Hi,
Some TVs also use the SCART socket's composite as a synchronisation signal - if this is missing (which it will be when going from the Amiga's 23-pin monitor socket), you will not get a picture on such a telly.
The most basic test you can do from an A500 is to grab a lead with an RCA / phono socket at each end and go from the Amiga's composite video out to the TV's composite video in.
On the A500, this will be in glorious monochrome, but at least you'll be able to tell if it's working... To get colour composite video (and RF), you'll need an A520 video modulator as mentioned above.
- Ali