Modded a NTSC-U NUS-CPU-03 Nintendo 64 some time ago for RGB and had a SCART cable to test it with finally. Thing is, it is in black and white only. I've had red show up alone and blue will mix with red for a lovely purple but green simply didn't seem to do anything on its own until I redid the soldering on my SCART->BNC adapter and found it now did black and white only. The occasional yellow hue would take over. The cable is fine as the owner sent me a SNES jr to mod for RGB and the SNES jr looks lovely in RGB on the same displays (Sony PVM). There is an amp inside using a Texas Instruments chip with RGB tapped from C124, C125, and C126. I realize I could tap RGB directly from the video encoder but this seems like a better idea.
I've done a couple of these including my own so I might be of some help. Do you have 75 ohm resistors hooked up to all three outputs and do you have a 0.1 uF cap going across the voltage and ground pins of the THS7314?
It might be worth trying although I've never had a problem soldering to the caps underneath the board.
I think I know what is your problem; Did you test the Scart cable before modding the nintendo console ?
I did. Though I think I may have found the problem. It was almost black and white using composite (some color was visible). I removed the amp and redid the wiring and composite has its color back.
