In this video, I show you the process I used to test my ESCs, motors, and servo before introducing the KK2.1 board to the build. I wanted to make sure that everything was working as expected before adding the flight controller, that way it will minimize what to troubleshoot if there are any problems. I also test motor directions at this point, and finish up securing the wiring.
I did have a servo buzz, and it turns out it was because there was a little backwards pressure on the servo when strapped down. I show you the symptom, and the solution. Maybe it will be helpful if you run into something similar.