Ah, good, you found & fixed the problem yourself--well done!
For anyone else with a similar problem who might be reading this post, I would direct you to
THIS POST with its color-coded PCB map. The tone control portion is shown in green; you'll see C9, which was the problem in this case, right in the middle of the component cluster.
To the OP: That's a pretty gnarly looking set of rotary switch solder joints. Might never cause a problem, but I'd recommend gently scraping off the brown crud with a small flathead screwdriver and cleaning it off with an old toothbrush.
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