Have you got a A520 Modulator plugged into the back of your Amiga. Without it, it ain't gonna work on any TV I know of. Should look something like this.
RF out (wire connected from here to your TV)
__l_l__
..............l
...C<......l= Video out (nothing plugged in)
..A520..l
..MOD...l= Audio in (connected to left & right audio at back of ..............l Amiga.)
..............l
..............l.......................Audio left & right
______l_____________l_l__l_l______________________
.............................................................................................................l
....................................................................................Amiga.500.....l
It's hard to do ascii art on A.Org, I hope it helps.