I've recently been having trouble with a Yaskawa UGCMEM-04-MU11 Cup motor. It has an attatched TFUE-05ZC7 feedback unit. This servo would "jump" at a certain spot when jogging, sounded similar to a bad bearing. Through trial and error I found that the tach unit armature on the end of the feedback unit had a dead spot in it. I found a used UGHMED servo with the same feedback unit and swapped the armatures but not the entire feedback unit. This solved the jumping problem, or so it seemed. Do to another problem elsewhere on the machine(binding in the Z axis screw which I found later) The control/drive would throw an alarm 35 when at zero return. I talked to a tech and he thought it could be a problem in the servo motor itself. I took the motor apart, careful to mark the motor so I could get the main case back together the way it came apart. I found that the armature(similiar to a fiberglass substrate w/ inlaid copper) had small cracks in it. A guy offered to recoat it and I let him do so. It turns out he used a urethane clear coat. I put the motor back together only to find out that it would jump erractically when in rapid mode and wouldn't hold position. Even in jog mode it was not smooth. Another tech told me I had to get the urethane off and have it coated with Glyptol (sp.) which I did but it still exhibits the sames problems, although less in effect. Is it possible that the encoder is not aligned to the servo correctly? What about the fact I took the tach unit from the other encoder but not the entire unit, is it possible these are a matched unit and should not be swapped? I have a feeling I'll either have to find a used one or have it repaired. Unfortunately Yaskawa doesn't carry any accessible information on thier website for either the motor or feedback unit. I would really appreciate it if the experts on here could give me some advice and direction. Thanks!