Weird Mega Drive/Genesis S-Video problem

[Mangaman]

I'm not necessarily looking for a solution with this one, just some discussion since there are plenty of video gurus on this forum I've recently thrown in some S-Video mods to model 1 Mega Drives and have noticed some weird artefacts in the aftermath, and wanted to get feedback on what I might be doing wrong, or at least hear some thoughts on what might be causing the problems.

I did my first s-video mod by doing the shonky method (cap + resistor on pins 15 and 16 on the CXA1145) a few years ago and managed to get a reasonable signal - colour was nicer than composite and it was clearer, but the image was littered with rainbowing (cross colouration??). But for all intents and purposes, it worked, and was nicer than vanilla composite output.

Anywho, ended up selling it off on eBay to make some quick cash, and grabbed a very early model 1 Mega Drive and did the same mod. I didn't have much in the way of decent-quality multi-strand wires lying around, so I used single-core wires (actually from some telephone cabling I'd bought a few years ago [don't know why ] and hadn't used) to run the signals off the CXA1145, through the cap+res and to the s-video jack I added to the machine. When I plugged it in, it all seemed okay (colour was a bit dull - resistors must have been a bit too high), but in some games the screen would momentarily turn to black and white and distort - it happened on the "Sega" logo part in Sonic 1 and 2 before the title screen came up, Konami's Tiny Toons platformer suffered from it for the whole game, even NHLPA '93 suffered from it when you actually played the main game. It kinda looked like when you tune in an RF signal and it's kinda there but isn't, and the image suffers from screen tear and is in black and white instead of colour.

I was having a whole lot of trouble with this machine (50/60hz switch and language switches weren't behaving themselves - I think it was a BIOS thing though, as the machine was one of the ones where the 'Produced by or under license from Sega' didn't flash up before a game started, and by flicking the switches at the right moment I could bypass the tricky territory-sensitive games like Gunstar Heroes; I've done more 50/60hz and language switches than I can remember, and this is the only case where I've had trouble, and it's also the only super-early model 1 Mega Drive I've worked on), so I've shelved it and grabbed another, later-vintage model 1 Mega Drive.

So, I did the same mod after I had problems with the proper s-video circuit on the wiki introduced too many artefacts, and went back to the single-core wire, cap+res method. Lo and behold, the exact same problem reared its head! The weird thing is that this time I bothered to test RF and composite output as well, and the same artefacts appeared there as well (not to mention tons of cross colouration on the s-video output by default). So I took off the wires that were connected to pins 15 and 16, and the problem vanished on the RF and composite signals.

So, any idea what the problem might be? Am I simply rubbish at doing video mods? Is it the single-core wire? And is it possible to remove the cross colouration I always seem to get with s-video connections?

I thought maybe someone else out there has encountered this, or if not, at least this might be news to some of you modders out there and you might find it interesting!

-Mangaman-