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Help! Encoder code wheel/grub screw on CGEM motors

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#1 orlyandico


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Posted 21 October 2012 - 03:13 AM

I think I've resolved the wild slews of my CGEM - ironically because I was going to sell it, until I discovered that the wild slews hadn't disappeared after the firmware update.

I used "Get axis positions" in the Utilities menu to verify that both motor encoders were losing a huge number of ticks.

I was able to resolve this by spacing the encoder wheels very close to the photodetectors.

However, I busted the RA grub screw in the process. I recall some other folks have had this same problem.

So -

(1) what is the size of the grub screw? M2?

(2) where can I buy a replacement encoder wheel? anybody know the part number?

As an aside, I've always hated this CGEM, both for its immense bulk and its completely unreliable operation. Many times I questioned my sanity because I couldn't get it to work properly.

But now I know why it has been behaving the way it has, and I'm actually hoping I will now get the great CGEM / CG-5 experience that lots of folks keep talking about.

#2 Geo.



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Posted 21 October 2012 - 02:49 PM

Fortunatly, I haven't had to take mine apart. I think these are Indian made motors. Not sure about the encoders. They used to be USDigital E4P encoders. Check their web site for parts. Celelstron may have complete motors on their parts page.

#3 John O'Grady

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Posted 21 October 2012 - 04:07 PM

I've read on the CGEM Yahoo user's group that the CG-5 motors are the same servo motors as those used on the CGEM. On the Yahoo group they report that they can be used as a direct replacement (with some caveats of swapping out the motor bracket). I really can`t say, but on celestrons website, on the page for the CG-5 RA replacement motor, there`s a photo that shows an encoder. Now, I haven`t had my CGEM apart to look at the encoder, so maybe this isn`t the same encoder (one in the picture appears to be a shaft encoder)???

Since, I don`t know if its the same encoder (just speculating), I`d suggest you post on the CGEM Yahoo user`s group and ask if anyone over there knows.

Here`s the link to the replacement motor on the Celestron website:

(while the link and page don`t state CG5, the part number on the parts page that links to this page refers to it as Item: CG5-FOO-2)

Hope this is some what helpful.

Best of Luck.
P.S. I remain in a constant love-hate relationship with my CGEM. It`s an awesome GOTO scope but wish I had chosen something entirely different for imaging.


#4 orlyandico


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Posted 21 October 2012 - 09:31 PM

It is an E4 encoder (with the glass code wheel).

To clarify, my encoder is not busted. But I damaged the grub screw on the code wheel when torquing it down, so I will probably damage the wheel if ever I try to remove the screw to re-space it (not that I would need to).

The newer Celestron motors use an E4P encoder (with aluminum push-on code wheel instead of the glass wheel on the E4). I can buy the entire assembly from US Digital and plonk it on the existing motor, and avoid paying an outrageous price for the Celestron motors.

But for now I just want the grub screw replacement (and a new wheel if I end up destroying the old one when I remove the busted grub screw).

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