The Mark III was released two years later than the Famicom, and has slightly more powerful graphics hardware. But it still has worse sound capabilities. So Sega developed the FM unit for those customers who would be happy to pay more for superior audio. It adds in a Yamaha FM sound chip for Mega Drive-like sound effects and music.
The FM unit sits snugly on the recessed top part of the console, and plugs into the front expansion port.
A cable extends out the back of the FM unit to capture the console’s video and regular audio.
It plugs into the AV out, using five of the eight pins (so no RGB available)
You then attach your AV cable to the FM unit’s AV out, which passes through video, regular audio and adds in the FM audio for supported software.
Yes, supported software. Games had to be specifically coded to support the FM unit. This was displayed on the front of the box on the bottom left, see my copies of Kenseiden and After Burner below.
Is it worth it? You bet! It makes your Segamaku sound like a Mega Drive! Definitely a worthwhile upgrade, even though working FM units regularly sell for up to 10,000 yen these days.
Nintendo Sega Japan! 
Hi, thanks so much about your posts about Mark III. I would like make your rgb amplifier mod but I’m not good in that things 😭
Anyway I have a question about FM Sound Unit. Today just arrived my Fm Unit, I’ve connected (back and front) and works good with not compatible games but if I put a compatible game (Phantasy Star in my case) sounds like Fm unit is not connected. Any suggestion? Thanks!
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Hi rtypex,
I would try cleaning the front port of you Mark III and the connector of the FM unit with alcohol. And try unplugging and plugging it in a few times.
It’s basically like the reverse of a cartridge connector, so the same principles apply to getting carts running!
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Hi D.Lo,
Thanks for your answer!
I cleaned it when arrived but after your comment I’ve tried it a few times more and after one hour I’ve gotten it to work a few times. Maybe a cable is damaged or I still need clean more.
Anyway, thanks again! 🙂
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If it’s worked a few times that’s good. Yes you could get more thorough and test everything, but I suggest if you get it working, leave it if possible. It really is exactly like a cartridge connection.
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