When I first RGB modified an N64 I noticed that the RGB Scart Cable never switched the TV to the correct channel. I had always assumed it was because the N64 didn’t support RGB, but upon double checking the wiring of the Scart plug, it became apparent that it was wired wrongly.
Look closely at the following photo, you can see an orange wire going to pin 16 (the right hand side of the resistor), this wire should be going to pin 8 (the left hand side of the resistor).
Pin 8 is used to switch your TV onto the correct AV channel when you switch your console on, pin 16 is used to switch your TV into RGB mode. With the Scart lead wired up incorrectly as above, not enough current is reaching pin 8, so it may not switch your TV to the correct AV channel. Simply unsolder the wire from pin 16, and solder it to pin 8, make sure the resistor stays in it’s place and is going from pins 8 to 16.
That was easy!