I have now changed the LED bar from a donor Cinema display and it works like a charm since a week already. It always starts immedeately and the display is dimmbar normally. LEDs age, and as such the amount of light that is emitted reduces. The LED’s in the LM240WU6 panel are made by LumiLEDs which is a subsidiary of Philips who in turn produced the panel under license from LG. The LEDs used are LXML-PWN1-0120 which should be operated at a forward voltage of between 3.0V & 3.6V @ 350mA and have a stated 39000 hours of life. The LED strip has a sticker on it: (1) P1803001D (2) P66L01746
I am currently just doing the same reflow process for my led bar on 24” model. Unfortunately I was not able to find the 24” inch model led bars on the market either and by mistake ordered the 27” led bar. Anyway I detached the led bar and unsoldered 4 leds but they all seem to work individually. Also wondering how this reflow process should look like? Just add flux and heat gun and that is it? Or really detach every led and apply new solder? Any idea how long this solution lasts? Is it worth the trouble? I mean the leds lifetime is like 4,5 years, so maybe changing all the leds (very tediouswithput the spare part) is a more longlasting solution. Just wondering…
I still today continue using my Mac display, but I have to occasionally unplug replug the power cable, so the problem still remains. I have ordered two LED strips to replace them when I get time to do it. I realised that adjusting the screen brightness to full before connecting the display to macbook helps for the issue often, but not always.
Update (2018/09) My display is still up and running after changing the Capacitors, but I reckon this is only coinsidense. I would advice you to have a look at James Stuart comment/explanation below. I think it makes the most sense and explains the cause of the problem well! Update (06/14/2020): I got some more equipment and returned to fix my 24” cinema display. So in the meanwhile the display was working like every second day by chance. In the meantime I found a donor cinema display with the same defect. Backlight shutdown after a flicker of light. So I decided to open my display again and took out the led strip totally to test it. I only have a 30V regulated power source so I could test the whole led array (60Volts needed). But I tested the leds individually and they seemed to at least light all one by one with significant differences in luminousity. Now I wonder how to find out which ones should I replace without desoldering all of them. I desoldered 4 of them with my new hot air station, but it is...
If the display is not even recognized by the system, I would try listening if even the fan starts to roll. Otherwise I would go further checking the power voltages first. But yeah as @SW said You might as well quick try and see the Panel with a flashlight if you have a weak image without background lighting.
I would guess that if the display is not connected to the laptop video output the display shutsdown/ power save mode => switch off the LCD voltages => I havent checked the TP11 but I guess it goes to the LCD or LEDs. if you read the issue: What components need replacing on the Logic board? You will find an an extensive description on the LED background not lighting. If you have any questions just ask and I ll see what I can find out but please read the thread carefully, I think you ll find your answer there.
Ich habe das gleiche problem gemerkt und dann habe ich in service manual gesehen das der verursache war falsch versteckte sensoren im Logic board. Der display temp sensor und der andere blower sensor sind vermutlich verkehrt.