Bosch pow 500 router?

My Bosch POW 500w ROUTER (GREEN MODEL) is making some strange poping & crackling noises and the body where the on/off switch is mounted gets very hot and there is an electrical burning smell. I have removed the case and the brushes are not worn but where they run on is black/grey in colour and not copper looking. Could the fault be something other than the brushes as they look OK to me. Many thanks, Bob