Facing TV Problems? Ask a TV Technician.
Hi and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, please post back with clear pictures of the circuit boards.
Still, consider an experiment (if have not been done). Power on the TV and set to a program/channel. If at all possible, darken the room where the TV is. Wait for about 5 minutes or more. Shine a flashlight at an angle (not directly) and nearly but not touching the screen where the volume level / menu display is usually seen. Press the VOL UP/DOWN or MENU button and note if a dark/ghostly image of the volume level or MENU could be seen.
The above is a test to determine if it is a backlighting issue.
If "...no back lighting visable..." then primary suspect would be the LED drivers that is part of the power supply board.
BUT since "...power supply board...replaced...", then this replacement power supply board would still be highly suspect.
On the otherhand, if the new power supply board is known to be good, then the primary suspect would be the LED backlighting strip.
It has to be posted that depending on the version, some LED strips are made to be an integral part of the screen.