I need the rotary hook only for a bernina 850 can anyone help me

adjust the top thread tension to tighter

Are you sure you have needle inserted with scarf facing to the back of the machine ? If so likely the shuttle/hook timing needs professional attention

A Bernia Artista 180 appears to be a typical single needle rotary hook sewing machine (from what I see in the manual). "J" hooks are used for overlock stitching. So I'm not sure what/where a j hook is in a BA 180 <?>. So what machine are you reffering?

Most likely, the thread has wrapped and twisted causing the bobbin case to hang up. If you can remove the needle plate, you may be able to access some of the thread and gently remove it with tweezers and small scissors. There are some great Yahoo Groups for Bernina machines. There is one in particular called "Bernina1630Devotees" where you can post questions and gather information from users. You will need a Yahoo email account, search for the group, request to be added as a member, and then post your questions.

The Sew Whats New Archive

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Give this web site a try.

http://www.sewingpartsonline.com/needle-threader-singer-503277.aspx

If you still need a copy of the manual, I can scan mine and e-mail it to you.

Your Bernina 180 is a rotary hook model and there is a bobbin case retainer mechanism below the sticth plate that has a long spring on it - it sounds like this has become detatched at one end and may well be affecting the stitch quality. Take the stitch plate out and you will see a black rectangular metal bar in front of the rotary hook mechanism - there is a small silver part in the middle that fits in a slot in the rotary hook bobbin basket - the spring should fit on this and then extend the length of the black bar and pull onto a retainer at the other end so that there is fairly strong tension on the silver part. If not this spring, then it's probably something that's dropped-off from elsewhere and maybe, possibly the feed-dog cam retaining spring. Let me know what you find.

clean out rotary hook area following manual instructions on how to do this, usually removing bobbin holder and bobbin, then the rotary hook area by undoing the latches each side.

Could be a thread jamming in there causing the error message.

If you remove the hook on a 640, the timing of the machine will need reset, and that involves pinning the machine for precise timing!

The tensions are not calibrated properly. To get by, or if you do not want to take it back to the shop, just lower the upper tension a little at a time until it looks ok.
I have 25 years experience working on sewing machines