Hi,
If you can open the case to look at the circuit board, it looks like you can inspect pin 20 on the video encoder IC to determine whether the unit is PAL or NTSC.
The pictures on
http://amiga.resource.cx/exp/a520 shows the encoder is an MC1377P. According to the datasheet for the chip (
http://www.datasheetcatalog.com/datasheets_pdf/M/C/1/3/MC1377P.shtml), it supports either NTSC or PAL. To configure for NTSC, pin 20 is grounded. For PAL, pin 20 is left disconnected.
I don't have a handy way to highlight pin 20 in the pictures, but it's one at the bottom right of the chip in the pictures showing the back side of the PCB. Use the top side pictures to see where to find the chip, then flip to the back side to find the two rows of 10 pins.
Both of the boards shown on amiga.resource.cx appear to be PAL units, as there is break in the trace that connects to pin 20 before it reaches the ground trace. On my NTSC unit, this break isn't present.
I'm not sure if they used the MC1477 on all units.
The 520 is kind of annoying to open. There are four cylinders molded into the top case that have a friction fit onto pegs in the bottom (think lego block). There aren't any clips or screws, it just takes careful prying around the edge to separate. There's an RF shield that would need to be desoldered to inspect the top side, but since all the PCB traces are on the bottom, that's not necessary.
Robert