So at best, there was an fault somewhere which had cleared coincidentally when the hand controller was reconnected.
You probably had corrosion on the pins of the Y cable and possibly on the motor box as well. Inserting the cable a couple of times might have wiped the pins somewhat clean, but there might still be a poor contact. You might want to clean all the pins of the cable and mating connectors with a contact cleaner and perhaps add some anti-corrosion lube to them to prevent problems in the future. Pins can deteriorate over time, especially if they are subjected to high humidity.
In these mounts, runaways are almost always caused by poor connection between the motor encoder and the CP servo controller. Reason is that there is very low level signal with almost no current and fairly low voltage being run on these signal lines. Therefore if there is no current to speak of, the corrosion acts as a fairly good insulator. By contrast, the two power connections to the motors carry a fair amount of current, so the corrosion is easily bridged by the voltage present on those pins.
Runaways occur when the encoder signals don't reach the servo controller, so the controller thinks that the motor is not turning and supplies more and more voltage and current to turn the motor shaft. Of course, the motor shaft is turning, but without the encoder feedback, the controller runs open loop.
From: M. Collins <aegle_observatory@...>
Sent: Thu, Sep 16, 2021 12:36 am
Subject: Re: [ap-gto] Erratic behavior, possibly contact related
As I noted, the mount has operated without any errors over many hours in the past week, so at the moment there is nothing to investigate. Prior to exchanging any of the electronic components, I did check to see that all of the connections were sound, including the three associated with the Y cable. While I didn't remove the shells to inspect the soldered connections to the pins on the three connectors, the cable shows no sign of damage and it seems likely that an intermittent contact between the motor controller and motors would have resurfaced by now. So at best, there was an fault somewhere which had cleared coincidentally when the hand controller was reconnected.
Also, the RA axis exhibited the same erratic behavior as reported for the declination axis when I was initially investigating last Wednesday, running away or rapidly stopping/starting during a slew. Whatever was causing problems appears to have been common to both axes. Perhaps a cable fault could cause such a thing. No other single element (other than the hand controller, which you've ruled out) was in place each time failures occurred. (And to be clear, only the hand controller, Y cable and power were connected to either the CP2 or CP3 during the initial investigation. The USB/serial translator was not connected until after the mount had operated using the hand controller for more than half an hour after the problems had disappeared.)
If we see further issues, I'll start by checking the connections through the Y cable.