Vintage Singer

How do I fix the zig zag stitch in my vintage Singer sewing machine?

I've had my Singer for about 6 - 8 months and its a great machine. Its a vintage machine and I have the manual so have read through it many times and went over the instructions to doubly make sure I'm not missing anything.
The problem is the stitches are all mixed up and it won't do a zig zag stitch at all.
The machine was a gift so I have no idea of where to take it (plus its an old lady now) and would prefer if possible to try and fix it at home.
Is there anything that might be an alarming simple solution or would this probably require the attention of a rather pricey repairs shop?

Thanks in advance!

It may just need a through cleaning where only a service person should go.

It may be rusted or just clogged with lubricant and lint/dust. An old trick for the latter problem is to warm the area where the levers are with a hair dryer to soften the old gunk.

You did not mention cams, so it would seem the stitch selections are determined with a lever only and not a combination of lever settings an a specific cam.

Some of the older models used cams for decorative and zig-zag stitching - http://www.ecrater.com/p/6989689/vintage-singer-top-hat-model-600-603

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