tell us what comcast cable box you have and where you got it. a comcast cable box must be authorized for the system it is currently being used on buy them. if you did not get it from the park your at or the local comcast office it will not work. are you trying to use a coax cable conection to the tv and tuning the tv to channel 3 or are you using a hdmi or rca cable from the box to the tv. are you getting a picture from the box and it says no signal or what. we need this info to help.