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Andy Cuffe
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Default imac shuts down after power on

I'm working on an orange slot loading imac that powers on normally with
the turn on chime, but shuts down about 5 seconds later. The power
light stays on (green), but the monitor and hard drive shut down. The
5v and 3.3v supplies to the logic board drop noticeably when the monitor
shuts down, but this may be due to the sudden loss of most of the load
on the power supply (the B+ to the monitor remains constant). Has
anyone seen anything like this before? I suspect a monitor/SMPS
problem, not a logic board problem. It has HV and vertical deflection
before shut down. Apple's service manual is useless.
Andy Cuffe