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No power on pc

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July 29, 2010 12:10:33 AM

Ok my girlfriend had a Dell Desktop PC they had an electrical storm and when she came home the computer wouldn't power up. I tested the switch and power supply and both were fine. I concluded it was the motherboard so we got a new one at MicroCenter. It didn't fit the case so I got a new case and transferred all the components to the new case along with the new motherboard and it still doesn't power up. Tested the power supply again. It will spin a fan and it gets power to all the pins so the power supply is fine.
I've never had one that refuses to show any signs of life to the motherboard after making sure I have a good power supply. I am wondering is it possible that the CPU is fried? Would a bad CPU not let the system power up? I get no fans no lights. Like it's not getting any signal to power up or any power? I was thinking if the CPU was bad you would get life at the motherboard or some fans or something??? :pt1cable: 

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a c 144 ) Power supply
a c 156 V Motherboard
July 31, 2010 6:40:03 PM

CPU failures are actually pretty rare. The most likely suspect is the PSU.

Work through our standard checklist and troubleshooting thread:
http://www.tomshardware.com/forum/261145-31-read-postin...
I mean work through, not just read over it.

Breadboard - that will isolate any kind of case problem.
http://www.tomshardware.com/forum/262730-31-breadboardi...

The breadboarding thread has a paragraph about how to build and test a PC in stages.

Breadboard with just motherboard, CPU & HSF, case speaker, and PSU. You do have a case speaker installed, right? If not, you really, really need one. If your case or motherboard didn't come with a system speaker, you can buy one here:
http://www.cwc-group.com/casp.html

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.

At this point, you can sort of check the PSU. Try to borrow a known good PSU. 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.
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.

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 or long single beeps indicate a problem with the memory.

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.

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.
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August 1, 2010 8:50:54 PM

I really do appreciate the post. Made me rethink the PSU. I tested it with the paperclip method and checked for the 3v,5v and 12v's with a DMM prior to my post. I've also assembled a few PC's over the years about 10-20. I know the beeps you should get when there is no memory or other type problems like no input or output connected. That is why this is so puzzling to me. I've never had one pass those checks and still act like a dead PSU. No power up at all. No fans no lights no signs of life at all.

I am however now thinking that maybe the PSU when under load doesn't produce or maybe the gray wire. I've never paid attention to all wires just verified it was getting 3v, 5v and 12v on the various wires.

PSU is the cheapest and best route. I thought of buying a new CPU just because I am stumpped, but an extra PSU is a good thing and relatively cheap.

I will post and let everyone know when I do find a cure for this problem.

Thanks! :bounce: 
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a c 144 ) Power supply
a c 156 V Motherboard
August 2, 2010 3:42:47 PM

You should check the gray wire. You can have all of the supply voltages, but if you are missing the 5 volt control signal, your PC will not boot.
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August 3, 2010 11:25:54 AM

sharpshooter33 said:
Ok my girlfriend had a Dell Desktop PC they had an electrical storm and when she came home the computer wouldn't power up. I tested the switch and power supply and both were fine. I concluded it was the motherboard so we got a new one at MicroCenter. It didn't fit the case so I got a new case and transferred all the components to the new case along with the new motherboard and it still doesn't power up. Tested the power supply again. It will spin a fan and it gets power to all the pins so the power supply is fine.
I've never had one that refuses to show any signs of life to the motherboard after making sure I have a good power supply. I am wondering is it possible that the CPU is fried? Would a bad CPU not let the system power up? I get no fans no lights. Like it's not getting any signal to power up or any power? I was thinking if the CPU was bad you would get life at the motherboard or some fans or something??? :pt1cable: 



Have you checked the the wall socket? is anything else plugged into the same power outlet and is it working?
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August 7, 2010 8:19:05 AM

Checked the gray wire got 5v (actually 4.79v) Unless I got a bad MB from MicroCenter I only have the CPU left from the old system and the DIMM, but the DIMM would atleast give me a beep. I get no power to the board
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August 8, 2010 3:38:25 AM

Solved! Ok picked up a CPU and an extra power supply today from MicroCenter. CPU made no difference. Plugged in the new PSU and bang!!!! Powered right up. That's weird. All voltages checked ok, but it wouldn't power up the board. That's a new one for me. Mark that one up to experience. Not sure what that was all about since the the PSU checked all the basic test of voltage with a paperclip shorting the on. Must be something to do with adding load to the power supply.

Thanks for everyones input I wouldn't have went back and tried the PSU without your input.
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August 18, 2010 6:41:14 PM

Best answer selected by sharpshooter33.
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