Sorvall RC-5B Pluss spinn up to approximately 7k rev/min, then lose power and stabilize at app. 2.5k rev/min

Initially the centrifuge had some kind of power related incident, where the cooling exhaust become full of debris from the brushes, and the mains fuse (in the building) opened. This led to full motor service with new brushes/bearings and milling of the commutator. The centrifuge have been run at low speed for some hour so run in the new brushes. When trying ro get the centrifuge up to speed, it seems that the breaking circuits activate when it reach right above 5-6k. If i lower the setpoint of the speed (after the loss of power), the centrifuge seems to run free, but if I try to increase the setpoint above the actual speed, it seems to brake instead of accelerate down to approximately 2500 revs. I am new and alone at the electronics workshop here at the institute, and the company that have sold it (many years ago) have merged into something else since that, and can not help me... and I have not been able to find the right service manual. Does this seem familiar for any of you?