Someone posted here on amiga.org or amigaworld.net a guide to make the a520 modulator into outputting svideo instead of composite. This guide seemed simple enough if you have some soldering and some basic electronic skills. They also showed pictures of how it was done and also screen grabs for comparision.
Edit:
http://www.amiga.org/forums/showthread.php?t=25836The guide itself:
guide
