I have been working on a 75" Samsung UN75RU8000FXZA that is in eternal boot loop. Removing power from mainboard allows all LEDs to light normally (LEDs good). Removing ribbon cables from mainboard (TCON and mainboard now in a single board) allows TV to boot- I can hear the boot sound and TV stays on. Tried a replacement mainboard with the same results. Upon digging, it looks like part of the display is faulty, causing the boot sequence to stop and try to restart upon an error. I will try to isolate whether its the left side or right side of display. From video I found online - you can then slide plastic under parts of the ribbon cable to try and isolate the bad section of display but still allow it to boot. From the video I watched- the tech suspected the chip-on-film going from buffer board to display has gone bad on one section of the display.