by RayL » Mon Sep 26, 2016 12:24 pm
Bill,
Can you feel the whole switch moving when you try to change speed?
If you can, then it's possible that movement in the past has fatigued a wire connection to the point where it has broken. Solution - re-solder wire.
Or is there no resistance when rotating between the settings? Rotary switches of that vintage often have a ball bearing pressing against a spring to give that resistance and to aid location. Solution - look for the ball bearing and put it back.
Or maybe the switch is simply broken. Solution - replace the switch.
I don't do repairs these days, but if you can get the amp chassis out of the cabinet we could meet at, say, St Pancras (convenient for Luton trains) and I might be able to spot the problem.
Ray