modding a coax input tv to s-video

[N2IXK]

You need to be VERY careful attempting a mod like this.

The "ground" point on the TV chassis is actually connected to one side of the AC powerline (either directly or through one side of a diode bridge), whereas the ground point of an external video input needs to be actual earth ground referenced.

Your circuit would need to incorporate proper isolation (optical most likely) to get the video signals across the potential difference between chassis ground and earth ground. Otherwise, you will have a severe shock hazard, and potential sparks and smoke when you connect an external device to your new input connector.

[Zenith26kc20]

chances are there is no easy point to inject video if the set uses a "jungle chip". Also, a number of this era set used opto coupling to prevent any chance of grounding the source thru the power supply.Can you post part of the video schematic?