hello, i have an old briot 5000 S edger with a problem that i cannot resolve yet. in old post ive asked for help but i cannot find it. i would appreciate if anny suggestions cause i dont have more ideas on how to solve the issue.

the issue:
when i put a lens on the holder to be worked the hoders put a lot of pressure on it and damage the coatings
ive checked if has a way to calibrate the pressure but dont find anything that can be adjusted. i think that the holder has to close and retain a little pressure over the lens but not so many.
ive checked connections and noted thar te voltage applied to the motor who closes the holder is near to 20V, when the holder pplies force to the lens the voltage goes down to 4-6 volts but allways is appliing voltage (i think that is the problem, the motor never stops because allways is receiving voltage). when the edger lift up the lens to change to another weel (for veveling) the voltage in the motor goes up again and whoe comes down the voltge goes down again to 4-6v

i have photos of the circuit connected to the motor and a diagram too if wanted