Your tv has a built in tuner. When you say,"directly connected to the tv" does that mean no cable box? Have you gone to the menu/settings and selected a "source" for your cable signal? You need to do that. If that doesn't work, call your cable provider for a digital converter box. Mine supplied one at no charge.
I bought a B650 Samsung TV last week. I have basic cable and it is connected directly to the TV. I get picture and sound for few minutes, somewhere between 2-10 minutes, before the screen goes blank and displays "weak or no signal" message. If I shut down the TV and restart after 10-15 minutes the whole process is repeated. Is this due to a problem with the TV or with the cable connection? How can I fix this is problem? BTW I have no issues watching movies on DVD.