2. Check the surface of the switch and its diameter.
If the surface is dirty or burnt, abrade it with emery cloth or lathe within the tolerances indicated, or resurface with #500 or #600 sandpaper.
If the commutator diameter is less than the limit, replace the rotor.
3. Measure the switch's runout.
If the commutator runout is within tolerance, check for coal dust or brass chips between the fins.
If the commutator runout is greater than acceptable, replace the rotor.
4. Check the depth of the mica. If necessary, cut through the mica with a hacksaw blade and bring the depth to normal. If the limit value cannot be maintained, replace the rotor.
5. Check if there is continuity between the commutator blades. If there is a break between any lamellae, replace the rotor.
6. Put the rotor on the stand. Bring the hacksaw blade to the rotor core. If the blade is attracted to the core or vibrates as the core rotates, the rotor is shorted. Replace rotor.
7. Using an ohmmeter, verify that there is no continuity between the commutator and the core of the rotor coil and between the commutator and the rotor shaft. If there is continuity, replace the rotor.