I too replaced the mainboard and powersupply
I am facing the same issue with my Samsung KS8500 , looks like it is the panel issue, I too replaced the mainboard and powersupply . I had checked all the voltages from powersupply and they were all correct , but just to take a chance changed the powersupply as well, it still having the same issue.
These TVs do not have any T-Con boards , they are part of the panel , so if T-Con is bad, there is no way to fix this.
It is the same issue as the OP posted in the thread.
"As of now, my after turning on my TV, I can see the backlight turning on for a second or two, then turning off for a second or two, and endlessly repeating this cycle."
I have tried changing both mainboard and powersupply and still facing the same.