I have repaired dozens of LG LN series TVs with bad LED backlights. Symptom is the LG logo flashing on power up and the TV is on with sound but no video because the backlight has shut down. Though a LED tester will confirm the failure, it is possible to confirm bad LEDs without one but you need to remove the panel to watch the LEDs flash.
The flash is caused by protection circuitry shutting down the backlights because an error is detected in the LED driver circuitry. Driver circuits are current sources driving the leds wired in a series string like Christmas lights. LEDs can fail short (most common) which will cause an undervoltage situation. If LEDs fail open, there will be overvoltage that the driver can detect as well. If there is an open LED, the backlights in that string will not flash. In non-smart LG models, there is no logo and there may just be a quick backlight flash.
The shutdown protects the rest of the LEDs and the power supply. It may also prevent the LEDs from burning up. I have seen LEDs burn up in LN series even with the protection shutdown.
There are a lot of youtube videos on panel removal and I will include one here from Shopjimmy. The panel is extremely fragile and you need to be extremely careful to not crack it. The boards flex connected to the panel can also be ripped off very easily. Damage to the panel or those boards is not repairable.
The TV will turn on with the panel disconnected so you can observe the flashing. By going frame by frame, I took a capture of the flash where you can see two LEDs not lighting up. If you have an LED tester, disconnect the strips from the power supply and probe each strip individually.
LEDs are likely to fail if the backlight level is set to maximum. They get hot and will burn out over time. Failure could occur between 2-5 yrs. On LGs, the default picture modes for Standard and Vivid modes are at 100% level. With the LN smart versions, apps may also default to 100% backlight and can only be changed in the Quick menu.
- Full replacement of all strips with NEW ones. This option is preferred to minimize future problems with your backlights. Reputable sellers include Shopjimmy and ebay seller meihua361000 based in China. I have had failures with Rzconne branded strips so best to avoid those.
- Replace bad strips with used ones. This option is cheaper if you have used strips but if you are buying strips, its only a bit more for new ones. Because there is wear on the existing LEDs, they may fail in the future.
- Replace the LED itself. The LEDs are pretty cheap but you could have an issue with centering the lens back on. If not perfect, it can cause bright spots. For the LN series, you can find 3V 2835 LG replacements on Aliexpress. https://techhometravel.com/lg-42ln5700-logo-flashes-no-picture-has-sound/
It is preferred to have your replacement strips in hand before doing the repair. If your tv has been disassembled for weeks waiting for parts, you may forget how it goes together. You also risk damaging the LCD panel if you haven’t stored it safely and prevented it from gathering dust. However, some models use different panels so may have different strips even for the exact same model number.
To ensure your repair lasts, you need to turn down the backlight level. This setting is in the picture menu. For LGs, 50 to 70% is recommended. With apps, you need to access the Q.Menu to adjust the backlight level. You need to do check on ALL inputs and ALL apps. I would also recommend turning Energy Saving OFF since this could increase the backlight level in a bright room.
Samsung TVs also have LED failure but they don’t have shutdown protection. LEDs will fail until there is damage to the power supply or the screen will get very dim. With Samsung TVs, you can disconnect the cable from the main board to the power supply and this will force the backlights on for testing.
You can also set the backlight level on Samsung TVs. Dynamic picture mode will crank the backlights to 100% (level 20/20). It is recommended to set the backlight level between 10-14 to make them last.
These models have a protection feature but there is no backlight setting in the menu. However you can turn it down in the service menu under Panel Settings. With the TV on, try Input-1-1-4-7 or Source-2-5-8-0 Be very careful with service menu settings since you can brick your tv. I would check every input and set the backlight to around 60. Some models will have an Energy Saving picture mode which runs the backlights at a lower level.
List of backlight repaired models
LG 32LB5600 42LN5200 42LN5700 42LN5750 47LN5700 47LN5750 50LA6205 50LB6100 50LB6500 50LN5310 50LN5400 50LN5700 50LN5750 55LB5800 55LB6100 55LF6000
Samsung UN40EH5000F UN40EH5300F UN40H5203AF UN46H5203AF UN46EH5000F UN46EH5300F UN50EH5300F UN55D6300SF UN55FH6030 UN55ES6100F UN58H5202AF UN60ES8000F
Haier 40D3505 55UFC2500 65E3550
Insignia NS-37D20SNA14 NS-55D550NA15
RCA/Proscan RTU5540-D RLDED5078A-D RLDEDV3988-A EP5585 SRC5565 RTRU6049