Very good advice. My first thought is that it was the pots. I cleaned them thoroughly and checked them many times. They appear to be OK. I have also gone over the tracks with a magnifier, but I have not rung them out with my meter. I will do this.
Thanks, Kev. I have been wondering about that zener. Does this look like a reasonable replacement? https://www.mouser.com/datasheet/2/427/tzxserie-241403.pdf