I have LG 42LN5300 which is reading 230VDC at led + plug and no back lights. All other voltages going to main board are OK, I changed main board because of flickering on and off screen, so is it MB

Hello !! first of all check for bad connection on the led strips if all connection ok , test the single led one by one , at the end Do not Hesitate replace the leds strips . Because problem could happen after heating .

Back - light problem. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis.
http://electronicshelponline.blogspot.com/
To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.
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Wow, a master replied, if you need to buy TV accessories, you can take a look at this website:www.abctay.com

Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis.
http://electronicshelponline.blogspot.com/
To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.
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The input board (T-con) was probably not the problem. Check the high voltage circuit on the power board first. After 120 volts enters the TV, it goes to a high voltage transformer and a capacitor there is where you can the high voltage - should be as close to 400 volts DC as possible. (380 volts is a failed high voltage circuit.) The video card is next, check those small capacitors. Good luck.

Chances are you have some open LED's, if one opens the entire strip will go out as they are wired in series and the power supply is shutting down the LED circuitry due to improper current or voltage

Check the various voltages coming from the power supply board and see if they are all ok, there is a legend on the power supply board to show what voltages are at what pins on the connectors,the 5 volt standby we know is good as long as the set has sound, but check the others

This is a common problem for LG 42LN5xxx & 47LN5xxx LED TVs, the problem is defective LED back lights strips, it has nothing to do with main power supply & the main board.

Back - light problem. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis. http://electronicshelponline.blogspot.com/ To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.

These blinks [flashes] are fault indication (Error Codes).From the number of blinks occur at one time switch ON, you can get an idea about the faulty circuit section inside it.This will make repair easy.These are called blinking codes. Make sure about the exact number of blinking of the standby/power LED, by switching On & OFF the set, for at least two times.Count it carefully.If you wish to get some details, check the site linked here.It has details about blinking codes, and fault locating procedure, to many popular brand TVs and other devices.http://electronicshelponline.blogspot.com/
Blinking codes details to many popular brand TVs are given.Surf the site with patience.If blinks continue without any pause, the main board is faulty.