I've always wanted to do an AV mod for the original Mega Drive but up until now never got around to doing it. I intended to do this mod a little different and avoid drilling holes into the casing for the S-video, composite & stereo audio output. The standard AV port doesn't provide enough pins to cater for the extra signals so the only other option was to use the EXT 9-pin port at the back of some of the early models. If your Mega Drive doesn't have the EXT port, this mod isn't possible and you'll have to resort to drilling holes.
The EXT port was originally used with the Mega Modem in Japan and possibly a few other peripherals. For the most part however, the port is pretty much useless. By utilising the existing port, it allows the use of the Power Base Converter which would otherwise be unable to be used as the AV output jacks would be in the way if installed in the casing.
Step 1: The Serial Cable
I went to my local electronics shop & picked up a standard DB9 serial cable with straight through wiring. The cable I bought was around 1m long, but you only need 20-30cm of it. Don't make it too long to avoid possible interference issues.
After cutting the cable I realised the plug wouldn't fit properly as the MD case protrudes a little further than the EXT port. So I chopped the sides off & folded up the metal edges. A piece of heatshrink tubing then finished it off neatly.
Step 2: Onto the EXT Port
So here's the port in question. As you can see there are a number of traces leading to it. You have the option to either cut those traces or remove the row of black components just behind it. It's a fair bit of work removing the components and as they have three pins each, I wasn't certain if they'd cause other issues if removed so I just cut the traces instead (marked in red).
There is also single trace on the top which leads to the green diode type component. You don't have to cut this trace unless you're planning on using all nine pins. We won't be in this mod, so leave it alone.
Just worry about cutting the traces to the top five pins, that is all we will need to cover composite, S-video & L & R audio output. Ground is covered by the outer casing.
Step 3: Connecting the AV Outputs
Before getting to this point, it's necessary to do the S-video mod if you haven't already, just pick one of the many tutorials for this on the internet. I opted to make my own little circuit on a piece of breadboard for neatness. I also installed the switchless region & PAL60 colour correction mods while I was at it, which is what the other wiring & components are for.
The wiring can be done in whatever order you like, I chose this particular order so none of the wires would overlap.
In order to proceed, the wiring pinout from left to right is as follows:
Pin 1 - Composite output, straight connection from the corresponding pin of the regular AV port
Pin 2 - Right audio channel output
Pin 3 - Left audio channel output
Pin 4 - S-video Luma output
Pin 5 - S-video Chroma output
Ground is connected via the outer casing. Easiest way to do this is to remove one of the screws holding the port in place, wrap a wire around it & then screw back into place. The other end of the wire can then be soldered to a ground point on the main board.
Step 4: Making the External Break Out Box
With the MD wiring done, it's time to get onto the break out box. I used a small Jiffy box as the enclosure and marked the holes with the aid of some painters tape, regular masking tape works just as well.
With the holes done, thread the cable through & secure with a cable tie so it can't be pulled out. Next install the S-video & RCA sockets and make a ground daisy chain & solder to the ground connections on each socket.
Next you need to determine which pin corresponds to which colour wire, a multimeter is a must for this otherwise it'll be next to impossible. For reference though, my cable was wired the following. Other cables might be different.
Pin 1 (Composite) = Red
Pin 2 (Right channel) = Purple
Pin 3 (Left channel) = Black
Pin 4 (Luma) = Grey
Pin 5 (Chroma) = Green
Once all the wiring is done, before reassembling the console, it's worth giving the system a test run to make sure everything is working properly. There's nothing worse than putting the console back together & then realise you've accidentally made a mistake somewhere or got one of the wires mixed up.
Step 5: Finishing Up
If all has gone well, you will have a brilliant picture & audio all while keeping the console's appearance as original as possible.
Time to play some games!