Loss of Power
On a brush motor this is carbon brushes until proven otherwise. They wear gradually, so the decline is slow enough that technicians adapt to it without noticing. Inspect the brushes; if they are visibly short or the commutator is scored, replace both brushes as a pair.
If new brushes do not restore power, the armature is the next suspect. Replacing the armature together with new brushes is a normal workshop repair and far cheaper than a new machine.
Vibration and Chatter Marks
Vibration transfers directly into the work as chatter marks. Two causes dominate: worn bearings in the handpiece, and a worn chuck that no longer holds the bur concentric.
Test by running the handpiece empty. Noise and vibration with no bur fitted points to bearings; vibration only under load with a bur fitted points to the chuck.
Overheating
A handpiece that becomes too hot to hold is usually either running with worn bearings or being pushed too hard into the cut. Check the bearings first, then the technique. Excess pressure is a common habit when a motor has been quietly losing torque.
Intermittent Cut-Outs
Almost always the cord, and almost always at the handpiece end where it is flexed most. A conductor breaks inside intact insulation, so there is nothing to see. Replacing the cord is a cheap repair on an otherwise sound machine.
If the cord is sound, check the foot pedal and its cable next, for the same reason.
What Is Worth Repairing
Brushes, chuck, cord, pedal and bearings are all routine replacements. An armature is worth replacing where the control box and handpiece are otherwise sound. Beyond that, compare the parts cost against a new machine honestly.
Preventive Habits
Keep a spare set of brushes on the shelf. Change burs with the chuck fully open. Do not run a handpiece with the chuck closed and empty. Coil cords loosely. None of this is difficult, and together it roughly doubles the useful life of a bench micromotor.
