How to fix a broken micro motor carbon brush depends on the specific cause of the problem. Here are some common fixes:

1. Replace carbon brushes: If the carbon brushes are severely worn or damaged, they should be replaced immediately. When replacing carbon brushes, be sure to select the correct type of carbon brush and replace it according to the manufacturer's instructions.

2. Clean the carbon brushes and commutator: If the surfaces of the carbon brushes and commutator are dirty, it may cause uneven wear of the carbon brushes or poor contact. Therefore, the carbon brush and commutator surfaces should be cleaned regularly to ensure good contact between them.

3. Adjust the carbon brush spring pressure: If the carbon brush spring pressure is insufficient or too large, it may cause poor contact between the carbon brush and the commutator or uneven wear. Therefore, the carbon brush spring pressure should be adjusted to ensure proper contact pressure between the carbon brushes and the commutator.

4. Replace the motor: If there is a problem with the motor itself, such as burned windings or damaged bearings, then replacing the motor may be necessary.

Please note that before performing any repair operations, you should ensure that the motor is turned off and the power source is disconnected to avoid the risk of electric shock. In addition, if you are not familiar with micro motor repair operations, it is recommended to seek help from professional repair personnel.