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PSU probably dead -- need expert opinion

Last response: in Components
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August 13, 2011 6:30:36 PM

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?
a c 272 ) Power supply
August 13, 2011 6:40:02 PM

When you checked your main ATX connector your PSU was in an off state, there was nothing telling it to be on so only the standby rail was getting powered.

Do the paper clip trick as shown in this video and then test your voltages, if the power supply wont start up with the paper clip trick then its dead.
http://www.youtube.com/watch?v=5FWXgQSokF4
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a b ) Power supply
August 13, 2011 6:46:39 PM

i would check your heatsink is on correctly as lying it on its side to make it work could mean the heatsink wasnt properly contacting the cpu--lying it on its side means gravity helps the heatsink down--had this happen once
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August 20, 2011 3:33:38 PM

Thanks for the replies. Apparently I was wrong, because the power supply does start up (I can feel/hear the fan) with the paperclip test. Additionally, the heat sink was a bit loose. After securing it, the machine still doesn't boot.

What should I try next?
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August 20, 2011 9:21:07 PM

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!
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a c 272 ) Power supply
August 20, 2011 9:35:54 PM

Not having the 8 pin EPS connector plugged in doesnt actually tell you anything. It means that the CPU has zero power going to it so nothing else in the system can proceed. When you hit the power button the PSU starts putting out power and the motherboard bootstraps itself, but it cannot proceed with POST without a CPU so it gets stuck in a constant state of power on since it cant make any progress.

Have you been through the troubleshooting guide yet?
http://www.tomshardware.com/forum/261145-31-perform-ste...
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August 21, 2011 2:02:44 AM

Thank you, yes, I went through every step of that troubleshooting guide. I've tried two things since my last post:

  • I bought a brand new PSU, hooked it all up, and got the exact same result. (I'll be returning it tomorrow)
  • I took everything out of the case, hooked up the CPU with new thermal paste, heat sink, PSU, one stick of RAM (in the correct position) and system fans on a glass surface. I also got the exact same result.

    My remaining questions:

  • Is it safe to try my CPU in a friend's machine without damaging any of his hardware?
  • I have an i7. A different friend seems to think it will shut off automatically if it reaches a thermal limit. He thinks it's highly unlikely that the i7 would be damaged from the heat sink being loose. Is that correct?
  • Could the motherboard be having problems if the fans work, LEDs light up and power is getting to all the other components? (this situation occurs when just the EPS is unplugged)

    Again, thanks so much!
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    a b ) Power supply
    August 21, 2011 2:07:06 AM

    Reading this, I feel that something small but important is being left out. Just a gut feeling.
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