Some observations:
1- When unit is off and giving little force by my finger to the grating mirror frame it rotates freely without any resitance .
2 - When the instument powers up, after 2 sec. the grating mirror starts to rotate slowly in a constant speed and is very stiff for about 1 and a half revolution and then it stops in a locked/"braked" position giving the grating error code. The machine stays at this state until I switch it off, then the "brake" realeses .
One intersting observation is that during the 2nd step when it starts to rotate for about 1 an a half revolution, if I force my finger on the mirror frame , it goes free from the locked state and the grating errors appears upp directly after that. So I suspect that the motor control is working , the encoder detects thatthe mirror is stuck by my finger and realeases the locked rotational state to relaes the mirror free giving the same error.
also, if I unplug the encoder socket to the controll board and power upp, the mirror will try to rotate on start but in unlocked manner (swings a little and tatty) but stops directly giving the same error.
It seems some how the encoder and the motor circuit works (?) but not 100% correct since it can not ock the mirror in the correct position.