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Power supply


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#1
teddles

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My computer switches on momentarily then switches off. all fans operate.
I suspect the power supply.
How can I check this.
I have knowledge of electricity and electronics

Edited by teddles, 18 March 2011 - 05:48 AM.

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#2
Digerati

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Here is my canned text on testing power supplies.

To properly and conclusively test a power supply unit (PSU), it must be tested under various realistic "loads" then analyzed for excessive ripple and other anomalies. This is done by a qualified technician using an oscilloscope or power analyzer - sophisticated (and expensive) electronic test equipment requiring special training to operate, and a basic knowledge of electronics theory to understand the results. Therefore, conclusively testing a power supply is done in properly equipped electronic repair facilities.

Fortunately, there are other options that are almost as good. I keep a FrozenCPU Ultimate PSU Tester in my tool bag when I am "in the field" and don't have a good spare power supply to swap in. While not a certain test, they are better than nothing. The advantage of this model is that it has an LCD readout of the voltage. With an actual voltage readout, you have a better chance of detecting a "failing" PSU, or one barely within specified ATX Form Factor Standard tolerances. Lesser models use LEDs to indicate the voltage is just within some "range". These are less informative, considerably cheaper, but still useful for detecting PSUs that have already "failed". Newegg has several testers to choose from. All these testers contain a "dummy load" to fool the PSU into thinking it is connected to a motherboard, and therefore allows the PSU to power on, if able, without being attached to a motherboard - great for testing fans, but again, it is not a true load or suitable for conclusive testing.

Note the required voltage tolerance ranges:

Posted Image
NOTE: Disregard the -5VDC reading. It is no longer used.


Swapping in a known good supply is a tried and true method of troubleshooting used for years, even by pros. If you have access to a suitably sized, spare power supply, carefully remove the suspect supply and replace it with the known good one, and see if the problem goes away.

I do not recommend using a multimeter to test power supplies. To do it properly, that is, under a realistic load, the voltages on all the pins must be measured while the PSU is attached to the motherboard and the computer powered on. This requires poking (with some considerable force) two hard and sharp, highly conductive meter probes into the main power connector, deep in the heart of the computer. One tiny slip can destroy the motherboard, and everything plugged into it. It is not worth the risk considering most multimeters, like plug-in testers, do not measure, or reveal any unwanted and potentially disruptive AC components to the DC voltages.

Note the ATX Form Factor standard does not "require" specific color coding for power supply connector wiring. It has recommendations but manufacturers often do not follow them. Sadly, many testing guides or tutorials will refer to wire color only and that can lead to improper testing.

The voltages can be checked in the BIOS Setup Menus of most motherboards but they do not reveal ripple or other anomalies either.

As always, before working in the interior of the computer case, take necessary ESD precautions to ensure static buildup in your body does not discharge through and destroy any sensitive devices. Unplug from the wall and touch bare metal of the case before reaching in. And remember, anything that plugs into the wall can kill. Do not open the power supply's case unless you are a qualified electronics technician. There are NO user-serviceable parts inside a power supply. If you do not have a tester or a suitable spare to swap in, take the PSU to a qualified technician for testing.


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#3
teddles

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Thank you for your info.
I will take heed of your advice.
I did think it was the switch but ruiled that out.
Teddles
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#4
Macboatmaster

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I can add absolutely NOTHING to the PSU matter.
It is clear that in that regards the expert Digerati has spoken.

On a more general basis, having eliminated the power button switch, I would strip, taking usual anti-static precautions, to motherboard, graphics -enabling onboard if the motherboard is so equipped, that will eliminate a fault on the graphics, keyboard and monitor of course. I would disconnect from the motherboard, all drives, sound card if separate, etc and ram.

See if you can enter setup.
If so, go from there, renabling one at a time.

Edited by Macboatmaster, 19 March 2011 - 08:28 PM.

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#5
teddles

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Thanks will try that.
teddles
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#6
sujit-agarwal

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According to me, the main problem is the SMPS (Switch Mode Power Supply) box inside your CPU. Check whether the cables in-and-out the box are connected and plugged in properly or not.
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#7
teddles

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Thanks will also check that
Teddles
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#8
123Runner

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According to me, the main problem is the SMPS (Switch Mode Power Supply) box inside your CPU. Check whether the cables in-and-out the box are connected and plugged in properly or not.

There is no power supply inside the CPU. I suspect you mean the computer case. With that said, continue with Digerati tests.

There could also be a heat problem created by dust, buildup and/or a bad fan on the CPU.
When was the last time the fans were cleaned?
Make sure the CPU fan is operating.
Clean all the fans.
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#9
teddles

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fans are working just tere appeears to b e no power from the power supply could be switch or what ever the switch controls
Thanks
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#10
Macboatmaster

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At thi stage you really need to either try another PSU, easier said than doen if you do not have a spare or try the tests I recommended in post 4.
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#11
Geek09

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Check all the cables are plugged in correctly. If it still does it. Check the motherboard. I had a similar problem before and it turned out it was the motherboard.
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#12
teddles

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Thanks I checked they were all connected and still the same problem.
Teddles
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#13
Macboatmaster

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I thought you had already done that as a result of an earlier post. - your reply was post 7.

With respect you really need to follow the advice already given to you, - not just by me - but in my case in post 4.
To which you have never replied as to if you have proceeded with this.
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#14
teddles

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I have been trying to follow the suggestions.
But I have been busy with other matters.
I have fit the problems with computer whereever I can.
I have replied to most of suggestions.
If I have missed any I apologise.
Thank you for your input
Teddles
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#15
Macboatmaster

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No apology necessary I did not mean you had not replied.
I was suggesting that you proceed to

On a more general basis, having eliminated the power button switch, I would strip, taking usual anti-static precautions, to motherboard, graphics -enabling onboard if the motherboard is so equipped, that will eliminate a fault on the graphics, keyboard and monitor of course. I would disconnect from the motherboard, all drives, sound card if separate, etc and ram.


Now we know that you have the onboard graphics, you need to :
1. Try another PSU if possible
2. Strip down so that you just have the motherboard, all headers disconnected except front panel header for power switch etc.
Disconnect CD drives, Hard drives and just leave connected on the rear I/O panel monitor and keyboard.
See if the computer will then post. - that is switch on and stay on and go to the first screen.
If not it is either the motherboard or the PSU.
When I say motherboard it could be a faulty capacitor that as soon as it receives power goes open circuit, this can normally be seen by a swelling on the top of the capacitor, although not always.

Really your first line of approach is another PSU or take that one out and take it to a computer repair shop for testing.
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