At the Logic Board

With the iMac placed with the CRT face down on a soft surface, remove the bottom housing and the EMI cover to access the logic board.

Testing J9 for +5 V Trickle Power on the Logic Board

You will need a volt meter to test for power on the logic board. Refer to the picture on the next page to locate the necessary test point referenced in the following procedure.

Plug in the computer, but leave the iMac powered OFF. Ground the black probe on the volt meter to the logic board screw, as shown in the photograph below. With one hand behind your back, touch the red probe on the volt meter to the top pad (pin 1) at J9 (see the photograph below) on the logic board, to see if you have +5V on the logic board. A +5V reading on the logic board means the power supply is providing the necessary trickle voltage to the logic board.

Logic Board Voltage Inverter Macbook
Testing for +5 V Trickle Power at J9 on the Logic Board

Testing DCO (Down Converter On) Voltage at C10 on the Logic Board

In a No Power situation, first check for +5 V trickle power at J9 (refer to "Testing J9 for +5 V Trickle Power on the Logic Board"), then check the DCO (Down Converter On) voltage on the logic board at capacitor C10 (see figure below).

The DCO voltage should be present as soon as the power cord is plugged into the computer. Ground the black probe on the volt meter to the logic board screw, as shown in the photograph below. Touch the red probe to C10 (see the photograph below.) The DCO voltage should measure approximately -1.2 volts with the power cord plug plugged in. The voltage will jump to 21 volts after powering on the system. A reading of -1.2 volts up to +1 volt indicates the logic board is OK. Without a reading of -1.2 volts, verify the fuse at F901 is OK (refer to "Testing the Power/Analog Board Fuse" in this section). If the fuse is OK replace the power/analog/ video board.

Imac 400 Fuse
Testing DCO Voltages at C10 on the Logic Board

Testing J7 for Voltage on the Logic Board

The following voltages measure if the down converter is providing the required voltages and are only present if the system is powered on. If the following voltages are not correct, refer to the "No Power" troubleshooting symptom chart.

Measure the following voltages at J7 on the logic board:

  1. Pin 2: +12 volts
  2. Pin 4: +5 volts
  3. Pin 14: +3.3 volts
Imac 400 Logic Board
Testing Voltages at J7 on the Logic Board
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Responses

  • pimpernel
    How to test macbook logic board volt meter?
    7 years ago
  • Konsta
    What is apple logic board voltage?
    2 years ago

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