There were a bunch of retired brushless servos, 1st $60 ones & later $40 ones, all discontinued & in various states of disrepair.  They could have great value in pointing cameras, but only with the more precise motion of stepper motors.

1 x SPT5835W used an unknown D1669 chip  & 2 x TS-940-HG used the mighty Silabs F330.

Stepper mode servos have traditionally used external L6234's bypassing all the electronicals.  Driving the motors with L6234's had problems with the high voltage required by the L6234, the extra space, the cooling required by the L6234, & the L6234 being discontinued.  Ideally, their original microcontrollers would be reflashed with stepper motor firmware.  They would read a code from the UART containing the states of the MOSFETs.  

The SPT5835W was a lost cause.  Maybe it used an ASIC or an unknown chinese brain.  It might be possible to stuff another chip on a carrier board where the pot is, without increasing the size of the servo.

The TS-940-HG's will never be used as servos again because of glitches in their pots.  The SPT5835W might be used as a servo again.