I have a model 2380 and I HAVE NO IDEA HOW TO SET IT BACK TO A STRAIGHT STITCH. WHEN I SEYT THE DIAL TO "A" IT STAYS ON ZIGZAG. THE MANUAL DID NOT HELP

turn the ZigZag width dial (23) to Zero

https://www.etsy.com/listing/764828308/samb-2-nalco-necci-dressmaker-penncrest

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Here is a link to the Manual:

http://www.consew.com/Files/112347/InstructionManuals/199R-1A.pdf

does it sew ok straight stitch but not zigzag?

What's the model? Do you have the stitch selector set on straight stitch, or on a stitch with a width component, like a blind hem stitch?
Some machines consider straight stitch to be a zigzag stitch of width=0, while others have separate settings for a straight stitch and a zigzag (or other) stitch.

I don't know your model but most machines have 2 common setting devices, namely a stitch width dial (how wide the needle swings) and a stitch length dial, how far apart the stitches are. So a normal straight stitch is "width" set to zero and "length" set to 2.5. If the machine has several different stitches then there may be a "stitch selector" too where you can choose the stitch style, ie zig-zag, 3 step zigzag, blind hem stitch etc.

You could try to locate a manual or threading diagram at various websites or visit sewing.about.com which gives good general information on threading and use.

sounds like you have a broken zigzag gear in the top of machine. could be something in the dial is broken . not sure.

You have 3 knobs/dials on the machine, the top left dials the stitch, the top right sets the length and the bottom right set your width. set it 0 for straight.

The brother website has a users manual for it that can be downloaded. Use this link to get it:

http://welcome.solutions.brother.com/bsc/public/us/us/en/doc/manual_index.html?reg=us&c=us&lang=en&prod=hf_ls2125eus&type2=5&flang=English&dlid=

with the Shark Euro-Pro X machine, you need to bring the stitch width to zero it's the nob on top of the machine the manual states not to use it wider then 3 with the double needle cause it can break the needle. but either stitch selection "A" or "S" will give straight stitching with width at zero.