Yes! Mine played correctly when I moved the arm over and then back. Very interesting! Maybe they are correct that the end of rail microswitch needs to be looked at?
Update: cause of problem found. Belt replacement did not help. I deduced the problem with the position tracking as increased commutator/brush resistance due to long term oxidation of the commutator (arm motor). I reached this conclusion experimentally by cleaning the commutator and observing the results in the unit – no fancy probing and measuring.
This primarily happens when the motor has not been used for a very long time or possibly related to the environment it was stored in. In this case the history is not known. The commuter surface has only slight wear, if any, on brush paths and no evidence of arcing/sparking on edges. No significant carbon residue on brushes.
Grease: I have a fairly heavy amount of grease on the rail and it works fine now. Light grease on the worm gear. But the choice of lube is another topic at this point. I don’t have the concerns about grease I had mentioned above related to the early auto return, unless it’s the 30 year old original grease.
Warning - a long read follows:
(Disclaimer: much of what follows has an element of conjecture in order to explain what is happening regarding the malfunction. It’s very difficult to actually observe all of the cause and effect). The increased resistance of the commutator affects low speed motor operation more than high - uneven start and stop under load - complicated by the fact that the motor is almost over-geared for the low speed operation during arm angle tracking. That is generally supported by torque - speed - efficiency curves. The servo does not control the motion well under these commutator resistance conditions and a low level of position tracking errors occur - around 5 % in my case. It follows the arm angle ok, but the computer loses track of where the arm is positioned such that the arm has not traveled as far as the sensor output would indicate. That is due to something like intermittent bounce on the window edge timing within the backlash of the worm gear. It does not affect the arm angle so much since the servo is able to maintain it albeit less smoothly just not to a degree that the eye can detect at the arm. As I mentioned earlier I did see the worm gear moving backwards briefly at the end of a forward movement under servo operation.
The exact momentary reaction that causes this is not entirely clear. But I can imagine the motor stalling right before the set point is reached as it comes near (during deceleration). Then the reaction of the servo bumps it enough to get it going again which creates an overshoot of the set point (hysteresis in motor stop/start). But the exact reaction that causes the worm gear to bounce back is not clear. I think it might be coincidental detent torque. Over gearing is a major point in this scenario. But they had to use one motor and gearing for all arm modes and the arm position sensor resolution is limited in the same sense.
I can’t say if something like the SL-15 escapes this age related problem with the arm position errors and programmed play track location because it seems to have the larger motor of earlier designs. Note that it uses a position switch for end auto return like the early designs (SL-10, etc, and SL-QL1 as mentioned previously in the thread). The elimination of the end switch is a bit of a curiosity. They don’t use one in some fully auto pivot arms either where it’s done by opto sensor integration. I would guess it is purely cost related.
I hope this keeps a few of these out of the dump.
