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TCL 43S431 powers on, Logo screen flickers on, then screen off

I just replaced a mainboard on a TCL 43S431 that wasn't getting any image. Now, when it comes on, it shows the TCL load screen for a few seconds and then goes black. After several minutes, the speakers start playing music. I assume the board is in demo mode. Do you have any idea how to keep the screen from going black?

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