I recently purchased a Tarot 690s hexacopter , it was partially assembled (e.i the ESCs and UBEC were already soldered to the PDB). The Flight controller is an Arducopter APM 2.8 and the TX/RX combo is The RadioLink AT10 and RadioLink R10D. The problem I have is, upon take-off the copter immediately tips over backwards and the rear props hit the ground. I've tried everything I can think of. Double checked the props and motor rotation, the signal inputs and outputs, everything. Reinstalled the firmware for the APM. Updated the firmware for the transmitter. Still, nothing worked.One would think that you could just trim something like that out with the Transmitter, but I tried it. This led to the discovery that the copter isn't responding to any input other than the throttle. It doesn't respond to the yaw, pitch, or roll inputs from the transmitter. Nor does it react to its own internal gyros or potentiometers, when you tip or rotate it manually. Nothing but the throttle. In mission planner it shows the the transmitter is sending all of the signals to the apm, but it doesn't react to them. I don't know how to fix this at all.