I have 2 that recently started dripping a bit; installed in 1989. They have a single large plastic knob. This is in a shower stall, not a tub

A. How do I get the knob off? I don't want to destroy anything while experimenting.

B. Can the drip be repaired by tightning something or does the complete valve have to be replaced?

C. If replaced what is the part number? I can't find any reference on Google to this particular model.

I can supply a photo if wanted.

Geoff