Sega Mark III external RGB amplifier with FM audio break-in

The Mark III’s composite output is pretty bad.

M3RGB_0001

It does have RGB Out, using the same pinout as the Master System and Mega Drive.

Mark3FM2_1620

Unfortunately, unlike later Sega consoles, it’s an unamplified signal. So while Master System and Mega Drive RGB cables fit, the picture comes out far too dark.

I didn’t grab a picture of the native RGB output, but it looks something like this:

M3RGB_0005a

Rather than tamper with the console internally like I usually do, I had a theory I could use the 5V and ground pins of the output to power an external RGB amplifier. I grabbed a cheap RGB cable and spliced in a THS7314 based amplifier circuit (commonly used to RGB mod Nintendo 64s), powered and grounded by the console.

M3RGB_0002

Success!

RGB

The only issue was that like many older Sega consoles, it has a weak sync signal, and on my XRGB Mini some sync dropouts occurred. I needed to boost the sync as well. Rather than do this myself, I ordered a Mega Drive RGB cable with boosted sync built into the scart plug. It’s also powered by the 5V output of the console, but I’ll wire my amp in parallel and there should be plenty of current available.

An additional complication is that when using the FM unit, you only get composite video. You route video through the FM unit, which mixes in the FM audio when appropriate and outputs both video and audio from its own AV out. But it’s only a 5 pin din, so no RGB.

Mark3FM2_1623 Mark3FM2_1621

I did some quick tests, and due to the way the circuit of the FM unit is designed, you can tap the FM audio from both the input and the output. So I’ll be able to use the existing cable of the FM unit as my FM sound source.

M3RGB_0003

Works perfectly! But doesn’t look too nice like that…

M3RGB2_9999

So now I have to get the whole thing into a little project box. I drilled the holes and sliced up my new (nicely shielded) RGB cable. In this pic you can see I’ve added a 5-pin DIN socket to one side, with four of the output pins removed. This is where I’ll splice in the FM audio.

M3RGB_0011

 So many wires to fit in. Red, blue, green, sync, 5V, audio… luckily most of the rest are ground, so I can just solder them together and connect them with a single wire. I could just leave them disconnected, but I want good grounding to prevent possible interference, which is common for poorly grounded scart cables.

M3RGB_0012

With the video circuit working, I lined up the wires and hot glued them in place, so nothing can be pulled out by the inevitable cord-trip that will happen sometime in the future. I also glued the chip down.

M3RGB_0013

Disaster struck just before I finished – the picture was too bright! It turned out my new scart cable was wired differently, and was missing some 25 ohm resistors the old one had on the RGB lines. So I pulled them from the old one and spliced them in carefully, in series with the 75 ohm resistors already built into my amp circuit. Now what was once a halfway neat job became a mess again, oh well.

 I also completed the audio circuit, spliced in the FM audio pin of the din socket, and closed up the box.

M3RGB_0015

It connects like so. I have oriented it toward the right of the console, because the power cable connects on the left.

M3RGB2_1631

Very neat.

M3RGB2_1632

And the results, all through an XRGB Mini to my Panasonic Plasma:
M3RGB2_1648 M3RGB2_1645 M3RGB2_1641 M3RGB2_1635

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4 thoughts on “Sega Mark III external RGB amplifier with FM audio break-in

  1. Steve December 20, 2016 / 11:33 am

    I just bought a Mark III, could you make me one of these?

    Like

    • D.Lo December 20, 2016 / 12:35 pm

      Hi Steve, it would be quite expensive, the parts would only be $50ish but there is a lot of labour involved.

      Like

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