No signal on pc tv tuner with analog

I am a long-time Comcast customer with basic (analog) service and cable internet. I recently purchased a computer with an internal TV tuner combo (receives analog or digital). Everything is hooked up correctly. When I run the set-up in Media Center, it detects the card and sets up all my local channels, but none of them have any signal.

I tried calling Comcast technical support, but they had no idea what to tell me. Can someone please help?