My Elna SU (from the '70s) has three dials across the top of the front which face you when sewing.

The left-most dial is where you choose the style of stitch. Set it to "1" for a regular zig zag AND set the middle dial to 2 or 3; you should get a nice zig zag stitch.

You can choose other styles of zig zag with other numbers on the left-most dial, all the way "7" which then uses the cams.

No matter what that left-most dial is set to, if the center dial is set to "0" you should get a straight stitch. The center dial sets the stitch width.

The right-most dial sets the stitch length.

I hope this helps.