Apple Cinema HD Display 23" (M8537) backlight issue

[Xandros]

Hi. Here's an issue I've brought up in another thread relating to my PowerMac's PSU though it seemingly turned out this problem was the specifically something to do with the monitor.

It works fine, when it's working, but sometimes at seemingly random it will go black and remain that way, usually until I disconnect it and leave it for a few hours. Sometimes it will stay on for quite a while before going black, other times it will do it within 30 seconds of being turned on, though recently it doesn't seem to stay on for very long even when I've had it disconnected for a long time.

It might be worth noting at this point for anyone not in the know about these types of monitor that it is not externally powered, it is powered by the computer via its display connector (ADC). However I've ruled out an issue with the PowerMac's PSU being the problem as I say, so whatever the issue is, it is to do with the monitor.

When it does go black I have verified that the LCD panel actually still works by doing the old flashlight trick and can see the desktop, so the panel works just the backlights turn off for whatever reason. When it does go black, the monitor's LED power light blinks "short short long" which according to Apple's diagnostics indicates "a problem with the backlight or inverter board".

However Apple's service manual for the monitor includes a section for this sort of problem by testing various pins on connectors on the main board (I've labelled them in the last picture of the main board). When I tested them the first time the screen was actually working. The first two voltages checked out just about. The second two were lower than they probably should be:

Verify 24-28v at input of J702: I get 24.7v
Verify 24-28v at pin 10 of J701: I get 24.7v
Verify 5v at pin 11 of J703: I get 4.8v
Verify 18v at pin 6 of J703: I get 17.9v

The manual states on the last two if they don't verify at 5v or 18v respectively to "replace the main board". I left the thing running for a while until it inevitably turned off. Then I tested them again:

J702 input was 25.07v
J701 pin 10 was 25.05v
J703 pin 11 was 4.95v
J703 pin 6 was 17.93v

So, any ideas what could potentially be the issue?

[Xandros]

Re: Apple Cinema HD Display 23" (M8537) backlight issue

It was the inverter board, though I won't bother explaining how I ended up at that conclusion as it's too long winded. No idea what is actually wrong with it but given that it's got two teapo caps on it and not a whole lot else, I'd hazard a guess one or both of them are crapping out under load after a given length of time. I suppose it could be one of the transformers or a transistor or something but, I've seen plenty of people badmouth teapo caps on here and the one bulged cap I came across in my Mac's PSU when I was messing around with that in the other topic before making this one was also a teapo. So that's what my money is on.