madbadger said:
I watched it playing outside the unit and when it 'sticks' half way across the record, i noticed the tone arm drive motor had stopped, a gentle nudge and it started tracking again until the end of the record.
All the tone arm wires were running smooth in their runners and all the contact switches looked ok.
I just replaced my tone arm belt and noticed that fast-forward/rewind would work, but not track during playback. So, you hear about 30 seconds of music before the stylus gets so cock-eyed it pops back to where it started. The quick small turns the motor does during tracking can't seem to overcome the same resistance full-speed mode can, because I could nudge it during tracking like you did and get it to move.
SBM4.0 belts are the recommended replacement, which is what I had on there. They are 62x62 mils. However, the originally specified belt under PRB was SCQ4.0, which a bit thinner at 57x57 mils. As it turns out, 4" belts fit just right on a drill chuck, so I used double-stick tape to secure the belt in both axes for sanding until they easily fit through the gap on my caliper. I noticed that I still had issues at 57 mils, so I took it down to a little less than 50 and it began to work.
It's possible the issue is from some belt unevenness or a dirty worm gear. But if an SBM4.0 or SCQ4.0 belt gives you this problem and your worm gear is clean then it might help to try an SBT4.0 (51x51 mils) or SBS4.0 (47x47 mils).