A friend was complaining about a 'THRUMMING' sound his 543 always makes even at the higher speed although it does run and operate well.
He wondered if the fact that his machine seems to have a plain washer between the base of the armature and the lower sleeve bearing and not a fibre type as shown in a 541 parts list of 1923/4/5 ?
My friends machine was a @ mid 1930's Perivale rebuild ??
Comes to the Rescue!
