Lab 6.3: Removing and Replacing a Power Supply

Power supplies are not usually considered to be repairable, but frequently they need to be replaced. Sometimes, defective power supplies cause intermittent troubles that are difficult to pinpoint.

Power supplies come in many styles and in many power ratings. The power supply you use as a replacement must be the same style as the power supply being replaced, or it will not physically fit the computer case. The power supply you use as a replacement must also have a wattage rating that’s the same as or higher than that of the power supply being replaced.

Upon completion of this lab, you will be able to:

• Remove a power supply from a computer

• Install a power supply in a computer

Set Up

For this exercise, you will need an operational PC.

Exercise

In this lab, you will remove the power supply and replace a power supply.

Removing a Computer Power Supply

To remove a computer power supply,

1. Record the CMOS hard-drive information:

   Heads:

   Cylinders:

   Sectors:

2. Sketch the back of the computer and show where each cable is attached. Then, disconnect all the external cables from the computer.

   Warning

   AT computers have 120 volts applied to the panel-mounted power switch any time the power cable is attached to the computer. The voltage is a safety hazard to the technician. If you come in contact with the 120 volts, serious injury or death could occur.

   Warning

   ATX computers have 5 volts on the motherboard any time the computer is attached to the power cable. The voltage is a safety hazard to the motherboard when the case is open. If a screw or some other object drops into the computer and short-circuits the motherboard, the motherboard will be destroyed.

3. Remove the computer cover to expose the power supply. As soon as the cover is removed, ESD (electrostatic discharge) precautions must be used.

4. Disconnect the power connector from the motherboard.

   AT computers have two power connectors (P8 and P9) on the motherboard.

   ATX computers have one power connector on the motherboard.

5. Disconnect the other power-supply cable leads from the drives, fans, and so on. Each piece of hardware that you disconnect must be reconnected to the replacement power supply. You should make a list of the hardware items as you disconnect them so you can use the list as a guide when you reassemble the computer.

6. The power switch is located on the front panel of most computers. Generally, you should remove the switch from the case and leave it connected to the power supply.

7. Remove the power-supply mounting screws and then lift the power supply out of the case.

   Warning

   Do not plug the power supply into a wall outlet unless it is connected to a motherboard! Doing so could produce high voltage and cause the power supply to self-destruct.

Installing a Power Supply

To install a power supply,

1. Wear an antistatic wrist strap to protect the computer from ESD, and make sure no external cables are connected to the computer or the power supply.

2. Place the power supply in the computer case’s power-supply support bracket, and then install the mounting screws.

3. For an ATX computer, the power switch connects to the motherboard.

   For an AT computer, route the power-switch cable in a safe location, and attach the power switch to the case.

4. Attach the power supply to the motherboard.

5. If you have an AT computer, attach connectors P8 and P9 to the motherboard with the black wires next to each other.

6. On an ATX computer, the motherboard power connector is keyed and will fit only one way.

   Warning

   The P8 and P9 connectors are connected with the black wires next to each other (black to black). If the P8 and P9 connectors are reversed, the motherboard will probably be destroyed.

7. Connect the other power supply output leads to the drives, fans, and so on.

8. Remove your antistatic wrist strap, install the computer case, and connect the external cables to the computer.

9. Boot the computer.