Hi @Ryan Feeney
Try adjusting the backlight dimming setting and see if this alters anything.
Go to Menu → Picture → Backlight and vary level (0 – 20)
If no backlight changes observed:
Check the Panel Connector pin voltages and BLU_PWM voltages (CNM801) while changing backlight level.
If Panel voltages don’t change, and BLU_PWM changes, replace SMPS.
If BLU_PWM doesn’t change replace Main/T-Con Board
I'm a bit concerned that you can change things by using the "percussion test" as usually board problems aren't affected by doing this unless it is a loose connection.
Here's an image that shows how to test the backlights independent of the main board.
If the backlights are still dim in sections of the screen the problem is either the power board or the backlights or the connections between them.
Turn off the TV, disconnect the power to the TV and then follow the direction to remove the cable between the mainboard and the power board. Reconnect the power to the TV and check the backlights. Also see if they alter brightness. when using the percussion test. They should be full on and stay on.
(click on image)