I have a HF R62 that I tried to use an audio input from a mixer. I plugged in the connector and got a great sound from the mixer, but the internal microphone did not cut off, so I got a double audio feed. The internal mic was just a hair delayed so it sounds like an echo.
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Most mixers will send out a line-out signal, which isn't quite the same as a microphone signal. Your camcorder was not designed to accept audio from a mixer. Instead, it allows you to connect a self-powered or battery-powered microphone like the optional Canon WM-V1 Wireless Microphone.
If problems persist when trying multiple microphones, please elaborate on what you're seeing and what microphone accesssories you're using.
The internal mic should have been cut off automatically when you plugged in an external audio source.
To test that this isn't an artifact of the mixer itself, either turn off mixer power or unplug the mic from the mixer to see if the camcorder is still picking up audio from its internal microphone. If so, it is possible that the plug from your mixer is maybe a little shorter than industry standard and isn't properly moving the contacts in the mic jack that disconnect the internal mic when an external mic is attached. Or the jack in the camcorder itself may be damaged/defective.
On edit: I don't have a schematic for your camcorder but the most common mic jack wiring uses the tip of the microphone plug to open a set of contacts between the internal mic and microphone input line so if the plug isn't long enough or isn't fully inserted into the mic jack then your internal and external mics are run in parallel.