If you are using a cable or satellite box you need to adjust the settings there.
For example, on a standard Comcast cable box you need to power on the box. Press the red Menu button which will bring up a menu with choices on HD resolution and what to do about SD channels. Options should include 480 and stretch. If you want the SD channels to fill your screen to need to choose stretch.
Press the Menu button after making your selection and now power on your cable box. Go to an SD channel and the image should completely fill the screen.
Hello Everyone -
Thank you for taking time to assist. It is appreciated. How do I get my screen to fill completely with non HD channels? My HD Channels fill the screen completely, but the Non-HD channels only fill the middle, leaving a lot of BLACK all around the picture. It is very frustrating to have a big tv with a little picture in the middle. I just recently purchased the UN55C7000WF and so far have been very pleased with the performance and picture. Any assistance you can provide would be greatly appreciated. Thank you.