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  1. #1
    WS Lounge VIP mrjimphelps's Avatar
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    How to test an ATX power supply

    I have an eight-year-old computer that just died. It appeared to be the power supply. I removed and opened the power supply, so that I could check the fuse, but there is no fuse in this power supply! I was about to order a new power supply, when I figured that I should test this one before buying a new one. But I have never tested one before, so I looked up information about testing ATX power supplies.

    I found the following website, which gives excellent information on how to test an ATX power supply that you have removed from the computer:
    http://marcomiltenburg.com/2011/09/1...-power-supply/

    The author gave a diagram of the 24-pin plug on the power supply; mine has 20 pins, so I didn't know how to convert his chart to my situation, until I realized that he eliminated the four bottom pins in order to make it 20.

    Here's another website which isn't quite as clear, but it gives additional helpful information:
    http://www.techsupportforum.com/foru...er-151526.html

    I tested the power supply and found that it works perfectly. Therefore, it has to be the motherboard which is bad. If I can find a cheap replacement, I'll probably fix the computer. If not, then I've just acquired some spare parts.

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  3. #2
    WS Lounge VIP mrjimphelps's Avatar
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    I also tested my power-on button, to make sure that it wasn't the culprit. I traced the wire to where it was plugged into the motherboard, and unplugged the plug from the motherboard. I found that there was a place on the plug where I could easily put my multimeter probes. I set the multimeter to the lowest ohms setting and touched the probes to the plug. When I pressed the power-on button, the multimeter needle jumped. So I know that the power-on button is not the culprit.

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    WS Lounge VIP mrjimphelps's Avatar
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    I decided to take the CMOS battery out for a bit, to see if that would clear up any error on the mother board. While I had it out, I checked it with my multimeter - it was dead!

    I bought a new CMOS battery and installed it, and lo and behold, the computer fired right up! So the culprit all along was a dead CMOS battery.

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    Quote Originally Posted by mrjimphelps View Post
    I bought a new CMOS battery and installed it, and lo and behold, the computer fired right up! So the culprit all along was a dead CMOS battery.
    Long ago, in a galaxy far away... Oops, wrong movie.

    I used to do a lot of computer hardware support and this was one of the things that was almost never on anyones radar to check. These batteries were cheap even then but no one ever thought about replacing them from time to time.
    Graham Smith
    DataSmith, Delaware
    "For every expert there is an equal and opposite expert.", Arthur C. Clarke (1917 - 2008)

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    WS Lounge VIP mrjimphelps's Avatar
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    I already had my multimeter out for testing the power supply, so it was very simple to test the CMOS battery.

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    For non-laptop computers the CMOS battery can be relatively easily tested while in the circuit by placing one lead of a digital multimeter on the (usually positive) visible surface of the battery, and the other lead on a nearby motherboard mounting screw. If the voltage is any less than 3 volts then it's past time to replace that battery.

    I've encountered some (badly designed?) motherboards in which the CMOS battery discharges at a high enough rate that the battery needs replacing as often as every two years. A more common occurrence is that a battery will last for five years or even longer.

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    mrjimphelps (2016-08-13)

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