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Andy Cuffe
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Default imac shuts down after power on

rubik wrote:

On Thu, 13 Nov 2003 06:04:58 GMT, Andy Cuffe

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.

try this, open the back and unhook the video connect and connect it to

an external monitor if it powers and boots normall then its a problm
with the flyback transformer which would have to be unsoldered and
replaced, this was a common failing with a bunch of imacs.


I spent some more timeon it today and I seem to be making some
progress. For what ever reason, the monitor doesn't shut down now, but
there are still problems. As soon as the monitor warms up it displays a
raster with the pin balance badly misadjusted. The picture sides look
like this )). The picture also has poor white balance and smears a lot
unless the G2 is increased until the picture is much too bright. I
connected the display adjustment memory jumper, booted from a CD and ran
the display adjustment utility. As soon as the utility started (before
I tried to adjust anything) the picture returned to normal. Gray scale
and geometry were all reset to being perfect, so it couldn't have just
restored the factory defaults. The good settings hold until the
computer is restarted, then is returns to the way it was before (yes, I
did save the settings in the adjustment utility). Obviously there's a
problem with the monitor control circuit, or EEPROM. It seems that
some, or all the factory settings get read incorrectly from the EEPROM
on turn on, but they are still accessable to the display software. I'm
going to investigate the CRT board which seems to contain all the
control electronics. It can't be a software problem becuase it happens
before it's even started to boot.
Andy Cuffe