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Troubleshooting Power-Supply Problems

Typical symptoms associated with power-supply failures include the following:

  • No indicator lights are visible, with no disk drive action and no display on the screen. Nothing works, and the system is dead.

  • The On/Off indicator lights are visible, but there is no disk drive action and no display on the monitor screen. The system fan might or might not run.

  • The system produces a continuous beep tone.

Checking Dead Systems

Special consideration must be taken when a system is inoperable. In a totally inoperable system, there are no symptoms to give clues where to begin the isolation process. In addition, it is impossible to use troubleshooting software or other system aids to help isolate the problem.

When the system exhibits no signs of life—including the absence of lights—the best place to start looking for the problem is at the power supply. The operation of this unit affects virtually every part of the system. Also, the absence of any lights working usually indicates that no power is being supplied to the system by the power supply.

  1. Check the external connections of the power supply. This is the first step in checking any electrical equipment that shows no signs of life.

  2. Confirm that the power supply cord is plugged into a functioning outlet.

  3. Verify the position of the On/Off switch.

  4. Examine the power cord for good connection at the rear of the unit.

  5. Check the setting of the 110/220 switch setting on the outside of the power supply. The normal setting for equipment used in the United States is 110.

  6. Check the power at the commercial receptacle using a voltmeter, or by plugging in a lamp (or other 110-volt device) into the outlet.


Before changing any board or connection, always turn the system off first. In an ATX-style system, you should also disconnect the power cable from the power supply. This is necessary because even with the power switch off, some levels of voltages are still applied to the system board in these units.

Other Power-Supply Problems

The absence of the lights and the fan operation indicate that power is not reaching the system and that at least some portion of the power supply is not functional. This type of symptom results from the following two likely possibilities:

  1. A portion of the power supply has failed, or is being overloaded. One or more of the basic voltages supplied by the power supply is missing while the others are still present.

  2. A key component on the system board has failed, preventing it from processing even though the system has power. A defective capacitor across the power input of the system board can completely prevent it from operating.

The DC voltages that can normally be expected in an ATX PC-compatible system are +3.3V, +12V, +5V, –5V, and –12V. The actual values for these readings might vary by 5% in either direction. The black wires in the power supply/system board connection are the ground reference wires. Figure 3.6illustrates the correct measurement of voltages at this connector.

Figure 3.6Figure 3.6 Measuring system board voltages.

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