Originally Posted by
patchwork
Most of the time the bobbin is the culprit. 90% of the time. Now that being said, your top thread is coming to the bottom freely. You might try putting the thread TWICE around the tension assembly. This will tighten up the top thread. I need to do this on some threads on my A-1. But first rethread everything and rethread and good luck.
I've rarely seen the bobbin as a problem.