Well, without the diagram, you will have to trace out using Ohm meter from one end of the fuse on the T-CON board to the main board to find out which component has the lowest resistance reading, that will be the switching device (more likely to be MOSFET).
I will guess some thing in that red section I marked.
I just found out from the guy I got the TV from that when the TV first stopped working having no picture or sound that he bought a refurbished main board and installed and had same results. Based on what you see so far do you think the problem is in main board.
I traced from TCon to main board then to where the ribbon comes from the power supply to the main board . Only had 5 volts on three pins all other read m volts. Traced ribbon to power supply same readings.
The ribbon cable from the T-CON on the pin that the 12V from the main board is fed into will get the 12V through a switched device (3-pin device), that is the device you need to find on the main board.
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