HOW DO YOU CHANGE THE STITCHING FROM STRAIGHT TO ZIGZAG

Looks better where doubled over, but still not right. Likely fabric type and wide needle plate. As a test, try a stabilizer or layer a stiffer fabric on the bottom to see if problem goes away.

http://www.shop.sew-classic.com/media/Slanted%20Stitches.pdf

see if this link will help you out.
Zigzag Stitch iFixit
or this one.
Bernette 705 to go from straight stitching to zigzag stitching Google...

You bobbin tension CAN be adjusted. First, thread your machine with different colors in the top and bobbin, set your top tension at its default, and run a line of straight, and then a line of zigzag stitches, and compare the results at the top and bottom. You will be adjusting your bobbin to your top tension. If the top tension looks like a straight line instead of stitching and the zigzag looks like "chicken tracks" instead of a zigzag, but the bottom looks like a zigzag, your bobbin tension is too loose.

Remove the bobbin case, and locate the straight-slot screw. There are generally two; one is a Phillips and one is straight--you want the straight one. Leave the other alone.

The rule is "lefty-loosy, righty-tighty." If your stitching displays the above symptoms, it needs to be tightened, so turn 1/4 turn to the right--no more--then test again. You will need patience, but don't turn more than that. It is easy to do, but if you don't do it systematically, you can make things worse, and very slight adjustments can alter the stitching significantly.

Hi Susie,

I can't tell whether you mean straight stitching and then turning your work 180 degrees to stitch atop the straight stitches in zig-zag, or continuing around the corner of your straight stitched side using zig-zag. But you can do either. You can mix up straight and zig-zag stitching at will.

Just ease up to a stop, ensuring the needle is positioned down inside the work when you stop.
Lift the foot.
Adjust to your desired stitch setting.
Lower the foot.
Turn the wheel toward you manually to ensure the stitches will be placed where you want them, and
Slowly engage the power again (via foot or thigh lever).

Hope this helps.

If you can grab the hand wheel and move it right and left, then it probably needs a plastic collar that goes next to the left upper shaft bushing.
If not then the cam timing needs to be adjusted.

does it sew ok straight stitch but not zigzag?

You need to backstitch over the beginning and end of your stitching, Jodie. When you start out, manually turn the wheel to put the needle into the fabric. Push the foot pedal to stitch forward a little. Now, reverse over those stitches to nearly where you started stitching. Use the foot pedal to go forward again, stitch to where you want to end. Reverse again, just a 1/4 inch or so. Then take the fabric out and cut your stitching.

This backstitching keeps your thread from pulling straight out. You should always use it with straight stitches and general zigzag stitches. Don't use it with decorative or embroidery stitches, though, as it mars the appearance of them. But if you're doing any type of topstitching that uses straight stitches, backstitch in the area that will be in the seam allowance, ever so slightly, or your top stitching will pull out, too.

Hope that helps!

Your gears are not aligning correctly inside the chassis. I would suggest taking your machine to a certified repairman, because if you touch the wrong gear or tighten it in the wrong position, it may do a zigzag, but then your other designs will not work properly. Simple fix for the right repairman. If you open the chassis, make sure it is UNPLUGGED.

If there is no speciafic selecting tool for straight and zigzag and assume it is a mechanical machine;
try making the stitch width zero, this is how most mechanical machines work; hope this works.

It must be a design flaw- things don't always last like they are supposed to. I would personally check the physical reason it is not responding... knowing what it would take to make it stop moving- or whatis causing it to. *perhaps the switch isn't working to stop it- or the mechanism that controls it is worn down. If it was new- I'd return it- but there are only a few options at this point.