Try flashing the fields.
This operation should be covered in your manual.
I would need an exact schematic to tell you how to do this.
The big 20 is a unusual beast as it uses the motor alternator for excitation.
In the future, do not polish the slip rings. the copper oxide developed on them
is highly conductive.
Do be sure, however, that the brushes are in firm contact.
Good Luck
brande
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