Keep your Bernina commutator clean…
The achilles heel of any older Bernina is the commutator and the carbon brushes. Brushes wear away and the commutator becomes coated in the carbon and as a result gets too hot and burns.
If your carbon brushes wear away the springs that push them against the commutator will keep the motor turning but gouge out a nasty channel.
I would advise that in most cases (I know some will say they do this themselves) this is a job for your Bernina dealer during a general service.