Powers on, audio on, back light on, but no picture at all. I replaced the main board. Still didn't fix the issue. I originally thought it would be the T-con board. But this tv apparently does not have a T-Con board.
It has a Tcon board, but when those go bad it's not usually a black screen. Have you checked the power supply outputs? They are usually labeled with what voltage they should produce.
Even if it does not have a Tcon, then you have replaced the main board, logically you would actually check the power supply first. Nothing will work right without all it's outputs working. The power supply has several outputs. All need to work.