It may still be your TV but in order for an Amiga RGB cable to work there has to be between 1v and 3v on pin 16.
Pin 18 also has to be wired to ground.
Sigh, Nostalgia moment. I discovered this info in about 1990 when I bought my cable from a shop in Preston, Lancashire, UK. It was a little back street electronics shop down Cannon Street. I am sure it isnt there anymore, dunno what it was called.
They were always making Amiga cables and didnt work.
I doubt anyone will know where I mean