I have had some problem with bobbin case on the 7700. It happens when the chase jumps out of its proper place. I am not sure I can explain it clearly. On mine there are two areas on and in the bobbin are that if they are not aligned will cause the bobbin thread to jam. There is a tiny metal spring type on the outer ring that must be on the right side of a protruding catch on the case. If not the thread will catch and pull the case out of alignment. I hope this is clear. I can do it but not sure I can explain it.