Have you checked Picture Mode(Power Saving Mode) setting?
Symptom
If the screen is dim even without adjusting brightness, definition, color, and color density, it may be set as Power Saving Mode or the Picture Mode is set as Eco.
<Dim screen> <Normal screen>
How to fix
Non-Smart TV
① Adjusting Picture Mode: SMART → SETTINGS → PICTURE → PICTURE MODE→ Standard or Vivid
② Adjusting Power Saving Mode: SMART → SETTINGS → PICTURE → POWER SAVING MODE → Off
<Smart TV Netcast ‘12~’15>
① Adjusting Picture Mode: SMART → SETTINGS → PICTURE → PICTURE MODE→ Standard or Vivid
② Adjusting Power Saving Mode: SMART → SETTINGS → PICTURE → POWER SAVING MODE → Off
