A bad power supply can be the root of many PC problems. Experience can help a tech diagnose problems caused by a faulty power supply that would normally be overlooked by a novice. This article outlines how to diagnose a faulty supply by testing its voltage outputs, as well as how to replace the defective unit.

The symptoms
Just about any intermittent problem can be caused by a faulty power supply. I normally look there first when the symptoms are a little weird. Common power-related symptoms include:

  • Any power-on or system startup failures or lockups
  • Spontaneous rebooting or intermittent lockups during normal operation
  • Intermittent parity check or other memory-type errors
  • HDD and fan simultaneously failing to spin (no +12V)
  • Overheating due to fan failure
  • Small brownouts that cause the system to restart
  • Electric shocks that are felt when the case is touched

There are also some obvious clues that should lead you to suspect a faulty power supply. These include:

  • A system that is completely dead (nothing happens when the system is turned on)
  • Smoke
  • Circuit breakers popping when the PC is turned on

Measure the output voltages
One of the simplest tests you can perform on a power supply is to measure the output voltages. This will show whether the supply is operating correctly and if the supplied voltage is within the proper tolerance range. Note that you must check the output voltages while the supply is under a load, which means that it will be installed and running in the PC.

Caution: Power supplies can be dangerous

I do not recommend that any inexperienced person open the cover of a power supply. Even when unplugged, a supply should be considered dangerous. Capacitors can maintain a line voltage charge for a considerable period of time. Shorting one by mistake feels just like being electrocuted by 120 volts from a receptacle. If you are unsure or uncomfortable with working around high voltage,don’t!

Use the right equipment
Most of the experienced technicians I know use good quality DMMs (digital multimeters, see Figure A). These can range in price from just under $100 to well over $300. The price normally reflects the number of special features built into the meter. This type of meter is preferable to the older analog VOM (volt-ohm meter) because the DMM injects only 1.5 volts into a circuit when conducting continuity tests, while a VOM typically injects 9 volts, which is enough to damage a PC’s sensitive electronic components.

Figure A

Back probing
Measuring voltages on a running computer can be tricky. Since you cannot disconnect the power plugs from the drives or motherboard while the power is on, you must use a technique called back probing. This involves inserting the test probes of the meter into the back of the Molex power plug while it is still attached to the drive. There is usually enough room to fit the tip of the probe into the rear of the plug and make contact with the metal pin on the end of each wire it contains. I have one set of leads (shown in Figure A) that are bent almost 90 degrees to allow me to insert them and avoid the bundle of other cables and other components that are normally in the way.

A quick note about polarity: All the voltages you’ll be looking for are DC. Look at any of the power plugs in a PC and you will see that the wires are color-coded (see Figure B).

Figure B

Meter leads are also color-coded, with red being the positive (+) and black the negative (-) lead. To test the output voltage at the motherboard, place the black meter lead on a black pin and the red on the Power_Good pin (P8-1) of AT, Baby AT, and LPX supplies, and pin 3 on ATX 20-pin connectors. It should read between +3 to +6 volts DC. If you do not see this voltage, the supply is bad. If you do see an acceptable voltage, continue testing the rest of the pins on the motherboard and drives.

Any voltage within 10 percent of the specified voltage is acceptable for testing purposes. The tables below show the pin-outs for AT and ATX supplies (Figures C and D) and the Molex 4-pin drive connector (Figure E).

Figure C

Figure D

Figure E

Always have a spare
Some problems cannot be found through direct measurement, so having a spare supply available for substitution is essential. If the problems disappear with the installation of a “known good” unit, you have just confirmed a diagnosis.

Replace the bad power supply
If testing reveals that the power supply is not providing the proper output voltages, then it should be repaired or replaced. Since power supplies do not contain many user serviceable parts, for most people, this means replacement. Before you start, make sure the new supply has the correct form factor and power rating. A replacement supply should provide at least as many watts as the old one. I normally go up one size when replacing power supplies.

Replacing a power supply is pretty straightforward. Unplug all the cables from the back of the unit. Open the case and unplug all the drive power cables and the power cables feeding the motherboard. Check the power lead to the CPU fan. (This is usually a small-gauge pair of wires that can break if you pull too hard.) On some PCs, you must also remove the power switch.

Unscrew the supply from the case after all the power leads are free and remove it from the case. Slide the new supply into the case and connect all the power leads, starting at the motherboard. Don’t fully secure it in place until you power up the system for a function test. If everything seems to be working, shut it down, finish securing the new power supply and the case, and put it back in service.

Stay tuned
For those of you who feel they just have to open up that old power supply, my next article will outline how to change the fuse in the power supply, repairing it for about $2.

Share your power supply tips

Do you have a great tip or strategy for troubleshooting power supplies? If so, share it with you fellow TechRepublic members. Post a comment or write to us.


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