The Power Mac G3 or Power Mac G4 (AGP Graphics and PCI Graphics) computer does not turn on; no fan movement. To start up, the Power Mac G4 logic board requires a "trickle" power of +5V. If the system fails to start up, follow the procedure outlined below to determine whether the problem is related to the power supply.

This article covers repair procedures for authorized service providers on selected Apple products, but we have published them in the Knowledge Base for the general public for educational purposes. At this time, there are a limited number of repairs that a customer can perform and these are covered under the customer installable parts program http://www.info.apple.com/installparts . If you have not been authorized to perform these procedures, please contact Apple (888-APL-CARE in the USA) or your local authorized service provider to schedule a repair.

Symptom

The computer will not power on.

Products Affected

• Power Macintosh G3 (Blue and White)
• Power Mac G4 (AGP Graphics/PCI Graphics)

Solution

DANGER! This procedure exposes you to high voltages that can result in death or serious injury. DO NOT ATTEMPT THIS PROCEDURE IF YOU HAVE NOT BEEN CERTIFIED AS AN APPLE-AUTHORIZED TECHNICIAN.

Refer to the procedure below and Figure 1 to determine whether the problem is related to the power supply.

Figure 1 Power Supply Connector

1. Plug in a known-good power cord into the back of the computer. Do not turn on the computer.

2. Connect the black lead of the volt meter to pin 16 of the 20-pin power supply connector. Connect the red lead of the volt meter to pin 9 of the power supply connector. The volt meter should measure approximately +5V. If you do not get a reading of +5V, re-check the volt meter connections and measure the voltage again. If voltage is still not present, replace the power supply; otherwise, go to the step 3.

If you do measure +5V on pin 9, the power supply is likely OK. Go to the next step for further verification.

3. Start up the computer by pressing the power button on the front of the computer. Note : Verify that the power cable is plugged into connector J30. If the computer starts up normally, the power supply is OK. If the computer does not start up, go to the next step.

4. Check to see if the power supply fan is spinning. If it is not, go to the next step.

5. Connect the black lead of the volt meter to pin 16 of the power supply connector. Connect the red lead of the volt meter to pin 1 of the power supply connector. The volt meter should measure approximately +3.3V. If you do not get a reading of +3.3V, re-check the volt meter connections and measure the voltage again. If voltage is still not present, replace the power supply; otherwise, go to the next step.

6. Connect the black lead of the volt meter to pin 16 of the power supply connector and connect the red lead of the volt meter to pin 4 of the power supply connector. The volt meter should measure approximately +5V. If you do not get a reading of +5V, re-check the volt meter connections and measure the voltage again. If voltage is still not present, replace the power supply; otherwise, go to the next step.

7. Measure the voltage of pin 10 on the power supply connector. The voltage should measure approximately +12V.
If you do not get a reading of +12V, re-check the volt meter connections and measure the voltage again. If voltage is still not present, replace the power supply; otherwise, go to the next step.

8. The testing is complete. You have just verified that the power supply is not the cause of the "No power" symptom.