I have a 55D6900 that has vertical lines, no video. Sound ok. No splash screen, but backlight is on. It stays blank but leave it on, lines slowly appear and get worse over time. Started by the picture freezing then "melting". Replaced main bd, t-con bd. Heat/ cold no effect.
Power supply voltages show 17.6v on 18v line, 12.7 on 13v line.
Power supply? Ribbon cable?

Leave it on long enough it looks like the pic