I got a 24" cinema display that had the same issues.[br]
Since it was thrown out of the office, I decided to have a look at what the the problem was (as I see a lot of these screens on my local auction site with the exact ssame issues).[br]
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TL;DR : [br]
- The only options you have are :[br]
1. replacing the entire LCD panel with a working one.[br]
2. replace the LEDs on the LED strip inside the LCD panel. [br]
- Replacing the LED is extremely hard because of the thick aluminium strip to which the LED strip flex cable is attached to, absorbs all the heat. My rework station did not produce enough heat so you need something like a heating plate.[br]
- Cheap CCFL-to-LED backlight conversion kits don't produce enough light (15W instead of the needed 61W).[br]
- I have not been able to find a seller of replacement LED strips.[br]
- I tried using other LEDs but the LED driver seems to be configured for the LG LCD panel LED strip.[br]
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My screen sometimes flickered for a second and then would turn off, even at the lowest brightness setting.[br]
USB and sound was OK.[br]
I took a strong flashlight to confirm that the LCD panel itself was working.[br]
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I took the display apart and found that the LED strip was the problem.[br]
The LED strip has 3 strings (20 leds of 350mA per string at 60V = 61 watt) . [br]
The LED driver (HV9982) manages these 3 strings seperately.[br]
The display has a safety feature where the LED driver checks voltage and current of each of the strings of the LED strip. [br]
This safety feature is what kicks in and turns off the LED strip inside the LCD panel.[br]
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If the voltage/current goes out of a predetermined range, the LED driver will issue a fault status that turns off the string.[br]
The fault status is also monitored by a small CPU on the same logic board.[br]
This cpu is responsible for turning the LED driver on/off and provide the PWM signal for the dimming of the LED strip.[br]
(Note that the brightness setting only works when you also plug in the USB connector.)[br]
As soon as this LED driver goes into a fault state the cpu also turns down the dimming completely so that the LED string is turned off.[br]
That is why some people notice that the PWM signal is off and they try to bypass it.[br]
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My understanding is that when LEDs get older or faulty, they draw a lot more current (even though they are not producing much light).[br]
This is the primary cause why the screen LED is turned off.[br]
With a strong flashlight and moving a window with some text to the display, you can easily check this.[br]
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I also took the actual LG display (LM240WU6-SDA1) in an attempt to fix/replace the LED strip.[br]
The LCD panel is relatively easy to open by removing some screws and lifting some clips.[br]
Be extremely carefull with the sides of the panel. [br]
It has some electronic components on some flex cable sticking out on the side of the LCD panel. [br]
This flex cable is extremely easy to damage.[br]
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The LED strip inside was a big mess. [br]
According to some other post, the LEDs used are Luxeon Rebel LEDS (LXML-PWN1-0120).[br]
Some of the LED lenses of the SMD LEDs had fallen off and the same happened when I touched some of the other LEDs.[br]
Normally the lenses of SMD LEDs don't come off, no matter the age of the LEDs. [br]
I've took apart many TVs and LED monitors but I have never see a LEDs so badly damaged.[br]
Something is really wrong with these LEDs or the way that they are used inside this panel.[br]
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Since there are so many people with problems with this cinema display, [br]
I suspect there is something fundamentally wrong with the LEDs or the design of the panel.[br]
Compared to other LEDs they do give way more light but they also draw 2~3 times the current.[br]
That is a lot of current, lot of light and probably a lot of heat.[br]
The specs say that the LEDs should last 36000 hours but that is complete nonsense I think.[br]
I think due to the amount of stress to produce this much light, the LEDs don't last as long as the manufacturer claims.[br]
Or maybe because they are enclosed in the panel, they cannot get rid of the heat sufficiently, causing too much stress on the LEDs and shortening their lifetime. [br]
There are ventilation gaps in the panel but they have been covered by black tape.[br]
The datasheet of the LG LCD panel does not show that theese ventilation gaps are covered.[br]
So I guess it's Apple who put some tape on them to prevent any light leaking out of the panel.[br]
The Cinema Display has ventilation holes in the bottom of the frame, exactly where light would bleed out of the LCD panel.[br]
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The LEDs of which the lens had fallen off didn't light up anymore.[br]
The LEDS that did light up, did have very different brightness.[br]
Some were too bright to look at and some hardly lit up at all.[br]
This also makes me draw the conclusion that something is wrong with these LEDs.[br]
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Just for fun I tried installing a CCFL-to-LED conversion kit in the panel.[br]
You can get these kids for $10~$20 on Amazon or Aliexpress.[br]
https://www.amazon.com/Highlight-Dimable-Backlight-Adjustable-Monitor/dp/B00XFXM4NA[br]
That did not work at all.[br]
One strip gave far too less light. Even with the brightness turned on to max, it was too dark.[br]
The original led strip is 61 watt and 1 strip of the conversion kit is only 15 watt.[br]
I tried installing 2 strips. There is just enough space inside the panel to put 2 strips parallel.[br]
Oddly enough that didn't improve the brightness.[br]
My theory is that is you put 2 strips inside the panel, the LEDs are too much on the outside of the panel on both sides and thus not shining into the transparent back plate.[br]
So this attempt also failed.[br]
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I have searched a lot of the internet but I have not found any manufacturer who has a good replacement LED strip for the 24 inch model cinema display. There are replacements for the 27 inch version but these are not compatible with the 24 inch model.[br]
The 27 inch model seems to have 2 strips on both sides while the 24 inch model only has one.[br]
Maybe that is also way they put so much high power leds on 1 strip.[br]
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I tried using the LED strip of another monitor by modifying it to 3 strings of 20 leds.[br]
That also did not work. [br]
The string started flickering intermittently first and later only on the very lowest brightness setting it would stay on.[br]
Turning up the brightness 2 steps would result in the LED string turning off.[br]
It looks like the LED driver is made for particular LEDs with a certain current draw.[br]
The LEDs on my other LED strip were much smaller so I guess I hit some threshold of the LED driver again.[br]
Apple does seem to use the same schematic as mentioned in the HV9982 LED driver datasheet but the values for monitoring current and voltage are probably matched with the LED strip in the LG LCD panel.[br]
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