It looks like there are at least a couple of models of Sonata machines. There is a manual at
https://res.cloudinary.com/singer-se...ng_Machine.pdf I would pay particular attention to pages 43 to 46 of that pdf. It is possible that there is a thread or lint somewhere under the bobbin case or feed dogs. It is possible that it may have plastic gears that could have cracked. If that is the case, probably it would be more expensive to have it repaired, even if the gears are available.
I suppose it would be possible that there is something else going on. Do you still have both Sonatas? If you are into self servicing, you could probably take it further apart and oil some other places inside where metal rubs metal. Another thing that might help would be to take apart and clean the tension assembly.
Good luck on getting it back to working condition.
Janey - Neat people never make the exciting discoveries I do.
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