I know this is an older thread, but I had a similar issue with my 30" cinema display and just figured out the issue after multiple power bricks and about a year of off-and-on troubleshooting.
I researched many times about this issue and it seems it was pretty common. Although these monitors are a bit old nowadays, they are still great monitors. Most of the solutions that I read had to do with the power supply or brightness setting. There were also a couple that mentioned the monitor settings in preferences, and the infamous "taping of the ground connection" in the connection to the power brick itself. None of that helped for any length of time. My monitor was not waking up after being turned off at first. I set my computer to never sleep, which helped, but eventually my monitor would flicker and sometimes just turn on and off repeatedly and endlessly. So today I started testing the buttons on the side....and voila! It was the power and/or brightness buttons on the side of the monitor!!! They were contacting continuously for some reason. For the moment as a temp fix, I managed to get a sharp knife in the side of the casing where the power buttons are and pried it outward to create some space. Then I inserted a thin plastic clip to keep it in that state. The monitor has not malfunctioned since!!! I really hope this will help someone at some point, because I went through a lot trying to figure this one out. Good luck!

Thanks for this Unfortunately, my display doesn't have any magic buttons or switches on the side that might explain its weird behaviour. Anyway, I contacted some LCD suppliers and many of them said it was common with this model for the LCD back light to be a bit dodgy so I looked on Ebay and replacement backlight LED's are pretty cheap. I also found this page which states that the input voltage is 12v DC. Regrettably during my testing, before I found the probable reason for the displays dysfunction, there was a loud bang that originated from the power supply which indicates I must have blown a fuse or one of the capacitors on the power supply its self. As a result, in order to repair this display I have to find a new power supply to power the driver board and subsequently the LED strip as it will most likely not have the same electrical specifications as the original strip. Moreover, I would also need to find the power requirements for the logic board as well, and knowing apple I'm sure they wouldn't publish the specification sheet publicly. And then on top of that, I have no idea if by damaging the powersupply it also broke the logic board aswell. As a result, as much as I would like to repair this display I'm not sure if I have the skills or the patience to do all the troubleshooting etc.

 

I used this as power supply https://www.ebay.com/itm/351848919060 along with this schematic that I created

 

I also have a 23” Apple monitor that’s been sitting on a shelf for a while. It was broken when I got it - I was hoping to repair it. I’ve looked at it many times but never managed to solve the riddle.

It’s completely dead. I’ve opened it up looking for visual clues, but it looks perfect. Opened up the power supply as well, but can’t see anything wrong. Measured the output voltage - and here is the only thing that stands out - it’s 25.1V. It’s of course supposed to be 24.5V but I’m not sure if a 2% deviation is within tolerances or if it’s a sign of something wrong.
I’m of course hoping it’s the PSU that’s at fault, since it’s easy to replace...

 

After closer inspection the C4 capacitor ( the very first one after the fuse) on the backlight inverter board was shorted. I connected a power supply to the board and cranked up the amps. It didn’t take long for the cap to heat up enough to make the ethanol on my q-tip sizzle when I touched it. However, without proper schematics it’s impossible for me to order the right replacement. Anyone willing to de-solder and measure the capacitance? The board code is 6632L-0176A.

 

After closer inspection the C4 capacitor ( the very first one after the fuse) on the backlight inverter board was shorted. I connected a power supply to the board and cranked up the amps. It didn’t take long for the cap to heat up enough to make the ethanol on my q-tip sizzle when I touched it. However, without proper schematics it’s impossible for me to order the right replacement. Anyone willing to de-solder and measure the capacitance? The board code is 6632L-0176A.

Measuring the capacitance of a failed capacitor isn't worth the bother. Is there no labeling left? A clear picture might help.

That board appears to be a common LCD inverter. It might not be too hard to find more information about it.

 

;

 

Measuring the capacitance of a failed capacitor isn't worth the bother. Is there no labeling left? A clear picture might help.

That board appears to be a common LCD inverter. It might not be too hard to find more information about it.

No, I meant in case someone had a board lying around that had a working capacitor.

It’s an SMC capacitor in a 3225 package. The only marking on the board is C4. I’ve searched for schematics but they seem impossible to come by.

 

;

 

And to reply to my own question, I replaced the cap with a 10uF 25V one I had lying around, and the display powered up normally. Fixed!

 

And to reply to my own question, I replaced the cap with a 10uF 25V one I had lying around, and the display powered up normally. Fixed!

Thats beyond awesome. Well done!