This is a common issue with this 65" TCL roku 4K led TV that includes models 65s421, 65s423, 65s425, etc. The white indicator light at the bottom of the television will come on and blink when you hit the power, but there's no picture and the screen is totally black with No Backlight. But you still may have sound but no picture.
This video will show you how to replace the led backlights in the TV. This is a step by step DIY backlight replacement for all TCL 65 inch 4S series led televisions.

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