Take thread and bobbin case out. Clean bobbin area as well as you possibly can. If possible, use a magnifying glass to look for a tiny snippet of thread that may have become caught in the bobbin area. Take the bobbin out of the bobbin case and check the bobbin case for any tiny bit of thread or lint, especially where the spring is located in the bobbin case. Make sure the bobbin is tightly and evenly wound.
Clean between the upper tension discs by loosening the tension (make note of where it is before you change it), then using *unwaxed* dental floss to floss between the discs. If there is any tiny piece of thread or lint in there, this should remove it. Change upper tension back to where it was.
Replace needle, making sure it is *all the way up* in the shaft and positioned correctly.
Run your finger around the throat plate hole to check for burrs. The breaking needles may have nicked the hole just enough to cause thread to shred.
When re-threading, be sure presser foot is in up position and make sure that the upper thread gets positioned correctly between the tension discs.
After threading, run the machine very slowly and watch carefully (using protective eyewear!!!) to see what is happening as you move the machine. You can rig up a test sandwich strip to the side of the quilt to give you some space to test out the quilting; just pin a sandwich strip between rollers.
Honestly, what it sounds like to me is that the first break resulted in a tiny piece of thread getting caught somewhere in the mechanism -- most likely the bobbin area. These tiny pieces of thread can be *really* difficult to spot, but they are the demons that change the tension on you unexpectedly -- which is probably why you had to drastically reduce the top tension after the first needle break. When you do spot one, it can be difficult to remove even with tweezers. Soaking the thread in a drop of sewing machine oil can help soften it up enough to remove it strand-by-strand.
Edit: Also look for a tiny needle tip imbedded somewhere in the bobbin area.




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