mmmonkey already has a great guide, but I thought I'd post as much info as I can anyway.
Some things you'll need:
- A/V socket. A "U" shaped 8 pin DIN (on the LEFT of the picture) will allow you to use the Mega Drive 1 SCART lead, but you can use another kind if you want and make your own lead. A good place to get the DIN sockets is here. I thought I'd ordered 2, but received 10, so a pretty good deal.
- Single Pole Double Throw switch (SPDT). Slide type is fine too, I just didn't have the right screws for it at the time.
- Screws for the DIN socket. I got these on eBay. Can't remember if the nuts came with them or if I got them elsewhere. But you'll know what size to get from the screws. If you have any old computer hardware lying around you can actually use these. They're a bit more awkward to work with though.
- Some wire, solder, vero board, 4 x 220uF capacitors and 1 x 10uF capacitor.
I start off by making the holes for the switch and DIN socket. The hole for the switch is easy, but I don't have a drill bit big enough for the DIN socket so I have to sand it down, which takes ages. I do this by wrapping some sandpaper round a pen/pencil. You can remove the RF modulator and use the existing hole and make that bigger, but then you either need to have the wire go all the way to the right then back again or try and fit the board in place of the RF modulator, which I've actually done before it's just not very tidy.
For the 50/60hz switch you need pin 57 on this chip (315-5426). Wire it up to the middle point on the switch and then +5V & ground to the left and right points. I'm not sure if any Master System games are actually region locked so on my main console I just wired it up to 60hz permanently. You can use the negative side of capacitor C18 if doing this. For the switch I wired it to top-right capacitor.
There are some good pictures showing how to make the board and how to wire it up to the DIN socket on mmmonkey's site so I won't bother copying them over.
Here's what the finished thing looks like:
Here's where you need to wire it up on the CXA chip. You can use composite or C-Sync. Personally, using composite gives me jailbars so I use C-Sync.
Audio from negative side of C31.
Everything wired up:
...and how it looks:
