Chosen Solution
Model # lg-65LB7100 , when I start the tv I see light and sound both while Antenna or cable not connected. If I press menu backlight goes out in 30 second but I see the menu on the screen with the flashlight Update (11/24/2018) What happens when you do not press the menu. - If its on antenna or HDMi and I am not playing anything or changing anything on screen …..I can see light on full screen. when I try to change the source or play , it works for 10 sec and goes blank. Ultimately you do want to start by connecting your TV to a video source and then let us know what is happening. - I tried connecting to dvd using component cable - it worked for 5 - 10 second and blacked out. Please suggest the next step.
Akhilesh Dube “when I start the tv I see light and sound both while Antenna or cable not connected “ let us know what light that is. We need to know if it is your backlight or if you see bright regular images. Also, let us know what you see when you connect either cable or antenna. This “I see the menu on the screen with the flashlight would mean you are losing the backlight. this could be a power board issue or a backlight array issue. also, give us more information what happens when you do not press the menu. Ultimately you do want to start by connecting your TV to a video source and then let us know what is happening. LG TV’s are known for bad backlight arrays but it is to early to diagnose it since we need a bit more information from you. You will need at least a multimeter to test things if that should become necessary. Let us know the information needed first.
I was given (and repaired!) an LG 65LB7100 with intermittently failing backlight. Basically the screen would go blank after a few seconds or minutes, and the red LED would blink 4 times. I learned a lot about how the power supply board works while fixing my TV, so wanted to share that knowledge here: There are taps for the 8 LED- outputs, and LED+ and LED-. If the LED current or voltage is too high on any of the 8 strips, the board will disable that strip and flash error 4. Too low gives error 3. If more than 3 strips get disabled, the backlight board will turn off entirely. It is very hard to notice just one or two disabled strips as these are quite diffused vertically. The power for the LED’s is provided by a central dc-dc inverter to ~63 volts, and then boosted to ~75 volts, measurable at the LED+ and LED-. Less than ~72 volts or more than ~82 volts the backlight board will error out and shut down. If you see ~63 volts the booster is not boosting. In my case the board was producing ~82 volts and would sometimes work, sometimes not, with error 4. The voltage is set by a voltage divider with 6 “05D” smt 105K ohm resistors in series+parallel which is easy to find. The fix in my case was adjusting this divider to bring voltage to 75V by addition of a 330K resistor parallel with the last 4 resistors of the top of the divider. I could not determine the root cause of the voltage being off, and suspect it was internal to the controller IC. The LED strips are each driven from LED+ and PWM LED- to set the brightness of the given strip dynamically. In my case I tested all of the electrolytic capacitors and found one small one had failed outright. I replaced all the caps, but this made no difference other than providing a warm fuzzy feeling. One other thing to know, if you are experimenting with a shutting down board, the fastest way to power cycle it is with the power plug. (Seems obvious, but the power button otherwise takes forever!) You can test each LED strip easily using a ~500 ohm resistor from LED- to each LED tap and look for the glow while the board is in shutdown. If the strips appear to work, you most likely have an issue with the supply board. I hope this info is clear and helpful and the community gets some value from my learnings.
