I have dc motor with thyristor regulation on lathe .Electronics is AEG 029.295.
On low speed dc motor cause stop.This is problem.Collector and brushes on motor are OK.Tacho dynamo is feedback device.
According to your last post in Feb, it must be an ongoing problem?
Check the tach output if DC brushed, but usually if no output it causes runaway or Tach fault.
Some other options are try to find a schematic for the unit, reverse engineer it, or pick up one of the replacements available out there on the web.
Max.