Hmm...okay doesnt the Amiga itself determine whether it is PAL or NTSC and the A520 just converts RGB into RF? As in doesn't the A520 just take whatever signal the Amiga it is hooked up to regardless of whether it is PAL or NTSC and you just have to make sure you have the appropriate TV?
Most Amigas (ie., all but the very first models, with only the standard Agnus) can switch between NTSC and PAL video modes, either through early startup control, or by using a program like degrader.
HOWEVER, getting the actual video output is a little more tricky. Obviously if you're using something like a 1084S monitor, it will detect and sync automatically to either 50Hz (PAL) or 60Hz (NTSC) video modes. But your TV isn't so smart. If you're in North America, you'll need the NTSC version of the A520 to hook up to your TV. If you're anywhere else where they use Amiga's, you'll need the PAL version.
Take a look
here,
here, and
here for more info on the A520.
Regards,
Mike