This is an old post, preserved for reference.
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Many home computers of the 1980s were produced in various versions with television outputs to suit the region. Most had NTSC versions for North America, and PAL for the UK, and some also had SECAM for France. The differences were sometimes just down to a jumper, or needed various modifications, even different circuit boards.
Penultimate Cartridge, it turned out that Paul Rickards was in the USA with a selection of NTSC VIC20s, and was interested in a PAL one, so we concocted a plan to swap one. Rather than sending a whole VIC20, we just exchanged crystal, VIC chip and kernal ROM. That should be easy enough, just swap those bits over and away you go. Here's is Paul's proof that his NTSC VIC20 is working:
I have not been able to locate a decent schematic for the PAL version of the VIC20-CR, and only some ropey scans of the NTSC version, this is stitched together from two of those scans to show the video output section.
In order to convert my PAL VIC20 to NTSC, I need to get from that modified schematic back to the original. I removed the modified parts and assembled a pack of bits to send over to Paul so that he could do the reverse, and convert an NTSC VIC20-CR to PAL.
The PAL bits have now arrived in the states, and Paul's NTSC machine has now been converted. FB4 removed and one end of FB5 swapped over to the end of FB4 end a wire link fitted from the spare end of FB4 to the spare end of FB5, C13 removed and the new 220nF axial capacitor fitted linking to the top of R10, the track between pins 2 and 4 of the 7402 broken and a 1nF capacitor fitted between the two vias, and finally a 56pF capacitor added on the back across R5.