The cable feed is supposed to be grounded where it enters the building, this is a safety issue and in many areas is a code requirement, so wouldn't it be more professional to make sure the grounding is in place and working properly?
Here is an easy fix if it is a ground problem.
Cable drops should be grounded to the utility ground where they enter the building, but my experience is that it usually doesn't happen. The closest ground may be outside the building or even several poles away from the building.
Very inexpensive matching transformers are available ...used to connect 75 ohm cable TV to a set that has flat wire (300 ohm) inputs. Take two of them, hook the 300 ohm sides together. TV cable input goes on one end, and the TV on the other end. I've had the problem dozens of times in home theater systems and the this fix has never failed me. For convenience I usually use a standard matching transformer on one side, and a push on matching transformer on the other. The push on is the same electronics internally, but made to go the other way. (connecting a coaxial input TV to a 300 ohm antenna) Hook em together, use the push on on the TV input, and screw the cable into the other end and the problem goes away.
The TV can no longer see the cable ground through the transformers and signal loss through the pair of transformers isn't significant in any well designed TV distribution system. I'm in the TV and home theater business, so buying these pieces in bulk is under two bucks for me, but you could probably buy them at Radio Shack or any other retailer for less than five. It a whole lot cheaper than audio isolation transformers!
JC
Thank you for all the advice. When the cable and the s-video lines are both connected it cause the ground loop. very strange.
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