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CGEM RA runaway advice

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#1 petemumbower

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Posted 12 September 2012 - 01:17 PM

Thought I would see what the current state of thought on this issue is while I await word from the Celestron tech support portal.

Two nights ago I went to use my CGEM that is pier mounted in my observatory. I typically use NexRemote and awake the mount from hibernation. When I did this, it immediately started slewing in RA. I thought about the runaway issues I have heard about here in the past. So I switched over to the HC and got the same result. I followed this with the typical advice of reseating all the connections, using DC instead of my AC power source, reflashing the firmware for both HC and MC. Same results...runaway RA. The first night, I did pop open the panel on the mount and swap the connections for the RA & DEC and the problem seemed to stay with the RA motor.

So is it reasonable to think that something with the RA motor is bad? If so, is there anything I can do to 'fix' it? If not, how expensive is the motor? I did not see the motor on the Celestron parts ordering page.

Thanks for any advice on this, I feel like I lost a leg not having a mount to use.

#2 mich_al

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Posted 12 September 2012 - 03:30 PM

The motors are the same as the CG5's. They are $132 http://www.celestron...-small-dec.html

A spare mount is good to have around for CGEM owners.

You might also ask at : http://tech.groups.y.../CelestronCGEM/

#3 Alan S

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Posted 12 September 2012 - 07:25 PM

Pete,

I would recommend flashing the software and firmware...upgrading to the current version at the same time if you have not already...worked magic for me...

#4 RTLR 12

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Posted 12 September 2012 - 07:38 PM

Possible cause for run a way slews is encoder wheel air gap.

Stan

#5 petemumbower

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Posted 12 September 2012 - 07:59 PM

Alan - I did do the flash/firmware upgrade for both MC/HC and the same issue unfortunately.

Stan - I am positive the encoder is the issue. I can power up the mount and skip the alignment. Then go into utility menu and look at the axis position. When I get the motor spinning, the position numbers do not change. If I do the same test with the DEC motor...the numbers change. So for sure an encoder problem. Bad news: I was going to adjust the encoder wheel air gap and the set screw broke on me. I started to turn it and the slotted part of it bent back and fell off. That was with not much pressure. So now I cannot get the wheel off. Looks like a new motor assembly is in my future. Stinks, because the encoder is about $100 cheaper.

#6 shrevestan

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Posted 12 September 2012 - 10:37 PM

I had this problem the other night. About 2 hours in it started running away on me. I reset twice and quit having issues after I unplugged my USB hub from the battery pack. Could just be coincidence though.

#7 EFT

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Posted 13 September 2012 - 09:26 AM

Yes, definitely an encoder problem. I'm not sure why this happens. That tiny set screw is a pain. You can get a different encoder from US Digital but you will have to rewire the connections. The following encoder should work:

E4P-256-091-N-S-D-D-3
E4P Kit Encoder, 256 CPR, Single-Ended, Default Cover, Default Base, Individually Packaged

CA-MIC4-W4-NC-1
Cable Assembly, 4 Descrete Wires, MIC4 Connector to No Connector, 1 ft length

Total cost $39.05 plus shipping.

#8 petemumbower

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Posted 13 September 2012 - 09:34 AM

Any suggestions on getting that broken set screw off? The only option I can think of is to try to use pliers to rotate it since a screwdriver can no longer be used.

Any chance that doing an encoder swap is one of your services?

#9 petemumbower

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Posted 13 September 2012 - 03:08 PM

Talked to Celestron Tech support and verified the problem/part needed. So I order a CG5-GT RA motor online this afternoon.

Hoping it is not too picky about being put back together. (gear meshing inside the housing is really hard to see for me)

#10 RTLR 12

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Posted 13 September 2012 - 05:09 PM

(gear meshing inside the housing is really hard to see for me)


Even after removing the inspection screw?

Stan

#11 petemumbower

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Posted 13 September 2012 - 08:09 PM

Hmm, did not know that was there. I was too concerned with wondering what the problem was to really checkout how to get it meshed properly. This is the first time I have cracked open this mount.

#12 EFT

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Posted 13 September 2012 - 08:14 PM

Any suggestions on getting that broken set screw off? The only option I can think of is to try to use pliers to rotate it since a screwdriver can no longer be used.

Any chance that doing an encoder swap is one of your services?


I don't any particularly good ideas for how to get the set screw out since it is so small and the encoder disk is very fragile. Since you have another motor on order, you don't need to worry about trashing this one and you might just try to pry the encoder wheel off with a small screw driver. The set screw does not hold real tight on the steel shaft.

When I have some time, I will probably work on a procedure for changing out the encoders and getting the wiring correct.

#13 petemumbower

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Posted 21 September 2012 - 07:30 AM

Quick update to this. I received my replacement motor assembly(with encoder on top) yesterday afternoon. Last night I was able to work on getting it swapped out. It went pretty well, though I meshed the motor gear to the big gear in the mount too tightly and it did not turn at first. Actually I thought the motor was bad. But remembering about the inspection screw Stan mentioned above, I was able to get the meshing adjusted so the motor turned. It really is difficult to fine tune that meshing. Anyone have tips on that?

With the meshing adjustment made, I fired up the mount and everything worked as it should. Of course it was cloudy, so I could not actually use it, but I was expecting that.

#14 Geo.

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Posted 23 September 2012 - 02:46 PM

I aways try to get all the backlash out, but usually have to settle for a little to avoid having the mesh too tight.

BTW, a friend dropped off a late CG-5 with dual drives for me to look at (bent clutch shaft and dead hand control). I noticed it had ball bearings on the Dec worm axle. Is this now stock?






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