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PC problems...testing the PSU
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KennyLeg
Member
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19. November 2009 @ 11:14 |
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Howdy..myy father in-law's Pc crashed the other day and i was given the job of trying to sort it.
basically, it got the BSOD from nowhere..no hardware or software installed. When he tried to re-load windos from a CD it made an attempt to boot from CD then swithched off. Now, it tries to boot for between 5-20 secs then switches off.
i took out RAM, graphics card and same problem. I disconnected the power supply to test it and i'm getting the correct 12V, 5V and 3.3 V from the respective 20 pin connector.
I'm stumped...any ideas?
EDIT: WHile the 20pin connector was connect to the motherboard i was getting 3.3V on the 12v & 5 V rails. It was only when i disconnected the connector from the board that i got the correct voltages ( i shorted out psu_on to Gnd to spin up the PSU).
Ken.
This message has been edited since posting. Last time this message was edited on 19. November 2009 @ 12:30
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ddp
Moderator
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19. November 2009 @ 14:54 |
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switch psu's around to see what happens. look at the silver flat topped cans on his motherboard which are capacitors that they are not domed or leaking brown stuff.
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KennyLeg
Member
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19. November 2009 @ 15:49 |
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What do you mean switch psu's around? I only have one
ps the caps look fine
Ken.
This message has been edited since posting. Last time this message was edited on 19. November 2009 @ 15:51
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ddp
Moderator
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19. November 2009 @ 16:25 |
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try it on your computer if not using a laptop.
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KennyLeg
Member
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19. November 2009 @ 16:29 |
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I'm using a laptop and iPhone
Ken.
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ddp
Moderator
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19. November 2009 @ 16:32 |
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ok, so much for that idea. disconnect all cables to drives & let pc power up to see what happens if shuts self off or not.
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KennyLeg
Member
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19. November 2009 @ 16:39 |
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I did that too. It powered up and shut down again after a fee secs. When I disconectrd the +12v 4-pin connector it stayed powered up.
Ken.
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KennyLeg
Member
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20. November 2009 @ 09:05 |
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Any other ideas?
What i want to know is if i get the correct voltages from the PSU while not connected does that mean that its ok?
Similiarly...if i get wrong voltages while connected to the mobo does this mean the mobo is goosed?
Ken.
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Prof_666
Account closed as per user's own request
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20. November 2009 @ 16:31 |
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Originally posted by KennyLeg:
Any other ideas?
What i want to know is if i get the correct voltages from the PSU while not connected does that mean that its ok?
Similiarly...if i get wrong voltages while connected to the mobo does this mean the mobo is goosed?
The power supply could be breaking down under load,you could try connecting it to a dummy load and then testing,a 12v(car)bulb should do the job.
Prof
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20. November 2009 @ 18:51 |
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The answer to both of your questions is "not necessarily."
Originally posted by KennyLeg:
What i want to know is if i get the correct voltages from the PSU while not connected does that mean that its ok?
The ability of a PSU to maintain its rated voltage under its rated load is called "regulation." A PSU with good regulation will maintain its rated output voltage as its output load varies. Conversely, a PSU with poor regulation will see its output voltage decrease as the external load increases.
What you have is almost no regulation in that the PSU output voltage drops to practically nothing when placed under what would be its normal load.
This could be caused by two conditions: the PSU could no longer have the ability to regulate its output voltage under normal load. It isn't unheard of for a PSU to operate normally for a short period of time before losing its ability to regulate. This is a failure of the PSU regulator and calls for the PSU to be replaced.
Originally posted by KennyLeg:
Similiarly...if i get wrong voltages while connected to the mobo does this mean the mobo is goosed?
A second possibility is the motherboard is suddenly drawing so much current that it overwhelms the PSU's ability to regulate itself. For example, if your PSU was rated for 12 volts at 30 amps and your 12 volt load suddenly increased to 100-150 amps, no self-respecting PSU could be faulted for dropping its output voltage down around its ankles causing the motherboard to shut down.
The problem with the overload theory is that that much of an un-anticipated current load is usually accompanied by heat/smoke and occasionally pyrotechnics. Wires going to the PSU would quickly heat up and the distinctive aroma of over-heated wiring would be evident.
If I had to hazard a guess, it would be that the PSU 12-volt regulator is right on the edge and after a short amount of time jumps into the abyss, taking the output voltage down with it. That would also explain why your voltages look good when disconnected from the motherboard, because your voltage meter doesn't draw enough current to stress the PSU regulator.
I suggest you scrounge up a used PSU to switch out with yours to see if the problem goes away. If it does, then you can buy a decent new PSU as a permanent replacement. Most computer repair shops have lots of old PSUs kicking around and would probably loan you one if you explained what you were trying to do and promised to buy a new PSU from them if your test was successful. Or just buy a new PSU that you can return if the problem proves to be elsewhere.
Right now my money would be on a bad PSU.
Dick
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