I don't see many components listed for the main board. I only see a complete main board on the Samsungparts.com site for almost every model. This tells us you replace said board.
Remember, due to what you wrote I assume you are a TV Technician and are following the service manual on the product. But no, I'm not there to check your work so I won't be able to nail what's wrong.
Bob
I have a Samsung model # UN40D5550RFXZA
The backlight will turn on and off, on and off etc.
If you unplug the main board the backlight will stay on. So the problem is on the main board. Anyone know what components are failing on the main board?

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