SEGA Mark III external RGB Amp + FM Unit support

[gatsu25]

So, I have seen a few pages on the internet in the past about this RGB modification, but there is not so much info out there, and none of it is very detailed. This website here is where I have found most of my information:

Mark III Amp with Audio Break-In

I believe this is the cable I would be using for this project.

With this info, I have made an amateur attempt at making a schematic for it, so I can figure out if I understand this correctly, or if I am way off.

Mark III AMP + FM Audio Schematic

I have a few questions about things I am unsure of.

As for the resistors, the N64 amp, and most of what I see being used with this chip is 75Ω. However, he states that the cable he used, I assume the one I linked was too bright, and added 25Ω resistors to get a better result. I put 100Ω in mine, but I am unsure if this is correct.

THS7374 vs THS7314. The chip he used is the 14. Is there any advantage of disadvantage for using either one? The 14 seems to give a good result and maybe is less complicated.

Am I correct that the Sync and Composite video lines don't need to be connected to the amp at all? The pins on the Mark III are marked as Sync and Composite Video, I see a sync on the Scart plug, but what does the Composite video line run into?

Do I understand correctly that only one pin is taken from the FM Unit and goes directly into the Audio line of the Scart cable?

Am I close at all? This is the first time I've even attempted at drawing up a schematic, so any help would be greatly appreciated.

[ApolloBoy]

The THS7314 works fine, I believe the 7374 gives a slightly sharper picture but the 7314 is a little easier to work with (physically speaking anyway). I took a different route with my Mark III and did everything internally; the RGB amp is inside the Mark III itself, and then I modified the FM unit with an 8-pin cable and an 8-pin DIN jack. I left out composite video and instead wired up composite sync.

[theclaw]

Am I correct that the Sync and Composite video lines don't need to be connected to the amp at all? The pins on the Mark III are marked as Sync and Composite Video, I see a sync on the Scart plug, but what does the Composite video line run into?

Do I understand correctly that only one pin is taken from the FM Unit and goes directly into the Audio line of the Scart cable?

Am I close at all? This is the first time I've even attempted at drawing up a schematic, so any help would be greatly appreciated.

Yeah, Mark III internally amplifies composite.
You can put the composite signal anywhere you'd like. It just should arrive at the 8-pin DIN undamaged. (or reserved for future use like ApolloBoy's suggestion)

A self-contained external kit that does not modify either the console, RGB cable, or FM unit is doable if convoluted.
The steps will look something like this.

1: separate RGB to amplify
2: send composite and audio into the FM unit as normal
3: rejoin everything into an 8-pin DIN output
4: use standard Master System scart cable

[gatsu25]

Thanks for the help guys.

I will be using a Sega Master System Scart cable. As suggested on the site, added c-sync will help with any problems with the XRGB. Which is what my setup is using.

So for resistors, do you think that 100Ω would be better than the 75Ω, or is that better as a trial and error thing?

[theclaw]

Doesn't the Mark III have native c-sync? I don't think you do anything to that signal.
The console is mono by the way. Its official 8-pin DIN has only one audio output.

[hyuma]

I'm behind this mod too, but I've some issue to understand which pin use on the scart side, for now i know:

scart pin 7 = blue
scart pin 11 = green
scart pin 15 = red
scart vcc pin ???
scart sync pin ???

scart audio = 1-3 (wired together)
gnd = 13-9-5-4
sync and vcc gnd???