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Celestron Advanced Series GT CG-5 Mount Runaway Slew in RA

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#1 E-Ray

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Posted 23 November 2017 - 12:12 AM

Hello All,
I need some runaway slew advice here. I bought a used AS-GT C6-S which was in excellent cosmetic condition for an older scope. The last time it was aligned was in 2005 as the previous owner wasn’t into astronomy.  It’s been 6 years since I owned a CGEM mount so I knew the CG-5 mount was glitchy before buying it.

The mount immediately goes into a runaway slew after pressing Two Star Alignment without waiting for me to select the first star it chooses, Vega. A second symptom is that the Rate 9 slew will suddenly stop or drop to a very slow slew (the slew bar on the LED screen still rotates the whole time) for about 15 seconds and then resume before happening again. It happens multiple around the same location on RA from 0 to 180 degrees.

Something else I discovered is that the DEC motor Index marking are on the opposite side of the mount housing (the power switch side.) So the dovetail tightening knob is on the right.  I don’t know if this matters or not which side the index markings are on. But it doesn’t make a difference for the runaway whether I set the index on the left or right side of RA housing.

The DEC motor works without issues. However, I cannot disconnect the cable from the power board connection (seems seized up in the plug.) Could this be the issue?
I am using a fully charged Celestron Power Tank to provide the 12V juice. The power tank  works fine on the much heavier CGEM II mount.

What I have done to resolve the issue:
-          Set the DEC index to the correct side – no difference
-          Used 120V AC cable – no difference.
-          Used a DC cable from my CGEM II mount – no difference
-          Flashed hand controller to latest revs  for HC to GEM 4.21 and MC 5.20/5.20 – no difference
-          Cleaned the cable pin ports with alcohol (except the DEC input connection side – no difference
-          Changed HC and MC to original revs HC GEM 4.10 and MC 5.01/5.01 – no difference
-          Used a different hand controller and flashed to latest GEM rev – no difference
-          Tried selecting a different first star, Deneb
-          Opened the RA motor housing to inspect the gears and motors. Everything look clean.

So at this point I am guessing that the RA motor control board needs replacement.
Any suggestions on other things to test before I order the RA board? The selling price is $168 plus tax and shipping.

Regards, Ed



#2 ishorx

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Posted 23 November 2017 - 04:06 AM

hi,did you try factory reset?



#3 E-Ray

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Posted 23 November 2017 - 09:24 AM

Yes. No difference.

#4 rmollise

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Posted 23 November 2017 - 10:02 AM

Unfortunately, a runaway slew is usually a symptom of a bad motor/encoder...IF...

 

Your power/power cable/connections are good. Use a freshly charged 12-volt battery, and check all connectors. In particular, spread the two halves of the pin on the mount-side power connector slightly.



#5 E-Ray

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Posted 23 November 2017 - 11:18 AM

Thanks Uncle Rod for the reply!

I knew about the pin issue on the power connector from my previous Celestron C8-N scope (the second scope I bought 8 years ago!) This mount has a solid pin so that is not an issue for this scope.

I just pulled the motor board (NXW432, REV-A) to see if there were any obvious short marks and it looks clean front and backside.

 

I did finally manage to pry out the DEC cable from the power board connector. It looks clean too.

 

Regards, Ed

 

NWX432 board

 


Edited by E-Ray, 23 November 2017 - 11:20 AM.


#6 rmollise

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Posted 23 November 2017 - 11:19 AM

The usual culprit is the encoder. It's an integral part of the motor, alas.



#7 E-Ray

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Posted 23 November 2017 - 11:21 AM

Hi Rod, 

So the encoder is on the motor board, NXW432, correct?

 

Ed



#8 SKYGZR

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Posted 23 November 2017 - 03:17 PM

Do believe that the encoder wheel (on this mount) is "hidden" inside the motor housing. I've disassembled & hypertuned one of these, as well as an old Meade LXD-55. On the LXD, the encoder wheels are exposed on the end of the motor, and if not careful, can easily be damaged,bent,broken.

 

From memory, when having the GT apart, no encoders exposed, so less possibility of damage...yet that does not preclude that they can't get damaged somehow.

 

Would have to pull apart the motors to check them. On one end of the motor is the drive gear that has to come off, then two small screws to remove the shaft cover to access the internal gears. Be careful when extracting the cover as the gears are small, and there's a real tiny washer/spacer that goes on the end of the gear stack. It may come off with the cover, as it might be stuck w/the sticky lube to the inside of the cover. Keep track of how the gears are arranged. Cleaning the goopy/sticky factory grease off these gears & re-lubing is often over looked when hypertuning these mounts.

 

I do believe that the slotted encoder wheel is on the opposite end of the motor, where the power leads are. That end ought to be removed so one can physically inspect their condition. (This is from memory...might be incorrect though.) As Rod has pointed out, it may be integral, and not serviceable, yet would actually have to inspect..I think I remember being able to somehow take a look, yet can't remember for sure.


Edited by SKYGZR, 23 November 2017 - 03:21 PM.


#9 rmollise

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Posted 24 November 2017 - 10:39 AM

Hi Rod, 

So the encoder is on the motor board, NXW432, correct?

 

Ed

 

No. The encoders are part of the motors. They are electro-optical devices that measure rotation.



#10 Phuzzie

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Posted 24 November 2017 - 11:57 AM

I had the same problem with my CG-5GT earlier this year. It was doing run-a-way slews in RA if remember. It was also giving me a "No Response 16" in the hand controller. I could not complete a star alignment. I did all the things you have listed, fully charged power supply, factory reset etc. I tried a firmware update but it did not work. I gave up and sent it back to Celestron. I figured if they could solved the problem and it did not cost too much I would get it repaired. It cost $82.53 to ship it. It cost $50.00 bucks to fix it plus $20.00 to ship it back. Here's what the invoice read when I got it back....CLEAN AND REGREASE GEARS, ADJUST DEC AXIS, ADJUST RA AXIS RESET CLUTCH KNOBS, FLASH MOTOR CONTROL AND HANDCONTROL TO CURRENT FW. I have had no problems and it has worked like new. All total $152.53. I'm glad I got it fixed. The only thing that I see that is different here is that your mount did not give you that "No Response 16" message. I still don't know what that message means. If it has something to do with encoders or Dec or RA motor control boards they did not mention it on the invoice. This has been the only problem I have had with it in eight years of usage. Once it arrived back at Celestron it was about two weeks before they took a look at it and they kept me up to date. It was about six weeks when I got it back. Good luck and I hope you get the issue resolved.



#11 rmollise

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Posted 24 November 2017 - 01:00 PM

No Resonse 16 means the hand control cannot communicate with the RA board. Chances are your problem was a matter of poor connections internally.


Edited by rmollise, 24 November 2017 - 01:03 PM.


#12 E-Ray

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Posted 24 November 2017 - 03:27 PM

Thanks Phuzzie and Rod for the responses. It's interesting that cleaning, greasing, and resetting RA clutch knobs would solve the issue.

 

I have opened a ticket with Celestron Tech Support and should hear back from them by early next week. I'd gladly pay $150 to solve the issue.

 

Another observation today is that I disconnected the DEC cable from the RA mount and it still went into RA runaway as soon as I pressed "Two Star Alignment." I'm surprised I didn't get a "No Response 16/17" because it wasn't communicating with the DEC motor at all. Making this odder is that I can switch from the first align star the HC selects, Vega, to Deneb which would require the DEC motor to move (as it does when the cable is connected) without the No Response 16/17 error.

 

The RA motor will keep rotating 360 degrees with the occasional slew pause as mentioned previously. The slew pauses seem to be in the same location each time.

 

Regards, Ed



#13 rmollise

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Posted 26 November 2017 - 01:43 PM

Thanks Phuzzie and Rod for the responses. It's interesting that cleaning, greasing, and resetting RA clutch knobs would solve the issue.

 

I have opened a ticket with Celestron Tech Support and should hear back from them by early next week. I'd gladly pay $150 to solve the issue.

 

Another observation today is that I disconnected the DEC cable from the RA mount and it still went into RA runaway as soon as I pressed "Two Star Alignment." I'm surprised I didn't get a "No Response 16/17" because it wasn't communicating with the DEC motor at all. Making this odder is that I can switch from the first align star the HC selects, Vega, to Deneb which would require the DEC motor to move (as it does when the cable is connected) without the No Response 16/17 error.

 

The RA motor will keep rotating 360 degrees with the occasional slew pause as mentioned previously. The slew pauses seem to be in the same location each time.

 

Regards, Ed

 

I would guess that in the process of doing that, they connected/reconnected one or more internal connections, fixing the problem.



#14 E-Ray

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Posted 08 December 2017 - 08:41 PM

OK, so just an update. I am sending the mount to Celestron for inspection and diagnosis. 

They gave me a list of test to try (swap DEC and RA cables on motor board, etc.) and it failed all of them.

The final finding I had was the RA Motor is stuck in Slew #9. 

 

I'll keep you posted to what they found and how much $$$ to repair.

 

Thanks! Ed




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