LG TV Turns Off After 20 Minutes
This TV was brought in to me by another teacher at the college where I teach automotive apprentices. His complaint was that the TV would shut off after 20 minutes.
Removing the cover of the TV allowed testing during both the working and non-working period. I could see the backlights operating at the back of the TV. These little holes were a great and easy guide to when the backlight was working. After waiting 20 minutes, sure enough, the screen would turn black. However, sound could still be heard.
By using a torch on the front of the screen, the video could be seen, so this pointed toward a backlight issue. However, keep in mind that testing took 20 minutes between the backlight working and failing.
As the unit was powered up, a flashing LED provided a fault code. – 4. After looking up the code, I determined that it was related to the backlight.
At that point, I decided to make a list of voltages at the LED backlight connectors while the TV was operating correctly, so that I could see which voltages were missing when it faulted.
Fortunately, not only did I find a very detailed service manual, but also this interconnect diagram, which provided details about how to independently start up the power board without a signal from the mainboard. It also showed how the backlight was controlled by a PWM signal and how to bypass the driver.
Service Manual – https://www.manualslib.com/manual/670990/Lg-55lm6200.html?page=19#manual
Service Manual – https://elektrotanya.com/lg_55lm6200-ue_chassis_la22e_mfl67460404_1202-rev00.pdf/download.html
LG Interconnect LED 55LM6200 https://www.electronica-pt.com/esquema/tv/lg-esquemas-tv/lg-interconnect-led-55lm6200-84244/
However, even with the bypass done, the backlight would stop after 20 minutes. I have done a few repairs with backlight issues, but this seemed different to me.
I contacted Jestine Yong and he stated that he had seen backlight issues after 1 hour or even longer. He suggested that I get a LED tester to independently test the LED backlights.
This tool proved useful, but due to the design of the backlight, the light was too dull to identify any issues.
I examined the datasheet for the backlight driver, which proved to be both interesting and helpful toward the diagnosis.
Unlike other backlights I have repaired, this system uses only 2 LED strips with a total of 132 individual LEDs. This was difficult to see if any LEDs were out.
Using a 200Ω resistor, I could power up each ‘block’ of LEDs to section up the backlight. Once again, due to the diffusers being used, I could not isolate faulty LEDs.
The only option was to totally disassemble the TV. After removing the screen and sets of various diffusers, I now had access to the LED strips.
I soldered 220Ω resistors to each block, so that I could power it up, earth the leads with the resistor inside and test various blocks and see what the possible fault could be.
Of course, there was also the 20 time-frame for it to fail, but finally, on the last block, a single LED showed itself as faulty…….after 20 minutes.
The LED driver was using a protection program in the event of an open circuit, as in this case. That is why the backlight would totally shut down when this single LED open-circuited.
For my own personal interest, I was able to power up the individual LEDs on the old strip and yes, the faulty one would work for an extended period of time – that’s why it was so difficult to be confident with my diagnosis.
After replacing the LED strips and reassembling the TV, I ran it for 8 hours without a single fault.
The customer was delighted to get his TV back, fully operational. If you are interested in this repair, you can see a video on my channel following the link below:
This article was prepared for you by Mark Rabone from Australia.
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Note: You can check out his previous repair article on Astronaut Travels The Universe Again
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Albert van Bemmelen
April 12, 2025 at 3:56 pm
Thanks Mark for this well presented LG TV repair and for all included service manual download links!
Always something to collect and keep for future access when needed. Also in the event these are no longer shared online.
Parasuraman S
April 12, 2025 at 6:44 pm
Excellent article delving into the most difficult task of replacing backlights and bringing out the intricacies involved and steps to be taken for diagnosis. Very informative! Many thanks for this detailed and exhaustive write up, which will be very helpful to techies like me! Well done, dear Mark!
Mark
April 13, 2025 at 8:18 pm
Hey Parasuraman,
Thanks for that. It really tested my skills & I learnt so much during the diagnosis.
Yogesh Panchal
April 12, 2025 at 6:11 pm
Good job! Mark,
It is really difficult to catch the culprit under load.
Here Reading / Knowledge & experience worked.
Mark
April 13, 2025 at 8:20 pm
Hey Yogesh,
You are right, so much time was spent waiting for the fault to show it's ugly head.
Mark
April 12, 2025 at 6:29 pm
Thanks Albert,
You are right - especially with later appliances. It is getting more difficult to find helpful information.
Mark
April 13, 2025 at 8:21 pm
Totally right Mark.
Waleed Rishmawi
April 12, 2025 at 6:36 pm
yes, LG TV set are well known for their backlight failure and that is why I just replace the whole back light and never bother with testing unless I have to. thanks for sharing and have a blessed day
Mark
April 13, 2025 at 9:45 pm
Hey Waleed ,
I have learned a lot from this repair. Now that I have this knowledge, I would maybe carry out testing in a different way or feel more confident just going ahead with the backlight replacement.
muykit
April 13, 2025 at 12:05 am
Mark, pleasant repair, and exellent article. Once the culprit LED light was spotted, did you revert to Engineer Jestine Yongs method to find out if the 'caught' faulty LED was giving out light any differently ? During tests, using a 200 ohms resistor makes sense to me from a component protection perspective. But it beats me how you determine that it should be a 200 ohms resistor. Show me the maths please, once a teacher, always a teacher. I too have some automotive technician friends who would rather be taught what they dont understand in our mother tongue, quite challenging. At one time I really loved reading automotive articles by your country men, say hi to them.
Mark
April 13, 2025 at 10:02 pm
Hey Muykit,
As I showed in the video, after I had completed the repair, I went back with the backlight tester and checked the faulty LED. It lit up and so it would still take the 20 minutes before it failed.
I also included the interconnect diagram in the article which explained the use of a 220 Ohm resistor to bring on the backlight and individual light blocks.
I hope that explains my diagnostic methods.
Mark J
April 13, 2025 at 3:01 am
Great article. Thanks for service manual download links.
Mark
April 13, 2025 at 10:03 pm
Hey Mark,
Glad I was able to find them and share.
nick greek
April 13, 2025 at 5:17 am
excellent job congratulations
Mark
April 13, 2025 at 10:03 pm
Thanks Nick
H.khalilian
April 13, 2025 at 10:13 pm
thanks a lot
Mark
April 14, 2025 at 7:37 pm
You are welcome
Imoudu
April 14, 2025 at 4:22 am
Good repair narrative, Mark,it worths keeping for reference, however how did you arrive at using 200ohms in calculations for testing those led stripes.
Mark
April 14, 2025 at 7:38 pm
All the instructions are shown in the interconnect diagram shown in the article below