Do you have any composite (yellow rca) cables connected to the the rca's by your S-video? If you do this could be the problem, some tv's require you to pick one of the two (s cabel or yellow rca type). If you do find this then disconnect your rca and connect just your s-video and see it will work this way. If you have already thought of this then I am sorry to have wasted your time.
Pete
Pete