I have an AF300 engine and tender I just repaired with several new parts I got from a vendor at the last local show. I ran new wires from the tender to the engine, thoroughly cleaned and polished the tender wheels and axle contacts, disassembled the E unit and cleaned and gently wire brushed the E drum, inspected the contacts on the drum for holes and springiness, and lubricated all moving parts.
I had the engine running very well and the E unit was operating surprisingly well too, even on very low voltage. Then the engine just stopped working. The E unit still rotated and buzzed but the motor wouldn't turn. I tried giving the armature little pushes with different E unit settings (as I didn't know when or if I was in neutral at any particular time), but the motor was dead.
I disassembled the rear of the motor and removed the armature. I had noted on my initial repair that the armature commutator was slightly scored and somewhat blackened in a circular pattern where the brushes rotate. I cleaned this up a bit with the Dremel prior to reassembly but never removed all the scoring. During this second dis assembly I gently wire brushed the commutator again and used 600 emmery on it for a smooth finish. It is now pretty smooth. I also cleaned up both ends of each brush and slightly stretched each spring to afford better contact with the commutator. In addition I re soldered the three armature wires to the commutator segments, as the solder looked a little sparse.
I set the chassis up on test blocks and ran it at slow speed. Initially it ran very smooth, however I could see a very slight arc coming from one of the brushes. I have noticed this on many motors in the past, and since it was so minor I didn't give it much thought. My goal was to run the engine for a period of time at slow speed in order to "seat" the brushes. After a period of time you could tell that the motor was not running as smoothly as it did at the beginning of the test. A small spray of silicone contact cleaner on the commutator while it was running at slow speed would momentarily pick the RPM's up and then they would drop back down and you could tell that the motor was "missing" very slightly.
Again I pulled the rear end of the motor and noted that there were some slight black marks across the sector gaps, which was evidence of slight arcing on the three commutator sections. I cleaned the sectors and brush ends up again and reassembled the motor. I set it up on slow speed test again and it was still running, although not perfectly but not too badly either after an hour at slow speed.
I have two other engines from the same era with similar motors that don't seem to do this and I have serviced those motors similarly. Has anyone seen this before? Is there a way to "arrest" the spark at the brushes perhaps with capacitors? Thanks for any assistance. Lenny