I'm guessing it's the Sega Master system, right?
But I could be wrong. I don't know of any systems older than the master system that are capable of RGB output. But I just wanted to be sure.
These are the systems that I know output a RGB signal:
1. Sega Master System - The Master system has the same audio/video plug as the Genesis, so if you have a Genesis RGB cable you can use that on the Master system as well. Of course you can also get the Master System converter that plugs onto the top of the Genny and get your Master system games in RGB that way too.
2. TurboGrafx-16 - The Turbo's RGB signal is kinda weak, and I think you need a RGB cable that has some aplification chips in it, to a get a great RGB signal from the Turbo. I think TurboDuos need to be modified internally to output the RGB signal.
3. Sega Genesis - The Genesis has a solid RGB signal, I don't even think you need any special chips to amplify the signal. In other words if you can solder stuff, then you can build your own. To get RGB from a Genny 2, you either need a specific Genny 2 RGB cable, or an adapter for the Genny 1 RGB cable that converts it to plug into the Genny 2. I'm almost positive that the Genny 3 doesn't have any RGB capabilities.
4. Super Nintendo - The SNES has RGB output, but the SNES 2, or SNES jr had the RGB capabilities removed.
5. Sega CD - Sega CD outputs it's RGB signal through the Genesis a/v port.
6. Neo-Geo - Neo-Geo has RGB output
7. Atari Jaguar - Jaguar has RGB output
8. Sega 32X - The 32X has RGB output, but you need to have a Genny 2 RGB cable or an adapter for your Genny 1 RGB cable.
9. Sega Saturn - The Saturn has RGB output
10. Sony Playstation - The Playstation has RGB output
11. Nintendo 64 - The N64 needs to be internally modified to output RGB. Not all versions of the U.S. N64 can be modified, and I don't think any PAL N64's can be modified. Also, the N64 RGB cable needs some special aplification chips in it to amplify the very weak N64 RGB signal.
12. Sega Dreamcast - The Dreamcast has RGB output. But since 90 percent of the DC games are progressive scan, might as well use the VGA Box and hook it up to a PC monitor or a HDTV.
13. Sony Playstation 2 - You can use a PS1 RGB cable with a PS2 no problem.
14. XBOX - I believe that there is a XBOX RGB cable that you can get, but I don't know why anybody would play their XBOX in RGB. Might as well play it in progressive scan on either a PC monitor with a "TRUE" VGA Adapter (not the fake ones) or play it on a HDTV.
15. GameCube - There is a way to get RGB from the GameCube, but it is expensive. It requires somebody to hack Nintendo's special component cable. The Cube might need to be internally modified too, I'm not sure.
16. Game Boy Advance - If you have the necessary items to play your Cube in RGB, with a Game Boy Player for the Cube you can actually play Game Boy Advance games in RGB on a RGB monitor!!!!!!!
That's all the systems that I can think of at the moment. I might have left something out, like the SuperGrafx or something like that. I'm not sure about the FM Towns Marty or the NEC PC-FX.
Notable systems that lack RGB capabilities:
NES - No RGB output, although I've heard of an super duper expensive hack of a Play Choice that will allow you to play NES games in RGB. But I don't know anybody that can really confirm this or not. It's just rumors as far as I can tell.
Phillips CD-I - I don't believe the CD-I has any RGB capabilities but I'm not 100 percent positive
Panasonic 3DO - I know for a fact that playing 3DO games in RGB is impossible.