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There's a heat-related problem, or perhaps just a problem with overload to the power supply so that it shuts down completely in protective mode, I think. This, esp. if there is no blink code nor flash code after it shuts down... most likely a severe short in a board (or two doubtfully.)
The best way to find out what board to eliminate from the TV and to replace, would be to
1. examine all boards, one by one, both sides, and see whether any capacitors are bulging, or leaking fluid, or otherwise obviously overheated or defective.
2. also examine all boards, one by one, both side, to see if there are any brown spots under components, or any signs of overheating or arcing. Or, any burnt-out components, period.
3. After that, to get a good service manual for this model of TV, which could save you a lot in money wasted on boards swapped in that turn out not to be defective after all. If you are open to that option, please let me know, and I will research a source or sources for such an LG service manual.
Let me know as to the three points listed above, please. Thanks.