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

The symptoms
Just virtually any intermittent problem tin can exist caused past a faulty power supply. I usually look there first when the symptoms are a footling weird. Mutual power-related symptoms include:

  • Any power-on or system startup failures or lockups
  • Spontaneous rebooting or intermittent lockups during normal functioning
  • Intermittent parity check or other retention-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 ability supply. These include:

  • A organisation that is completely dead (goose egg happens when the organization 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 mensurate the output voltages. This volition show whether the supply is operating correctly and if the supplied voltage is within the proper tolerance range. Note that yous must bank check the output voltages while the supply is nether a load, which means that it will exist installed and running in the PC.

Caution: Power supplies can be dangerous

I do not recommend that any inexperienced person open up the cover of a ability supply. Even when unplugged, a supply should be considered dangerous. Capacitors tin maintain a line voltage charge for a considerable period of time. Shorting 1 by mistake feels just like being electrocuted by 120 volts from a receptacle. If you are unsure or uncomfortable with working around loftier voltage, don't !

Apply the right equipment
Most of the experienced technicians I know use practiced quality DMMs (digital multimeters, see Figure A). These can range in price from simply 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) considering the DMM injects just 1.5 volts into a excursion when conducting continuity tests, while a VOM typically injects nine 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 yous cannot disconnect the power plugs from the drives or motherboard while the power is on, y'all must use a technique called dorsum probing. This involves inserting the examination 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 terminate of each wire it contains. I have one ready of leads (shown in Figure A) that are bent almost xc degrees to let me to insert them and avoid the bundle of other cables and other components that are normally in the manner.

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

Effigy 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 blackness meter lead on a blackness pivot and the ruddy on the Power_Good pin (P8-1) of AT, Baby AT, and LPX supplies, and pivot 3 on ATX 20-pin connectors. It should read between +3 to +six volts DC. If you exercise not see this voltage, the supply is bad. If you do see an acceptable voltage, continue testing the balance of the pins on the motherboard and drives.

Any voltage within x percent of the specified voltage is adequate for testing purposes. The tables below evidence 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
Effigy E

Always have a spare
Some problems cannot be found through straight measurement, so having a spare supply bachelor for exchange is essential. If the problems disappear with the installation of a "known good" unit, you lot 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 ability supplies do not contain many user serviceable parts, for virtually people, this ways replacement. Before you beginning, make sure the new supply has the right form factor and ability rating. A replacement supply should provide at least as many watts as the old one. I normally become upwardly i size when replacing power supplies.

Replacing a ability supply is pretty straightforward. Unplug all the cables from the dorsum of the unit. Open the case and unplug all the bulldoze power cables and the power cables feeding the motherboard. Check the power atomic number 82 to the CPU fan. (This is ordinarily a modest-gauge pair of wires that can break if yous pull besides hard.) On some PCs, you must also remove the power switch.

Unscrew the supply from the case subsequently all the power leads are free and remove it from the case. Slide the new supply into the example 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, cease securing the new power supply and the case, and put it dorsum in service.

Stay tuned
For those of yous who feel they but take to open up upwards that onetime power supply, my next article will outline how to alter the fuse in the power supply, repairing it for about $2.

Share your ability supply tips

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