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Post: Answer to "Backlight issues here's how I fixed mine!"

Last Vote: Oct 13, 2019

Voters: Dan (Admin Decided)

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Robert Nawfal @robertnawfal

Rep: 187

Question

Backlight issues here's how I fixed mine!

Posted: Oct 13, 2019

@John &Io Nut

I have an LED cinema display 24" from 2009, I have excatly the problem that the screen blanks out after a 1 sec flash of the image (probably LED driver signal turns down). I was about to try the bypass solution above for the 3.3V drive input, but then wanted to ask first if the same backlight strip probem problem could be on my LED Display as well, because I would not want to loose the dispay dimming possibility.

update:

So I did take the LG LCD panel apart in search for the notorius LED Strip, see pictures:

Taking the LCD apart:

The LCD frame,

The Backlight panel,

The plastic frame,

The glass of the panel,

The back plate,

Finally getting down to the LED Strip

So I did resolder the LED connector (just added some solder to it and re-heated the 5 pins again thoroughly), but my problem still persists, the LED stripe only flashes quick at connection display port connection time. I am considering of doing some logic measurements tonight with my oscilloscope for the LED driver, but the willi fix for bypassing the 3.3V is starting to sound tempting (still trying to find out the real reason for this failure, and I have to admit that the LED stripe fault sounds more plausible to me due to the drive signal LED shutdown after one second flash.

@io nut

I made some advancements with the display but the reason for the fault is still unclear.

All of a sudden the LEDs decided to turn on again.

I am convinced that it has something to do with the horizontal positioning of my setup. While lit I quickly checked the LEDs and realised that Led nr. 27,30,40 and 60 are out. Next day when I started the display the background lighting did not lit again.

I figured it might have something to do with grounding and tried out connecting the displayport without the USB cable. No effect the LEDs turn on with or without the other USB connector and/or the Powerconnector. I realised that the LED cinema display brightness can not be adjusted without the USB cable connected to the my macbook pro.

LED illumination pattern 2/3 Leds.

LED illumination pattern every third LED

So after all the MODs my display is running again. Eventually I ended up only changing 9 capacitors (3 pcs of each of the black ones in the pic, the orange ones I still have originals) on the powersupply,

and resoldering the LED stripe connector,

in addition I assembled and reassembled the display several times (loose connectors).

So Im still not convinced about the source of the fault and Im now testing the display for a week, lets see if it actually stays on.

My Apple LED Cinema Display 24" is still up and running.

Unfortunately I have to report that my LED cinema display died again after one month of fluent usage. So Im back to square zero. :!

I decided to try out if my display runs and it seems to run again :). So lets see for how long, but I am suspecting some sort of capacitor voltages that are incorrect due to capacitor ageing.

Robert Nawfal @robertnawfal_86

Rep: 513

Answer

Posted: Sep 11, 2018

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!

James Stuart at: https://de.ifixit.com/Antworten/Ansehen/...

“
I don't mean to offend anyone but you all seem to have missed the blatantly obvious and that is the LED backlight array.

LEDs age, and as such the amount of light that is emitted reduces. The LEDS 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.

LEDs operate in constant current mode that is the current through the device is adjusted by altering the voltage across it. In low current LED applications this done with a simple resister. In high powered applications this is done with a controller chip such as the HV9982. Most high powered applications work as a closed loop system, that is they monitor the current flowing in the LED string and adjust the voltage across the string accordingly. The circuit Apple adopted does just that, it monitors the voltage developed across a low ohm resister in the bottom of the LED string and adjust the PWM signal to the HV9982 that will adjust the voltage to the LEDs. The suggested fix above of applying 3.3V to the PWM input will remove this feedback and will apply maximum voltage across the LEDs and is likely to result in damage to the LEDs and driver circuit.

What most people are experiencing is the backlight driver circuit shutting down due to it reaching an overvoltage point as it tries to pump more current through the aged LEDs.

(picture in the original post)

The above plot shows the current through each LED with a forward voltage of 3.3V. As you can see there is a lot of variation (LED 1 shows as 0mA), none of them get near the 350mA that they should.

Conclusion, the only long term way to fix this problem is to replace the LED string as a whole or at least the weaker LEDs. The individual LEDs are available from DigiKey but they will prove difficult to replace as they are surface mount and are tied to a heat sink back plate. An alternative is to retrofit a low voltage backlight replacement string, they are available on eBay for as low as £4. These will require a 12V supply and will need to be turned on and off manually.

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 quite tedious to get one out.

I found the Lumileds to order, but investing like 130CHF (approx. 130USD) is a bit too much for the moment, for this project. Besides I tried unsoldering with hot air station that I ordered a few of those LEDs and it is indeed possible but quite tedious, so perphaps not worth the time and money.

This looks like the original one:
https://www.alibaba.com/product-detail/-...

From the original LumiLed Datasheet, the SMD LED component footprint:

Page 1 of 1

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Question

Backlight issues here's how I fixed mine!

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