On 30 Aug, 19:56, Owain wrote:

Open up. Identify pots. Values should be printed on the back, or on
the PCB. Just check which are logarithmic[1] (probably the volumes)
and which are linear (probably the tone controls).

Tone pots don't usually see enough wear to wear out, so just cleaning
will nearly always sort them out. Volume is worth trying for cleaning,
but you might well need to replace them.