Finishing and calibrating the K8200, I had problems with adjusting one of the motor drivers. The voltage that I measured was unstable, going up and down randomly. The related motor (it was the extruder) did not function well, it missed steps even without filament, it had a low torque, I could easily stop it by hand.
I measured the potentiometer on this driver, compared with the others, and concluded that it was faulty, causing Vref to float.
Luckily, I had a spare potentiometer. Altough the value that I had was not the same (10k instead of 20k, thus giving a different range in combination with resistor R5), I gave it a chance. I replaced it, and it worked.
I have seen similar problems with drivers on this forum, and perhaps there is a common cause: potmeters that didn’t survive the smd oven.