1 iMac 27" 2012 does not start, LED#1 on
my iMac went off by itself and now it does not start again. When I activate the power switch the HDD starts spinning up and the USB ports are powered, but there is no chime and the iMac does not boot. Only diagnostic LED #1 is on permanently.
I certainly tried the typical things, change RAM, resetting SMC and checking all connections, but without any success.
I tested the output of the power supply with a DVM. It constantly shows 12V (as long AC is connected), even if the HDD stopped. Also LED#1 is constantly on.
From the fact that LED#2 does not come on I assumed the power supply needs replacing, but the 12V is present at the main power output from the PSU to the logic board, so it seems to work.
I am not sure what else I can test to determine if this is caused by the PSU or the Logic Board.
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Unfortunately replacing the power supply did not solve the problem which was a bit disappointing. I had to replace the Logic Board to get the iMac starting again and the original Power Supply turned out to be fine.
The power supply in the iMac only provides a single voltage (12V 25A), all other voltages are produced on the Logic Board.
par Kurt Hardie