Re: 900 mount spurious RA motor slewing and stall

Roland Christen

You may have a failing motor encoder or bad connection on the cables going to the axis that is running away uncontrolled.

You can try this: plug the RA cable into the Dec axis motor, leave the Dec cable unplugged. Then push East and West buttons and see if the motor moves properly. If that motor also runs away, you probably have a failed Y cable. Check the wiring on the cable connectors to see if any of the wires are broken or frayed. Check the connector pins also for any oxidation which would cause bad contact.

If you cannot do these tests or can't find the problem, call George on Monday and he can run you thru a couple of tests that will determine what the problem is.

Whenever a motor runs away it is because the controller is not getting a signal from the motor encoder that the motor is turning. The controller then sends more and more power to the motor (which causes the motor to run faster and faster) until the software decides that there is an open loop from the encoder and it shuts off power to the motor. It usually gives the motor loop about 2 seconds before shutting the motor, in which time the motor will run at uncontrolled speed for a number of axis degrees, as you experienced.


-----Original Message-----
From: kensablinsky@...
Sent: Fri, Nov 27, 2020 12:31 am
Subject: [ap-gto] 900 mount spurious RA motor slewing and stall

Hey everyone, longtime AP900 user and lurker, with a problem that just cropped up.  I'm the original owner of a 900 mount for around 20 years now, and recently upgraded to the CP4 (with new motor cables and new 2-pin power cable).  It's been working pretty flawlessly with the CP4 and APCC pro for months, but all of a sudden is giving me a motor movement and stall problem.  Yesterday it would slew East/West OR North/South fine, but when both motors slewed together at any slewing speed, the motors sounded strained and sped up and slowed down in a jerky fashion.  Sounds like a power problem I thought, so I moved to a different power source (Powerwerx 30 amp supply, nothing else plugged into it, volt meter showing 14v output).  Same problem.  Today, it's much worse -- as soon as I boot up APCC Pro and click Initialize, the RA axis slews 10-15 degrees (OTA moving to the west) and the motors stall.  So I unplugged everything from the CP4 except for the motor cable, and pulled out my old AP hand controller, to make sure the problem wasn't coming from an external source. 

The first message on the controller is to select my location and then press go-to.  As soon as I press the goto button, the mount slews in RA by the same 10-15 degrees, and  again the motors stall.  DEC axis does NOT turn, only RA.  The next controller screen shows the options on how to unpark.  If I choose unpark from current location, I get to the main menu -- but the moment I press ANY direction button, the mount slews another 10 degrees in RA. If I choose 'new setup' and resume from park 3, it immediately does the same thing - slewing 10 degree in RA into a motor stall.

I have the APCC logs here to show the motor stall upon initialization:

But since it happens with only the hand controller connected to the mount, could there be a problem in the CP4 or one of the motors?  I don't have another 2-pin DC power cable to rule out a problem cable. Any ideas as to how to pinpoint the problem?


Roland Christen

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