Technical information

Troubleshooting Testing for Power - 7
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.
Testing for +5 V Trickle Power at J9 on the Logic Board