Remove thread caught on shaft Bernina 807 Minimatic?

Bernina Minimatic 807. I have thread caught on the shaft behind the bobbin raceway. I am hesitant to start unscrewing at random to get to the area--there are several screws around, but some appear factory sealed, there's a very large screw in the back that looks like a master screw--and I don't know if anything is spring loaded or if I will cause a Big problem. Any thoughts?