Modding coax game plug/adapter help

I have a DC coax cable adapter. the one that hooks up to the crappy antenna input of your tv to play games on channel 3 or 4. Essentially this is a reg A/V cable to support the antenna input. I took it apart and added A/V support. Works good by the way and now when i try to use the antenna hook up I get a fuzzy pic. Any thoughts on how to cure this problem. Also is this the standard making and antenna input of of the A/V? Id like to be able to use an old ninteno with a/v instead of the coax.