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The question of clock speed is an interesting one. I've heard that the difference in speed between PAL and NTSC consoles causes Pilotwings (European) to fail to work correctly on a US SNES, even when set to 50Hz mode! I haven't tested this myself, however, and I'm not aware of any MD or SMS game where it makes a difference (we're talking differences of the order of 0.01MHz here), so I think it's by and large a red herring. It was certainly never used for protection on the MD or SMS; I'm not sure that it could be detected at the software level.
Anyway, to cover all the bases:
SMS: You'll need to build a cartridge slot adapter in order to play Japanese Mark III/SMS/SG-1000 carts, since these have a very different shape and pinout to the European/US carts. If you have a SMS 2 you'll also need to build an adapter for card-based games. (or graft it to the board somehow, if you want to avoid the use of adapters).
In addition, you'll need to replace the BIOS, as the US and European SMS BIOS do a copyright check (similar to the later MD models) which locks out most Japanese SMS games and all SG-1000 games. You can program a 2764 EPROM with the Japanese BIOS (downloadable from SMS Power) and solder it in place of the existing one on the board.
It's probably easiest to start with a Japanese SMS 1 and mod that (the expansion port can easily be modified to accommodate US/European carts, and there's nothing else that needs doing besides the 50/60Hz switch).
A language switch mod is not essential - the language setting was never used for protection on the SMS - but 50/60Hz is.
MD: cartridge slot modification needed (the opening has to be enlarged slightly to accommodate Japanese carts; US ones are the same shape as Euro ones) in addition to the 50/60Hz and language switches. As ExCyber mentions, there is BIOS protection on MD2 and later MD1 models which locks out older unlicensed games, so you need to find an early version of the Mega Drive 1 which lacks this protection (you can tell an old version by the fact that it *doesn't* display "Produced By Or Under License From Sega Enterprises Ltd." on start up).
SNES: See comments about clock speed (which I haven't tested at first hand) but by and large, yes.
I'm not sure about the SA-1 chip; I have a US Super Mario RPG (with the SA-1 chip inside the cart) and it works fine on my European SNES with disabled lockout chip. I have heard of it *not* working though so it could be to do with different board revisions, or maybe even different revisions of the SA-1 hardware.
If that does prove to be a problem, you could use a switchable lockout chip (e.g. add a lockout chip from a US cart with a switch to flick between the two chips) or maybe program a PIC chip to emulate either version, if that's possible.
Mike |