Again on a few occasions telescope oscillations were noted and some of the motor brushes were replaced but the problem recurred intermittently. In the motor was partly disassembled, the top motor bearing support was removed to provide proper access to the where all the brush holders are mounted in so it could be replaces with a spare (originally from an altitude motor) containing properly mounted new brushes. During the procedure the motor was cleaned of carbon dust and metal shaving from the broken holders. Also the opportunity was taken to clean an old repair which may have been the cause of the problem being seen only over a limited range of the azimuth position. Afterwards it was noticed that the azimuth drive current while slewing had reduced to a more normal 3.5A from a previous high of 6A. No more problem have been reported since.
Clearly, the many repairs just replacing brushes without partly disassembling the motor to gain proper access do not work very well. It was decided to do more regular preventive checks including partly disassembling the motor. A complete set of 16 brushes have been received with extra parts to be able to repair any future problems with the brush assembly.
Thomas Augusteijn 2012-02-21