Re: The Upper Thread Sensor
It’s designed so that the machine can detect that the upper thread is threaded.
If it’s turned Off and the machine is threaded it will sew a few stitches, then stop and warn that the machine cannot sew.
One purpose of turning it Off is so that we can practice sew without thread to check if a design is properly placed.
Where is it located? I’m not exactly sure, But it’s somewhere near the thread uptake. We used to trick the old 11000 (That didn’t have an Off/On) by sliding an index card into the slot to do a practice test, so I assume it’s still in that area on all newer machines.
My analysis of your thread breakage is that you have a tiny burr somewhere in your upper thread path, maybe it’s even on the thread guide above the needle.
I helped someone figure it out on an 11000, by threading the machine, then we carefully kept pulling on the thread like it would move if sewing, until we could see and feel where it was catching up. When we located the spot we sanded it with crocus cloth.