The tricopter project lumbers on, delayed by faulty parts.
The recommended tail servo, a Hobbyking HK 922 Metal Gear Digital Servo, failed. It had a rough spot at about 70% of its range from new and which became worse quite quickly until the motor had insufficient torque to overcome it.
Above is the output shaft, and the final gear has a tooth which has broken / bent over jamming the transmission.
I did have another servo on hand, a TGY-113MG, which required some rework to use it (these parts are only semi-standard dimensionally). It was fitted, but has serious stability problems in this application, see the video. Measured Dead Band is 4µs, but the fundamental problem looks like an unstable internal control loop. (Dead Band on the original HK922 is 7µs.)
https://www.youtube.com/watch?v=cNbVwK3Fy3k
The model is flyable, but at low rpm the tail motor shakes severely. So, back to trying to procure a replacement HK 922 (which is presently out of stock).