I have inherited a Bernina Activa 140 and have the manual, have followed instructions to the letter but the thread keeps breaking, the bobbin is O.K. It sew for about 3 stitches and then the thread breaks

There is no difference to loading the bobbin spool. If you are talking about the thickness being a problem you will have to lower the tension of the bobbin. That is easily done by turning the screw on your bobbin case in a counter clockwise motion until the thread feeds with a tension similar to level four on the top tension.

Likley only by hand

Hold the top and bobbin threads with your fingers, straight out in back of the needle, and that should solve the birds nest problem, a common one across all brands.

12 wt thread generally will not work well in the bobbin...


"Aurifil 12wt Thread
Aurifil 12 wt thread is ideal for hand quilting, big stitch hand quilting and machine quilting. 12 weight thread creates a more defined and often primitive look. It's great for designs that use a longer stitch length if using a machine. When machine quilting with 12 wt thread, use 40 wt in the bobbin, increase your stitch length, lower your tension, use Topstitch size 90 needles and stitch slowly."

Fit a new needle, clean and oil the hook race, whether-or-not your manual says it doesn't need it and check that you have some tension on the thread coming out of the bobbin case and top tension unit. Are you threading the top thread with the foot up or down ? You should always thread the machine with the foot up to ensure the thread goes through the tension unit properly.

Are you absolutley sure you're not getting a problem with stitch formation on, for example, wide zig-zag. Check again, using a different colour top and bottom and make sure that the stitch is being formed properly with only the top colour on top and bottom colour on the bottom. It sounds as though the bottom loop is not being picked-up by the hook properly. Make sure you've thoroughly cleaned-out any fluff under the stitch-plate and remove the hook/shuttle and give the hook race a good clean and give it a single drop of oil, even if you've got the lightweight part-plastic hook/shuttle fitted.

If you go to http://www.bernina.com.au/webautor-data/888/0305295204_0303_a135-a145_EN.pdf you can download a manual for the Activa 145, I suspect that the instructions would be very similar for buttonholing.

go to the bernina website & click on the support tab,, you will see out of print manuals & will bring up several catagories to choose from

http://www.sewusa.com/Sewing_Machine_Threading_Diagrams.htm

There is also a tension adjustment on teh bobbin. It's just a little screw. Turn it about 1/4 to 1/2 turn and try again. Adjust as needed a little at a time.