I have a ~20 year old Necchi 525FA "thread injector" that I need to fix. It has been oiled, and the motor runs smoothly, but the inner hand wheel is stuck to the external hand wheel, and the bobbin threader won't spin. I think the two problems are linked.

I don't see any stuck material/thread anywhere, but am hesitant to pull the lower part of the casing off. Are any of you familiar with repair? After searching a while online to no avail, I might break down and buy a machine repair guide.