You can tell a Bernina bobbin by the number of holes "7"
9mm diameter and 6mm width
the holes will be superimposed on each other so that in the newer machines the light sensor will work properly
less expensive Taiwanese, Japanese etc bobbins will have more holes. Also the centre tube on a Bernina bobbin is slightly smaller and thus will wind more evenly than a the alternative
Bobbin thread breaks
1. There is a right way & wrong way to insert the bobbin in the case --when you insert the bobbin in if it doesn't stitch correctly turn the bobbin over see if that helps or bobbin case is not inserted correctly
2. Lint has collected in the bobbin holder
3. The bobbin is damaged & doesn't turn smoothly
4. Lower thread tension is two tight
here's a free manual
http://www.bernina.com/getdoc/48e8d09f-98f7-40ab-8663-a23663df4fae/Bernette_715_730_740E_manual
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