The 830 models varied so I'm not sure. My machine looks like this: Bernina 830 sewing machine feet pedal accessory box table instruction...
If you scroll over to the photo of the motor, there is a white plug which pulls out. Underneath, there is a screw. The carbon brush may be there. However, some 830s had the speed switch on the machine (right above the knee control) and others have it in the foot pedal.
I found the following information in my computer files that may be helpful: Bernina 830 Motor Brush Replacement
My machine is actually a Bernina 830 (mechanical 34 years old) I need to know how to get the housing off the motor so my hubby can replace the brushes
Re: sewng machine motor housing
DISASSEMBLY TO INSPECT OR CHANGE BERNINA 830 MOTOR BRUSHES:
Remove the big plastic cover piece on the bottom of the machine with the single screw in the center.
Remove the machine's end panel by removing the single silver screw from underneath the bottom middle of the side panel. Pull the bottom of the side panel out and down to remove it. It comes off easily.
Remove the two screws under the motor assembly (silver screws with black washers).
Swivel the motor assembly enough to remove the belt.
There is a another plastic panel on the motor assembly over the motor itself. Remove the two phillips head screws to remove this panel.
At this point you can see the commutator and inspect the "business" ends of the brushes. (In our case, one brush was almost gone and there was nothing left of the second brush except the spring!)
You can then slide the motor out of the housing. I pushed on the motor pulley and pulled on the other end of the motor with pliers. The motor comes out rather easily, but be careful not to disturb the wires any more than necessary.
With the motor exposed, unscrew the caps over the brushes and remove the brushes.
Our motor's commutator looks significantly worn and the motor may need to be replaced.
FWIW, if your machine hasn't been oiled for a long time, you should probably take time to do that. When the old oil solidifies, it sticks the moving parts together. Forcing the motor to try to run the machine can damage the machine. Use fresh good quality sewing machine oil or (I use) liquid Tri-Flow Synthetic Lube and apply a couple drops every place metal rubs on metal. (Wiggle the handwheel and watch where things move.) Do NOT oil belts, synthetic gears, cams.