I think I found the problem with that TV Boy. The live pin of the RF out socket seems to have broken free of its solder joint. It also seems to be coming into contact with ground.
I also suspect a previous owner has been trying to repair this themselves and hasn't even done a good job of it.
You might notice in the pic that the board itself looks absolutely grotty, and it's been like that since before I even attacked to with Isopropyl Alcohol and a toothbrush.