I may be mixed up, but I think you should have AC voltage from the transformer going to the rectifier, and DC voltage on the slip rings. Does the unit have a nameplate or tag with "exciter voltage" listed? That should be the DC voltage to the slip rings. If there's AC coming out of the transformer but no DC on the slip rings, then the rectifier is probably bad. If there's no AC, then you may have lost magnetism, or the windings feeding the transformer may be open, or the transformer itself may be open. Check the input to the transformer, and see if there is any voltage there. To restore the magnetism, do a search on threads about "flash the field".