iMac G4 17" 1.25 GHz DEAD. Help me, please.

@tiobe start off with what Apple suggest for your model in the No Power state:

No Power

No fan, no hard drive noise, and the screen is black.

1. Verify the power outlet is good. Plug a different device into the socket to ensure there is power, or plug

computer into another outlet.

2. Check the power cord. Use a known good power cord.

3. Check connection of the power cord on both ends. Verify that the plug is securely plugged into both the

A/C outlet and back of the computer.

4. Remove keyboard, mouse, and other peripherals such as speakers.

5. Disconnect the power cord, place the computer in the service stand, and remove the user access plate.

6. Reset PMU. Refer to “Resetting the PMU on the Logic Board” mentioned earlier in this chapter.

7. Using a voltmeter, check the voltage on the battery test point (see graphic below). If the reading is over

3.5 volts, go to the next step. If the reading is under 3.5 volts, replace the battery and test again.

8. Plug the unit in, but do NOT press the power button. Using a voltmeter, check the voltage from the test

point marked “Main” (see graphic above). You should get a reading of approximately 12 volts. If the

reading is 12 volts, go to the next step. If you don’t get a 12 volt reading, verify that all the cables are

securely connected and test the unit again. Pay special attention to the video cable.

9. Press the power switch on the computer. Using a voltmeter, check the voltage on the test point marked

“12v” (see graphic above). You should get a reading of approximately 12 volts. If the reading is 12 volts,

go to the next step. If you do not get a 12 volt reading, replace the main logic board.

10. Using a voltmeter, check the voltage on the test point marked “5v” (see graphic above). You should get a

reading of approximately 5 volts. If the reading is 5 volts, go to the next step. If you do not get a 5 volt

reading, replace the main logic board.

11. Remove the bottom housing. Warning: Whenever the bottom housing is opened for service, you must

do two things:

• You must clean the original thermal film from the surfaces joining the thermal interface layer and

reapply thermal paste to the thermal pipe.

• The bottom housing has four torx screws that must be tightened to at least 17 in.-lbs. If you do not

have a torque driver, you will have to make sure these screws are tightened by hand FIRMLY, BUT

NOT FORCIBLY. Or, purchase the service tool (076-0899) in order to ensure the thermal pipe is

firmly mated with the top base. If the bottom housing is not securely attached to the base in this

fashion, the CPU may overheat and become damaged.

12. Check the AC line filter. To measure power at the AC line filter cable, disconnect the AC line filter cable

from the power supply cable. Switch your multimeter to read AC voltage. Touch the black probe to the

pin of the brown wire, and touch the red probe to the pin of the blue wire (see graphic below). Do you

measure 120v (line voltage)? If yes, reconnect the AC line filter and go on to the next step. If no,

replace the AC Line Filter.

13. Connect the diagnostic cables (part 076-0897) between the upper housing components and the logic

board. Refer to “Service Cables” mentioned earlier in this chapter.

14. Check the connection of the video cable on the LCD flat panel, and try to power on the unit again. If

you still have no power, go on to the next step.

15. Check power supply output. Disconnect the diagnostic power cable from the power supply connector,

and measure power at the power supply connector by touching the black probe to the ground pin, and

using the red probe to measure power at the pins indicated in the graphic below. Did you measure +12v

at each point? If yes, replace the logic board. If no, replace the power supply.

16. Replace the inverter board.

17. Replace the neck assembly.