Ed, If the voltage plots you sent are correct it appears that C3 was shorted and the pot only connected when it was at the extremes of its rotation. This can happen due to the way the pot is made. C6 also COULD have been open. It seems very likely the pot is bad. if you have a couple of 12 ohm resistors you could wire them like this to substitute in for the pot. See the pic below for details. Normally you would set the pot, measure resistances, ?then substitute. In the case the pot is not working so just use 12 ohms for both resistors. This will simulate a centered pot.
I don't know how you made these measurements of of driver voltage, perhaps you can elaborate.
Also because of the network architecture, unless you are very experienced it is hard to measure some of the components in circuit. You have to unsolder at least one lead, two on the pot, to make more reliable measurements.
My career for 32 years was teaching folks how to make measurements correctly. It can be very tricky. There are lots of common mistakes.
Charlie Conger
