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[Chicago Astronomer - Astro Joe]

C11 Stepper motor issues on the Celestron/Losmandy mount

Spring is when I like to do deep maintenance on my equipment. No heat in the workshop, so winter is out and summer is chock-full of star parties and astro activities. Last fall, I started to develop some issues with the control pad, in that it would no longer nudge the OTA when pushing buttons. Tracks ok, but not the Dec motion. I had to manually correct or adjust with the slo-mos in the meantime till I figured out what's going on.

Before the Vernal Equinox Chicago Astronomer Star Party, I examined the motor. I could hear it trying to spin, but nothing was happening with the gearing. So I dropped it off the mount...

It spins freely when pushing the button, so I check the gear-box....

It does not kink nor has any resistance when I spin it by hand...feels solid. The teeth on all cogs look sound and the entire integrity of the gear box seems good.

I replace the the motor back in place, push the button..and again - nothin'...

Removing the motor, I push the control pad button and it spins again.....but, when I pinch the shaft gear with my fingers, it slips internally. The additional resistance applied to the gear is too much for this little motor and time to replace it. Perhaps the brushes are shot or whatever...just easier to replace.

I will search for one soon, so in the meantime, It'll continue to be a manual operation till repaired. Might as well replace both motors whilst at it.

[erikruud]

If it is really a stepper, there are on brushes. The shaft should have a set of permanent magnets. There are multiple coils around the perimeter that are energized in sequence to make the shaft turn.

When you spin a stepper with no power applied, it should cog as it turns. It will feel like it wants to jump from step to step. In a high quality stepper, the steps are small and the cogging feel will be small.

I wonder if the gear is slipping or I guess the shaft could be slipping inside the magnetic core.

[Chicago Astronomer - Astro Joe]

Erik,

I'll examine this motor again and perhaps video.

Let's see what we got.

[erikruud]

It does appear to be a stepper with all of the wires that go into it.

[skyluvr]

Hi Joe,

I would like to give you a little info on the motor.
The motor is indeed a stepper motor of the type unipolar. It has a 15 degree step angle and runs on 12 volts. And has about 120 ohms per phase(or coil). I suspect in your case that one of the phases is open(meaning the connection is broken or infinite resistance).
If by chance you have a voltmeter you can verify if that is the case. To do so look at the motor and you will see it has two half's, each with three wires coming out of them. One half has green, yellow, red, the other green, blue, black. Green is the "common wire" meaning it connects to the other phases or coils in each half. To verify if a phase is open, set your voltmeter to read on the ohms scale. Pick the half with the green, blue, and black, and find those pins at the connector. Now connect one of test leads from your meter to the green and the other to the blue. You should read around 120 ohms. Now connect the leads from the green to the black. Again you should read 120 ohms. Now do the same for the second half again using green as the common wire check the red and yellow. If any of the measurements read really high or off scale then that phase is bad and the motor must be replaced. SAIA (the motor mfg)is still in business but i think has been bought out by Johnson Electric. The motor is of the series UBB2. Attached you will find the data sheet for the motor. I can't seem to find this particular motor in stock on any websites. You might want to call them and ask about a replacement. It appears from the data sheet that the numbering has changed. The Johnson Electric 53466 is the closest thing I can find. Some of the local electronics houses carry SAIA products so they may be able to get it for you. Namely Newark and Allied.
Keep the old motor since you will likely need the pinion gear and/or the mounting plate which is removable.

Hope this helps

Rick

Attachments:

[Chicago Astronomer - Astro Joe]

Oooh,

This is good stuff.

I have not the chance to mess with the motors soince I posted this, but now - with this info, we can start to diagnose and start the repairs.

Here is is label on the motor...

Thanks Sklr...I will post the findings when I take the readings soon.

[Chicago Astronomer - Astro Joe]

Having other things on my mind, I had not attended to the failing motor of the Ci700 mount, but C.A. Steve has been on the look out for a replacement motor, but as Rick has also discovered...ain't one to be found just like it.

But, he did come across one that is similar on Ebay...

Looks decent and the price is right.

Arrived today...

MOTOR

UBD23U08LMZ1

625

FACTORY 156


120 OHM/PH

12V

2W

Let's see if this works, but a secondary problem has developed - as the leads to the circuit board from the control pad has been pulled out and a re-attachment operation is in order.

Never fails....sigh.