This project was something I'd wanted to do for a long time and finally getting it to work was a rush. A colecovision uses a Texas Instruments TMS9928 video processor IC which outputs color difference signals: luma (Y), R-Y, and B-Y. These are sent to an encoder IC (LM1889?) which produces composite video (luma + chroma) for the modulator. However, for s-video you need luma (black and white) and chroma (color info) separately.
My first step was to check the signals on the encoder which sits upside down in the shielded RF box. No chroma anywhere, just composite. Then tried various circuits to subtract the luma from the composite but that was too difficult and wouldn't have had the full sharpness anyway, because separating chroma from luma can never be as good as when they are separate to begin with. Finally, noticed that the luma didn't seem to be routed into the encoder anywhere, so cut the composite video output trace. Viola, had chroma on one side and luma on the other.
Another thing I was doing the whole time was investigating A1k0n's CV composite mod which is based on an op-amp. Couldn't really tell if it was better than my older mod which used a 3904 transistor, seemed about even to my eyes. However, I'm still recommending it because all the other consoles have 3904 mods and doing them gets boring.
Guess that enough blathering, here are the assembly instructions:
1. Do the composite video and audio mod at: A1k0n's Site (Now that's what I call out-sourcing!) but don't mount a composite video jack and make sure to solder the video cable to the test pad mentioned, not the row of pins on the side. 2. Verify that the composite looks okay, quit now if you're tired :) 3. Cut the trace between the test pad and the third pin up, verify it's cut with an ohmmeter. 4. Add pullup resistor 50k-60k to test pad, it's easier to do this at the encoder IC, just follow the trace back and connect it to the 12v line at the far left encoder pin. 5. You now have chroma on that line, but won't be able to see it without luma. 6. Luma is found on the right side (farthest from RF box) of L9 at the right. Give it a series 75 ohm resistor, then solder the cable on. 7. Solder ground shield of both cables to a close ground point to the two signals. You can use a continuous cable or mount an s-video jack according to taste. 8. After I did this, the picture was sharp with strong colors, but there was faint vertical lines, like an NES top-loader but not as bad. Still trying to find a fix for that, email if you know it. 9. The encoder might be touchy with the 12v pullup resistor, you can also try a 10k pullup to 5v (which is only outside the rf box).