Thinking back...my 301A has never had a thread jam ... never, ever... graduation present 1957, sews everyday.
However the Bernina 1630 and the Bernette have had thread jams but never so much that I had to take anything apart...they are ZZ and that is usually the mode they are in when they jam. I have a question about this..why would anyone turn the wheel backwards???? the wheel always turns towards you...written in stone !