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Computer won't power on - bad old PSU and bad new PSU??

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July 31, 2010 4:44:32 AM

Earlier this week my desktop PC I build about 4 years ago would not turn on. While plugged in I heard a high-pitched squeal coming from the PSU, even when all cables were removed from the mobo and components. I figured the PSU was bad, so I just replaced it.

Old PSU was an Antec Smart Power 500W unit that was included the the case I used in my build 4 years ago. I just got a new Antec EarthWatts 430W PSU as a replacement.

I hooked up everything to the new PSU - still no joy. I see the green LED on the mobo, but no power on.

So I went about testing both PSU's using a multimeter. Started with the new PSU. Jumped pin 15 and 16 on the 24-pin connector, but nothing happened on the PSU itself. No fan running or nothing. With 15 and 16 jumped, I checked voltage to the #24 ground pin across all the + DC voltage pins. Everything checked out.

Next I tried the old PSU. Same procedure. But on the old PSU, I only got voltage on the #9 pin, nothing else.

With the new PSU connected to everything, I checked the power switch on the case. I tried shorting the pins on the mobo itself where the power button wires connect. Still nothing.

I have 2x 1GB sticks of RAM. I tried switching those out, trying to power on with only one stick installed at a time in the Channel A DIMM 0 slot. Still nothing.

Anyone have any ideas? Mobo taken a dump maybe? Is it possible that my brand new PSU is DOA? Shouldn't its fan at least have come on when jumping #15 and #16 pins on the harness?
July 31, 2010 1:27:47 PM

Tried that, still won't power on. I checked the power switch wire and it does have continuity when the switch is pressed. Yet still nothing.
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August 1, 2010 6:30:51 PM

After a quick internet search I found that high pitch squeals are indicative of fan problems. Not turning on sounds like a power issue though, so maybe it contributes?

I'm also curious why you switched from a 500W PSU to a 430W. What are your computer specs?

Also, do you have access to another computer? You could always plug in the new psu to a different computer (preferably one without a video card) and see if it works.

I'd also look up other ways to test your new psu with the multimeter. If you're getting similar results then the new one might just be DOA. If you suspect this anyway and it's not too late, send it back and get a replacement.

Whatever the cause, the solution comes at a timely process, so I hope you're patient :) 
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August 2, 2010 12:11:35 AM

If you have anything more powerful than an ATI HD 5770 or NVIDIA GTS 250, you might run into problems with a smaller than 500W PSU.

Could you perhaps post your PC's specifications? Specifically the graphics card and CPU (if its something other than the Pentium D940 that's in your signature).
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August 2, 2010 3:32:54 AM

Yes this is the system in my sig. Video card is Nvidia Quadro FX540.

I figured the 430W should be adequate because my UPS reported power it was providing to the machine (including monitor) never went above 250W.

I tested both PSU's with a multimeter. Old one showed no power at all except for one wire. The new PSU showed the required voltage on all positive DC current wires. So I'm certain the new PSU is good.

Unfortunately our only other computer is a laptop. I have no other spare computer parts in the house.

One possibility I'm thinking of is a power surge, maybe from a recent storm. My APC UPS has had a dead battery for probably a year now. We just haven't had the money to replace it. I've had the machine plugged into it, though, just for convenience really. So I don't know if there was any surge protection being provided.

Today I removed the motherboard from the case, removed all peripherals, and just left the CPU fan connector and the PSU 2x2 plug and 24 pin plug connected. I saw the green light on the motherboard, but when I connected the power switch pins on the motherboard, nothing happened at all. No noises or anything. Surely 430W would be more than adequate for that. This is why I'm thinking the mobo is toast.
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August 2, 2010 12:20:06 PM

I'm not a motherboard expert, but examine it to see if there are any marks showing that it burnt up or something in a surge. It would also smell bad, too. You might have better luck asking the motherboard forum for this particular issue.
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a c 144 ) Power supply
August 2, 2010 4:19:52 PM

Patrick,

From one of my stock replies:
"Try to borrow a known good PSU. If you cannot do that, borrow 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."
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