With this sort of problem cannot see resistance amisss by checking with a
DVM.
Worn track but only worn down, not through. 3 sets of wipers per pot and 3
circular paths, conductor, 100 K resistor and low resistance inner ring of
about 20 ohms around.
Probably problem due to corrossion over most of the wiper fingers and this
central contact ring.
The pitfall is a not noticed/seen, presumably cartouche shape widget, that
locates that shape hole in the pot paxolin to the smaller same shape of axis
shaft. I must have removed both together thinking they were one part and
lost the widget before placing in tray or sticking to something else and
losing. Had to fashion one from a couple of transistor insulated washers and
epoxy.

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Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/