My mother has a Bernina record machine which has for the last 35 years not worked longer than a metre or so stitching on the most basic stitch without tangling the thread on the bottom spool. It has been to the repair shop a dozen times over the years and it always fails shortly after. Spools have been replaced and stitch samples created at the shop and it works OK. My mother must be doing something wrong?
Sewing Machines - Bernina
Answers & Comments
Try threading the machine with the presser foot up and see if this helps.
When the foot is down during the threading, the thread does not enter the tension control as it should, then when you start to sew, the upper thread has no tension and does not form a stitch with the bobbin thread.