Apple IMac - At the Logic Board

Troubleshooting Testing for Power/ - 8
At the Logic Board
With the iMac placed faced down on a soft surface, remove the bot-
tom access cover and then remove the logic board/mass storage
chassis to access the logic board.
Note : 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 (by inserting it into the metal chas-
sis). With one hand behind your back, touch the red probe on the
volt meter to the positive lead of capacitor C28 on the logic board
(see the photograph on the next page) 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.