@janbeta That was me the other day. I hot-unplugged the RGB cable between my CoCo3 and Tandy CM-8, both 40 years old, and it now only works with pin 2 disconnected (https://old.pinouts.ru/Video/Cm8CoCoRGB_pinout.shtml) and it produces a buzzing sound. I haven't diagnosed it.
CM-8/CoCo RGB pinout and wiring @ old.pinouts.ru
Pinout of CM-8/CoCo RGB and layout of connectorAvailable on the Tandy/Radio Shack Color Computer (CoCo).