Coming from the service manual troubleshooting for this problem. Here are the next steps to look at.
Things to check for.
1. Open circuit breaker
2. Open circuit between brush block and CR4 rectifier.
3. Brushes stuck in holder or not making good contact with slip rings
4. Defective CR4 bridge rectifier
5. Open, grounded, or short circuit in rotor, stator, or transformer
Solutions for each of the problems
1 ) Locate cause of overload and correct as required. Reset breaker
2) Check for continuity and correct if circuit is open
3) Release brushes if jammed in holder. Clean slip rings if dirty.
4) Test rectifier and replace if defective
5) Test each component for open, grounded, or shorted windings and replace if defective.