I have finally assembled my first tricopter, but have run into a problem with the yaw servo oscillating heavily on ground and disapears completely in air, as I've never seen or heard of this problem I wanted to ask you.
Here is a video showing the problem:
Is there a calibration step I've missed?
Try this, put your tri on a carpeted surface or some other "non-slick" surface and arm it and see what happens. Your desk is a slick surface, when arming the initial movement of the yaw assembly causes the tail to move slightly which the APM tries to compensate for, the compensation causes the tail to move in the other direction causing the oscillations. I had the same happen to me.
It didn't work, but it flies great with no trouble.