Once I have disconnected LCD my machine will run fine

I have the same problem and while no solution for you, here is some info for anyone else trying to get to the bottom of this problem.

First if you are a newb like me then here's how to get to your LEDs with this model (NB your screws might be different than those specified) https://www.ifixit.com/Guide/iMac+Intel+...

and also here's how to get your LCD off https://www.ifixit.com/Guide/iMac+Intel+... - I suggest taking the ribbon off the LCD rather than the PSU.

Once I have disconnected LCD my machine will run fine in Target Disk Mode (holding down T while turning on) and so you can grab anything important off your hard-drive.

The tech specs as provided by the everyman website suggested above provided no information on the PSU outputs but here is my serial number in case I'm doing it wrong 1784704tze4.

Having only a rudimentary knowledge of electronics my multimeter testing was as follows from the ribbon strip:

my reason for the pins selected for the common is based on this thread PSU providing enough voltage? for someone trying to work out a problem with the same power supply and as you will see they are vastly different.

The power supply is an OE Liteon PA-3241-02A1.

There does not appear to be any popped capacitors.

Below is the voltages I observed on the main power connector when the machine was in standby mode (LED 1 lit) with lcd disconnected, and in brackets with power on and 1-3 LED lit, lcd disconnected:

Pins 3-7 have continuity with earth pin on wall plug

• Pin #1 (closest to top of screen) Common

• Pin #2 0v

• Pin #3 0v

• Pin #4 0v

• Pin #5 0v

• Pin #6 0v

• Pin #7 0v

• Pin #8 +23.6v (+25.8v)

• Pin #9 +23.6v (+25.8v)

• Pin #10 +23.6v (+25.8v)

• Pin #11 +23.6 (+25.8v)

• Pin #12 +23.6 (+25.8v)

next

• Pin #1 (closest to top of screen) -23.6v (-25.9v)

• Pin #2 -23.6v (-25.9v)

• Pin #3 -23.8v (-25.9v)

• Pin #4 -23.8v (-25.9v)

• Pin #5 -23.8v (-25.9v)

• Pin #6 -23.8v (-25.9v)

• Pin #7 -23.8v (-25.9v)

• Pin #8 0v

• Pin #9 0v

• Pin #10 0v

• Pin #11 0v

• Pin #12 Common

next

• Pin #1 (closest to top of screen) 0v

• Pin #2 0v

• Pin #3 0v

• Pin #4 0v

• Pin #5 0v

• Pin #6 Common

• Pin #7 0v

• Pin #8 -23.8v (+25.8v)

• Pin #9 -23.8v (+25.8v)

• Pin #10 -23.8v (+25.8v)

• Pin #11 -23.8v (+25.8v)

• Pin #12 -23.8v (+25.8v)

next

• Pin #1 (closest to top of screen)

• Pin #2 0v

• Pin #3 0v

• Pin #4 0v

• Pin #5 0v

• Pin #6 0v

• Pin #7 Common

• Pin #8 +23.8v (+25.8v)

• Pin #9 +23.8v (+25.8v)

• Pin #10 +23.8v (+25.8v)

• Pin #11 +23.8v (+25.8v)

• Pin #12 +23.8v (+25.8v)

so there is probably a lot of irrelevant info there for someone who knows what they are doing sorry. Am I right in thinking that those are not healthy outputs and that the PSU is faulty?

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You looked at the diagnostic LEDS but didn't tell us what they showed.

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the LEDs show 3 solid green. With the screen disconnected the iMac seems to run fine ie. it appears on the network and I can access the HDD.

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