Apple Cinema Display 24" (LED!)

[TechChopExpress]

First time caller, long time listener! So, here's an ADC with a LED backlight issue that I got for cheap recently. It uses an LM240WU6-SDA1 LCD from LG.

Symptom:
Upon first powering on, backlight flashes on once for such a short duration that it's almost undetectable, but it does happen. After that, the display is dark. Unplugging/replugging the displayport connector does not cause the flash to reoccur, but cycling the power to the display does.

What I've done:
The LCD itself displays fine, determined via a flashlight test.
Visually inspected for any blown or burnt components.
As per the Apple Technician Guide for the display, I've checked the voltages at test points on the logic board for all the major functions, which checked out. (24.7V for points labeled as 24.5V, 12.15V for 12V points, 3.35V for 3.3V points and 1.03V for 1V points)
Checked continuity across all lanes on the LCD function cable.
Checked continuity across fuses on LCD PCB.

Additional Information:
On eBay I've seen quite a few used but working logic boards and PSUs "pulled from units with bad LCDs".
In reports of problems with these displays on other sites, people who took them in to Apple said the LCD and sometimes the logic board was replaced.
A similar LG LCD panel (Same model number except for the SDA1 suffix) that is used in iMacs suffers from LEDs not functioning, with the solution being to push on the lower left corner.
The technician guide can be found here: https://cdn.badcaps-static.com/pdfs/...e111828a87.pdf

All this information leads me to believe that the issue lies in the LCD panel itself, however, I wanted to get some input on other things I could check out. If the problem is with the panel, I'd like to try and fix that, too. There isn't a whole lot of info to be found on repairing LED backlit displays, but I have it and might as well try; might learn something!
Thanks in advance, guys.

Images:
1, 2: Logic Board
3, 4: Power Supply
5, 6, 7, 8: LCD PCB with arrows pointing to fuses
9: LED Driver cable. I couldn't test to see if it was lose while the display was on and connected, and I can't test for continuity without disassembling the panel. Could it be the problem?
10: Panel label

[jetadm123]

Re: Apple Cinema Display 24" (LED!)

Good work on troubleshooting so far. Since you tried the "flashlight test", I'm assuming you see an image on the panel, but no lighting of the LED string. The LED driver cable that feeds the panel LED's with power. With the monitor connected to a working computer, can you test to see if the voltage feeding the LED's is present? I understand you cannot tap into the panel end, but the the other end of the cable should be exposed, so that you can test for voltage there. It does not look like any of the connectors on the boards are labeled, so try using the on/off button on the front panel to help you identify which pin of that cable supplies the LED with voltage. If you're seeing voltage, I would suspect a problem with the LED's and/or wiring within the panel itself. If no voltage, then I would suspect the board that cable is connected to.

[cholzberger]

Re: Apple Cinema Display 24" (LED!)

Hey,

any progress on this issue so far?
i have the exact same problem with an 24" led acd.

Power etc. is good, but no picture and no backlight.
there must be something wrong on the lcd pcb circuits.

[Toad555]

Re: Apple Cinema Display 24" (LED!)

Hi

Apologies that my first post on this board is a load of questions but you guys seem to have a handle on the repair I'm hoping to perform.

I understand that these posts are very old now but I wondered if anyone ever found a solution.

I'm working on this very same screen with an identical panel, same fault.
The LED's fire up for a split second when the screen is first powered up then never again. I too can see a perfect display using a torch but no backlight.

I've done the voltage tests on the main logic board and all seems well.
I also read another thread on another board describing how there was a PWM fault on the 27" screens that caused the same fault, there was an associated fix which meant the screen was always at full brightness but this seemed to work.
I have not found a similar fix for the 24" model though.

Any further ideas ladies and gents.

[The Real Luo Ji]

Re: Apple Cinema Display 24" (LED!)

Hi all! Picked up one in a car boot sale (A1267) for 20 quid, so a winner!!! Had the same flashing thing at the start then black, but I have never failed to get it working if i try enough times. Sometimes its only three or four sometimes up to 20, but it always works. If I then plug the USB into the laptop that seems to 'close a circuit', and then no matter how many times i unplug the display, it fires up immediately first time. No idea what that means, but I would hazard a baseless guess that it is something to do with intermittent signal or power that is no longer intermittent once the USB is plugged into the laptop.Just did it 3 times this very second before pressing 'post'.

[The Real Luo Ji]

Re: Apple Cinema Display 24" (LED!)

Hi all! Picked up one in a car boot sale (A1267) for 20 quid, so a winner!!! Had the same flashing thing at the start then black, but I have never failed to get it working if i try enough times. Sometimes its only three or four sometimes up to 20, but it always works. If I then plug the USB into the laptop that seems to 'close a circuit', and then no matter how many times i unplug the display, it fires up immediately first time. No idea what that means, but I would hazard a baseless guess that it is something to do with intermittent signal or power that is no longer intermittent once the USB is plugged into the laptop.Just did it 3 times this very second before pressing 'post'.

***UPDATE***

I took the monitor apart completely. disconnected all cables and wires removed logic board, power supply and sub woofer and gave everything a really deep clean. Blowing out and toothbrush removing an enormous amount of dust from the fan. Reassembled checking all cable/wire connectors were well seated.

First time reusing took several attempts at connecting the mini display to get it to work, but then.... next time i connected it works straight away and at FULL BRIGHTNESS. MUCH better than before!!! I have now disconnected and reconnected the mini display 20 or 30 times and it connects every time, first time, WITHOUT usb connected! I have also tried every combination, unplugging monitor power while connected to mini display on macbook? Fine. Disconnecting mac, then unplugging monitor from power. Turning power back on then connecting to mac? Also fine. Every variation of connection and disconnection in all power states, and usb connected or not. Now what is REALLY interesting is that playing with the brightness slider for monitor, about half way the flickering happens, and then even return to full the monitor is MUCH dimmer. However, unplugging usb and then reconnecting mini display brings it back to full brightness immediately with zero issues. There seems to be some combination of software/hardware relating to brightness itself and its control that is involved here.

[waakye]

Re: Apple Cinema Display 24" (LED!)

Hi I know this thread is very old but I wanted to know if anyone remember or was able to fix it?.
cholzberger were you able to fix it?
Thanks all