I have a similar problem. TV screen turns black after a few seconds, sound still ok.
Today I tried to "bake" :) the mainboard in the oven as described in this video https://m.youtube.com/watch?v=c7p4qsTYXUc, but the carton I placed it on caught fire, the smoke smell still persists in the apartment after a few hours of leaving all windows open.
I will try to find a replacement mainboard, see if I can fix the issue like that. Diagnosing which component is faulty is an interesting challenge, just a bit above my current skill set :)