I bought my TWA used. It was sold for a discount due to having knob issues.
The first issue was that the one of then knobs spun freely on shaft. Inside the knob hole there aren't any serrations inside the knob to match the serrations on the shaft. I'd recommend changing knobs to one with serrations inside.
The second was lots of "tobacco" ash and little bits of other debris causing the knob to skip around. Easy fix with circuit cleaner.1
I'd recommend putting rubber boots over the body of the dial that goes to the base of the knob. I
Oh my goodness! That's rather dirty! Glad you got it cleaned up. A wrap of masking tape or two around the shaft on with the free spinning knob will fix allow you to keep using that knob!