My motherboard has a green and orange light on when it's all hooked up, and occasionally the fans will twitch if I hit the power button (usually not though). My thoughts are that my mobo is probably fine, but my PSU is dead.

    Does this conclusion make sense? Is there anything else I should try before buying new hardware?
  2. You may have been overloading it. What are your system specs? It sounds like you killed it. I think you should grab a spare power supply and "breadboard" that MB and CPU (run the different PSU, motherboard, and processor on a non conductive surface, like plastic).

    Please wait for another opinion before throwing anything out, as I am no expert.
  3. Ritwik7

    Ritwik7 TechSpot Chancellor Posts: 2,288   +7

    Do you get any beeps when you hit the power switch?

    I recommend that you clear the CMOS and boot the PC with only the CPU and a single RAM stick plugged in (If you don't have onboard video you'll need the GPU plugged in too). No HDD. See if the POST is successful.

    If the issue persists it could be anything between PSU, mobo and CPU. I'd start with the PSU. Plug it into a spare system if you have one and see if it's functional. Then proceed based on the result.
  4. Tedster

    Tedster Techspot old timer..... Posts: 10,074   +13

    The first clue should have been the random shut offs- serious damage could have been done by now. Replace the PSU using the correct wattage. Google PSU calculator and add 30% to the results for cushion.

    The only way to test a psu is under load with a PSU tester tool and a multimeter. In most cases it isn't worth the trouble.
  5. richardpianka

    richardpianka Newcomer, in training Topic Starter

    I think I've pinpointed it a bit further. After shorting the green wire with a ground on my 24 pin ATX connector, the power supply fans came on, so I think the PSU is fine.

    After playing around with different configurations of what's plugged in etc. I've found that a single point of failure is my 8 pin EPS power connector (the one right next to the CPU/heat sink on the mobo). If it's not plugged in at all, everything comes alive: LEDs light up, all the fans are on, I can open the optical drive etc. but of course there's no video output and it doesn't boot.

    I checked the 8 pin EPS power connector with a multimeter and each pin has the correct 12V value that it should. When it's unplugged (but everything else is connected), the fans twitch, only two LEDs light up and--like I said in the opening post--there's no post or boot.

    I'm making the assumption that this 8 pin EPS power connector is primarily for the CPU and heat sink. My guess at this point is that the CPU is fried. Is this reasonable? Could this situation arise from any other circumstances?

    And lastly, is it safe to try this CPU in a friend's machine, under the assumption that I cannot damage his hardware?

    Thanks everybody!

    edit: I did clear the CMOS with no effect.
  6. Cinders

    Cinders TechSpot Chancellor Posts: 1,313   +12

    Richard I have placed suspect CPUs in known good motherboards and come away with a still working motherboard and confirmation the the CPU was indeed fubar. I can't fully recommend you do the same because I'm not 100% sure you'll come away clean, but you can give it a try.


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