Finally found the solution. There is 6 large screw holes on the back of the tv, where you mount the wall mount. 4 of them are used to mount the tv and two center ones only holding the back cover. Looks like the guys who installed the tv used bottom and center holes leaving top ones empty. Longer mount screw went thru the frame pushing FB1 board in metal case toward the panel. Board under pressure of heatsinks broke after few years.
After I changed the board it still didn’t worked. while switching boards 2nd time I noticed that it doesn’t lie evenly in place. The metal stands under the board were bent down, right by the AU board, causing the two large connectors to not align correctly. fixed those and voila.
Interesting fact is that tv was on the wall in my living room for 5 years, then it was moved to basement also mounted, and it was working for 2-3 years, rarely used but working.