I had the same model with the exact same problem last week in my shop, it turned out to be a firmware issue, went online downloaded the firmware to a flash drive and into the TV and it worked fine after that, not saying this is your problem but I had checked both the power board and main and found nothing bad so that is when I reverted to the firmware issue
Your Main PCB is faulty. LG is version specific. There are additional letters after the "UA" in the model number located below were you found that one. The complete model number is also located on a small rectangular sticker near the edge of the TV on the back. Use the entire number when searching online for a replacement.
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.
SOURCE: LG 47LN5700-UH Sound no picture
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://electro-medical.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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