Thanks for choosing JustAnswer for your TV question!
I also own a Sammy, your Tv has several features that will "dim down" the picture.
One is energy saving mode. Many of these sets are Energy Star sets.
Look through all the menus to find these.
If all else fails, turn off the set, use the samsung remote and press:
Mute-1-8-2-Power in that order.
The set will turn on and go into "factory mode" or a service mode.
Select the "options" or "service" menu, and look for an option called "user reset".
select this option using "enter" or the right arrow.
The set should turn off and come back on (if not then turn it off then on again).
Your problem should be cleared up.
Hope that helps!
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