User Guide

Troubleshooting General/ Testing for Power - 4
Testing for Power
The iMac requires trickle voltage (+5 TRKL) in order to power-
on. If you are experiencing power problems in the iMac unit, you
will want to test for trickle power at various points in the system
to pinpoint the location of the problem and to determine whether
or not the power supply is functioning normally. If you have
trickle power in the system, the odds are your power supply is
OK.
In general, you should test for power at the AC outlet first. Next,
it is best to start at the end (in this case, the logic board) and
trace power backwards through the system. In this way, you will
be able to immediately pinpoint which component is failing.
Note:
It is highly unlikely that the power filter board will fail;
therefore, you need not test for power at this component.
Following is the order in which you should test for power in the
system: At the 1) AC outlet, 2) logic board, 3) power filter
cable, 4) power supply board, and 5) power supply fuse.
Testing for power requires that you remove certain parts from
the computer. Refer to the Take-Apart chapter for instructions on
removing parts.
At the AC Outlet
First, unplug the keyboard from the computer. Then, using a
known-good power cord, power-on the system using the power
button on the front of the computer. If the system powers on, you
can assume the AC outlet is good and the problem is with the
keyboard. If the system still won’t power on, try connecting to a
different AC outlet. If the unit powers on this time, you most
likely have a problem with the AC outlet. If you still have no
power, you will need to test for power at internal points inside the
iMac.
At the Logic Board
With the iMac placed with the CRT face down on a soft surface,
remove the bottom 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.