I've now tried to check all the wiring on the board, and everything seems to be in order...
I've tried to turn the two screws on the PCB to see if that changed anything - and it didn't :-(

The motor seems to be working (since it turns if I hold the chopper in the main rotor and give it power - then the body turns, and the rear-rotor spins out of control).

Do you have any other ideas of what it could be?