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Computer will not start

Posted on 2006-10-29
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Last Modified: 2010-04-26
Computer will not start.  Put in new power supply, cpu fan was working, machine has turned itself off before, thought it was power failure.  Nothing happens, lights, fans, etc does not start.  I have not touched or added any new hardware.  Could the cpu have overheated?  Is the mb burnt out?  Im baffeled.
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Question by:ricklath
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by:jasonr0025
ID: 17830340
when you press the power on button do any fans in the case run or try to run.  does the led on the motherboard light up.  If nothing at all happens on your pc, no fans no lights no cdrom blinks, it is a power problem.  double check the plug into the powersupply and the receptical.  Next make sure the 110/230 switch is showing 110.  Next recheck that the main connection to the mb is fully seated, as well as other connections.  If you have a 12 volt fan around with a molex plug try plugging it into the p/s and see if it turns when the pc is turned on.  Also, when you try to cut the pc on see if the fan inside the p/s turns?
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by:ricklath
ID: 17830380
power supply is a new one.  it is set at 110, power to p/s is good.  nothing happens on the mb, no led showing.  all connections are well seated.  
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by:jasonr0025
ID: 17830402
Lets start at the wall, especially since you have had problems in the past. do you have anything else plugged into the same wall recepticle that is working?  Can you measure the circuit with a meter to verify ~120vac/
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Expert Comment

by:waltzingguy
ID: 17830408
There are numerous reasons it might not turn on.  If you have a multi-meter the way to check if its the power supply is to disconnect all the power connections, motherboard, hard drives, cd drives, etc, and check the power, touch the negative (black to black) and the positive to the other voltages, you'll see a large variety of voltages, 3.3, +5, -5, +12, -12, etc.  Does the fan on the power supply run when its not connect to anything?

If so, you may have a short somewhere, start plugging in devices 1 by 1 and turning on the machine, try the motherboard LAST.

Good luck.
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Author Comment

by:ricklath
ID: 17830414
i relocated the computer to a diffrent outlet.  power is good.
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by:ricklath
ID: 17830418
thanks i will try that
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by:jasonr0025
ID: 17830452
some powersupplys will not run if there is no load present on the 5v and/or the 12 volt rail.  Also if you disconnect all the connections, especially the main m/b connection it will not turn on as it requires a closed loop between the ground (black) and pin 14 on most atx power suplies.  So of course if you unhook everything you will get nothing.  If you want to test it I can walk you through it as it has a few steps involved to ensure your saftey and the power suppies.  
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by:jasonr0025
ID: 17830456
The ground and pin 14 (green) is on the main 20 or 24 pin m/b connector.
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by:jasonr0025
ID: 17830461
seriously though--you will get nothing if you unhook everything from the pc.  The powersupply has to at the least be connected to the motherboard for it to even turn on!
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by:waltzingguy
ID: 17830479
Ah, I totally forgot about that, for some reason I was thinking the power switch would still work...  Though you can still see voltage on the supply to run the atx power button, 3.3 or 5 I think.

One thing you can do is once youve disconnected everything but the motherboard, start taking as much off the motherboard as possible, PCI cards, memory even(you'll get a POST error, but it will turn on), disconnect IDE cables.
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by:ricklath
ID: 17830625
if i want to check the p/s,  could i put a jumper on the p/s connector to see if it is working properly?
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by:jasonr0025
ID: 17830960
yes, BUT BE CAREFULL and follow this exactly.
Find a 6-10" piece of insulated copper wire-preferably solid.  Strip back the ends about 1 inch and set the side. --- Unplug the powersupply__ and unhook it from all periphials and mb-video -EVERYTHING>  On the m/b connector locate pin 14.  It should be green and the 4th one from the end and will be across from a red 5volt (red) line and will have a black-ground-on both sides.  (all this is with a 20 pin connector-the 24 will have 2 extral 12 volts-2 yellow and 2 black on the end.)  you will need to insert copper jumper between the green(start) and one of the blacks on either side of it.  In certian powersupplies you will need some sort of resistance on the 5v and/or the 12 volt line.  A easy fix is to place an old working hdd on one of the power molex plugs -or- a 10ohm 1 watt resistor would also work plugged between  a red and black on the 5 volt rail. Not all p/s require the resistance on the 5 or 12 but they do all require connection to ground to start.  So try the jumper alone first and plug it in the 120v rec.  if no go unplug and add the hdd or resistor.  Try plugging in again. If still no go you have a dead p/s. If it runs check the 12vdc rail (yellow and black) 5v is red and black for appropriate voltages.  These two voltage measurements will ensure mainly that the powersupply can be turned on by m/b and it is producing some voltages.  If you get this far and the powersupply works I would say that it is good.  I would then look to the motherboard (while it has no power) and place an continuity tester between pins 13 and 14 (I believe) and then push the power button on the front of the case. If the resistance drops when the button is pressed than it is working.  If there is no change in resistance then post back and I will give you next steps--my fingers are getting tired--lol

Also there are powersupply testers available to do this for you here is a link http://www.newegg.com/Product/Product.asp?Item=N82E16899129001
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by:ricklath
ID: 17831022
Thank you.  Before I do this I better check with you one more time.  The wiring diagram shows pin 14 as blue, -12VDC.  Pin 16 colour is green, PS-ON#.  Would it be pin 16 that I would use?
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jasonr0025 earned 250 total points
ID: 17831067
Yes you are correct.  I haven't had to tear apart many newer 24 pin p/s (if that is what this is) but yes it is ps-on.  And please remember unplug all hardware and unplug it from wall untill you get the jumper in.
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by:jasonr0025
ID: 17837869
Thanks, I hope it turned out ok.
Just for record, was it the powersupply?
Thanks again and good luck:}
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by:ricklath
ID: 17841337
The power supply was ok, the motherboard was dead, thanks for the help
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