This model has a common screen failure that causes the LED backlights to not turn on. One method to isolate the screen and test for this is to disconnect both the LVDS cables going to the screen and then turn on the TV. If the backlights come on now and they didn't before then the screen is the culprit and no board replacement will correct this.
IMPORTANT: If you experience WiFi issues after replacing the main board please perform the "Reset Network" function which can be found under the menu option General / NetworkPart Number:
BN94-12640X| BN97-13635A| BN41-02568B
BN94-12640X BN97-13635A BN41-02568B Main Board UN40MU6290FXZA UN40MU6290FXZC