old stepper motors stuck

Hello,

I am trying to continue to build an old mendel reprap kit which was in a box for two years (shame on me :-) )

The parts:
- Gen 3 electronics
- 4 steppers motors of type SY42STH47-1684MB

I am currently trying to test the stepper motors but I noticed that it is impossible to rotate manually the shafts of 3 of the 4 motors.
Does it mean they are broken ? Is it possible to repair them ?

I tested the only remaining motor and it was possible to make it rotate only for a short time after the potentiometer was rotated nearly completely clockwise.

The motherboard has been programmed with the teacup firmware.

Any idea on how to proceed from now ?

Leszek