Turbonanza PC Engine Mod High-Percentage Noise

[MaRC]

I have several PC Engine consoles—CoreGrafx 1 and 2—and installing Turbonanza requires cutting the metal shield that holds the spring-loaded balls. I didn't like this, so I redesigned the board for installation in the PC Engine. Everything remained essentially the same, but I removed the components responsible for Svideo. I output the image and audio to a replaced DIN9 connector, removing the resistors connecting the original audio to the connector. Everything works fine, the image has minimal banding, but high-frequency noise has appeared in the audio. It's independent of what's happening on the screen, it's not interference from the video. I use a very high-quality cable with shielded wires. If you re-solder the original audio, the high-frequency noise disappears. Is this a flaw in the original mod? Did I mess something up when redesigning it? Or is this noise coming from the additional channel the mod mixes with the main channels? When I get home, I'll try measuring the noise frequency with an oscilloscope.