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   >  Troubleshooting Intermittent Problems
   >  Diagnosing Hardware Failures


Diagnosing Problems with a PC Power Supply

Problems with PSUs

The job of the PSU is to supply a reliable and accurate voltage to all the various components. Problems with PC Power Supplies generally fall into two categories:

  • Catastrophic failure: this is fairly simple to diagnose -as the PC will usually not even power up
  • Random failures: intermittent problems that occur every so often

Catastrophic PSU Failure

This can often be caused by an electrical surge, which blows the internal fuse in the PSU. This is why it is important to protect your PC with a surge protector or - better still - an Uninterruptable Power Supply. Unfortunately, the fuse (-like any internal PSU component) is not replaceable and the PSU has to be returned to the manufacturer (-if under warranty) or a new one purchased and installed if this is the case.

The only course of action for a failed PSU is to exchange it for a new one

The only course of action for a failed PSU is to exchange it for a new one.

Diagnosing a Problem with a PSU

It is not just a case of assuming that the PSU is faulty if power does not flow to the PC: the problem could lie with the case switch or the motherboard. To confirm that the PSU is the root cause you can do any of the following:

  • If you have a spare PSU lying around, consider swapping this for the suspect one. A cheap PSU (-probably best confined to testing duties) can be had for around £10, at time of writing

  • You can buy a specialist PSU testing tool, such as the one below:

    A device to test PSU output

    Normally they are quite cheap (-again, around £10) and are the easiest way of testing out a PSU. Simply unplug the connectors from your motherboard and plug them into the device: the device will then tell you which voltages (-if any) are being output by the PSU. Type "pc psu tester" into any Search Engine to see what is available and the current prices

  • If you have one lying around, then you can use a multimeter to test the output of the PSU connectors whilst still attached to the motherboard by doing the following for each wire on the motherboard's main (24 pin or 20+4 pin) connector(s):

    1. Before starting work, deploy anti-static controls and ensure the power is off

    2. Remove the access panel(s) from the computer case: these are located on either side of the case and will either be secured with screws or thumbwheels

    3. Set your multimeter to a setting of 12V or more (-if it is not an auto-ranging one)

    4. Attach the ground wire of the multimeter to the bare metal chassis (e.g. with a banana/croc clip)

    5. Push the probe of the red wire into the power terminal that you want to test

    6. Once secure, plug in the computer and switch it on

    7. Check the reading on the multimeter matches the following, based on the wire colour:

      • 0V: Black
      • -12V: Blue
      • -5V: [Optional] - White
      • +3.3V: Orange or Brown
      • +5V: Red or Purple
      • +12V: Yellow
    8. Switch off the computer before moving the probe to the next wire and repeating the above procedure

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