That Bernina 830 is my favorite machine to service! To check the tensions, put a dark color thread in the top and a light color thread in the bobbin, then set the stitch width on 3 and the stitch length on 2. Sew with it and see if the top thread pulls to the bottom or the bobbin thread pulls to the top.
The fabric you're sewing on can create a crooked stitch. If you use a finer fabric and a smaller stitch length, the stitches will look straighter.
The 830 came with 2 different foot controls. If your foot control has a potentiometer on the bottom (looks like a small, black plastic screw), you can adjust it to get smoother power to the machine. If it stutters, no matter where you adjust the potentiometer, there's a capacitor that can be replaced to fix the problem.
If your machine doesn't have the potentiometer, you've got a carbon pile foot control. If there's a capacitor in that foot control, it can be a fire hazzard. Your Bernina dealer will have a free part (thermal switch) to replace that with.
You can usually check the motor brushes on an 830 without taking the motor out. Some models have a plug on top of the motor housing that pops out, then you twist the plastic screw, and the motor brush will pop out. Other models you have to remove the motor housing. You can clean the armature or commutator if it looks black, with a folded bit of sandpaper, while running the motor.
There are lots of little tweaks you can do on this machine to make it so fabulous! Needle/hook distance, thread passage, polish the hook, adjust the feed dog height. You have a great machine!