Chris Bacon wrote:


The segments should be solid copper
Just what I thought - yet underneath it looks more silver/grey!

- you could try polishing
the commutator with a dry cloth and some meths,

Tried that already. No difference.

and ensuring
the grooves between the segments are clear - don't scratch the
commutator surface, or undercut the segments.

What does the discoloured segment look like?

Kind of silver/grey as if the copper layer (?) had been taken off. Only
one segment, neighbours are fine.