The Japanese zz really had issues with the tension. I had a Kenmore parts machine and used its tension assembly. But first I had to remover the one from this machine. It was really stuck. I left the machine out in the sunshine for much of the morning and then put it in the car where it was very warm. By five oclock last night I was able to get the old tension assembly out and the new one fit perfectly. I also replaced the bobbin case with one from yet another parts machine.
Voila. Very nice stitches. I think this satin stitch is almost as nice as the one on the 237. I guess I didn't know how nice they were.