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Power Supply


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6 replies to this topic

#1 mbbtbg

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Posted 09 June 2009 - 10:01 PM

Windows XP computer (older model) will not power up. No lights - nothing. I am thinking power supply went out. How can I tell what kind of power supply in pc? I am going to try and replace myself. Pc was working fine. We did have a thunderstorm roll through then pc went dead. Open to suggestions as to what else to check.

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#2 fairjoeblue

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Posted 09 June 2009 - 10:23 PM

What brand & model is the computer ?

Was the unit on & in use when it quit ?
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#3 westom

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Posted 10 June 2009 - 12:02 PM

Windows XP computer (older model) will not power up. No lights - nothing. I am thinking power supply went out.

That failure can be almost everything in the computer. Shotgunning is a worst way to approach this problem - unless you like to spend money replacing perfectly good parts.

For example, is it the power supply or the supply controller? Simpler is to measure voltages with a 3.5 digit multimeter - to know in 30 seconds what is wrong. In your case, the first measurements are on purple, green, and gray wires where power supply connects to motherboard. Push the probe into that connector to touch each wire.

Purple wire voltage must exceed 4.87 VDC before power switch is pressed and power cord is connected. Green wire is above 2.0 volts before the switch is presses and drops below 0.7 when switch is pressed. Gray wire is near zero and rises above 2.4 when switch is pressed. Simply post those numbers here to learn which of many power components is defective and to learn what actually happens inside a computer.

If the power supply must be replaced, only way to know if the new supply is OK is, again, the meter. A defective power supply can boot a computer. Just another reason why that less than $18 tool is used by the informed.

#4 fairjoeblue

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Posted 10 June 2009 - 12:56 PM

If the unit is getting no power, nada, nothing, as in no fans or lights work it will be a tad hard to test a dead power supply, simplu replace it.

Testing with a multimeter will only tell you if the power supply comes on & give ballpark readings at at a idle.
A PSU can test "good" at a idle & still be bad.
A PSU is under it's heaviest load when the computer is booting up.
The only way to test a PSU & determine if it's actually good is to test it under a load.
That can't be done with a simple multimeter.
It requires an actual PSU tester.
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#5 ice2921

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Posted 10 June 2009 - 01:04 PM

If you dont have a multimeter you can always use a power supply tester as well.

#6 joseibarra

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Posted 10 June 2009 - 01:24 PM

You can make sure that whatever wall/power strip/UPS outlet is still working by plugging in something else to make sure power is even getting as far as you system. Could be a bad power strip. Could just be the cord even. Probably not though, but you don't want to be embarrassed later for not checking simple things.

PS are pretty easy to replace with just a #2 Phillips screwdriver and around here run about $35 with a 1 year replacement warranty. You can open your case and write down the PS specs from the hopefully visible sticker - wattage is what you want mostly, but write down everything or take it with you shopping. Never get less. Equal or more is better, but you don't have to go crazy unless you have extreme power hungry devices inside the box. PS is important, so don't buy the cheapest.

If you have never done such a thing before and take it out before you have the new one in hand, you need to either have a decent short term memory, take some notes, make a drawing or take a few pictures. Don't count on the new one having a manual, but you could prepare by downloading a manual for your motherboard and making a few notes in there before unhooking everything. It should have a picture of where everything goes. Some connections coming out of the PS may not be used.

If you don't at least have a power strip/surge protector for your valuables, you should get a good one for your computer, monitor, printer, etc. If you have more money, you can get a UPS which will allow you to perform a graceful shutdown if the power goes out. These things are to help protect form power surges/spikes - not a direct lightning hit. The UPS may help you survive a brown out or a temporary interruption.

It will probably take you longer to get your hands on a replacement power supply than it will take you to replace it :thumbsup:

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#7 westom

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Posted 10 June 2009 - 07:33 PM

Testing with a multimeter will only tell you if the power supply comes on & give ballpark readings at at a idle.
A PSU can test "good" at a idle & still be bad.
A PSU is under it's heaviest load when the computer is booting up.
The only way to test a PSU & determine if it's actually good is to test it under a load.
That can't be done with a simple multimeter.
It requires an actual PSU tester.


Multimeter reports massive information OR makes it possible for the informed poster to identify problems unique to that system. No numbers from the meter are why so many only post "try this and try that".

PSU tester is a complete waste of money. One reason: a defective PSU can easily test good with a tester. Tester can only identify some types of failure AND can never report a supply as good. Tester either reports a defect or an indefinite answer. Only useful answer is definitive - which means the meter.

PSU is not under heaviest load when booting. That is obvious when one learns how a computer boots. Heaviest load occurs during multitasking to all peripherals. That is when a multimeter provides the most definitive answer (and again why PSU testers are money wasted). Most useful information always requires numbers such as three digits from that multimeter. Those numbers are the only way to get assistance from the best technically informed posters.




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