Dim grey screen, no picture, good sound.

Hello,

I have an LG 55 inch smart tv, 55UK5090PUA. The screen suddenly went from a solid picture to a dimly lit grey screen. There was no transition where it was intermittent - it just stopped displaying a picture, although when I pressed hot buttons to get to a streaming service, I could hear the sound.

After trying the reset/troubleshooting, I replaced the Main Board with the same replacement part (EBT65513202), plugged everything back in, with the same results. I guess now I have a back-up.

Does anyone have a suggestion on what I should check next (and how)? With the back off the TV, you can see the back lights on for sure, so I believe there is juice moving through the power board.

Thank You in advance,

Evan