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CGEM AR motor stops working

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#1 grom

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Posted 29 December 2012 - 05:49 PM

I have had a CGEM for more than 2 years and it has worked quite well. Until yesterday.

Yesterday the AR motor didnt work when I switched ON the mount.
I checked again early this evening, and it worked alright. However, three hours later I switched on the mount and again, it didnt work.

Other data:
- No batteries involved
- DEC motor always works.
- Hand control seems to work, there is no error message shown.
- AR does not work either from the HC arrow keys or by itself as part of the alignment sequence (i.e. slewing to a star only DEC motor will move, but not AR)
- There is no noise at all coming from the mount, so it doesnt look like the motor is trying to move but the axis is stuck or something similar.

I searched the fórums and found very little. There is a similar problema described in
http://www.cloudynig...4050737/page...

However, most suggestions had to do with the power connector, and I fail to see how the power connector would ONLY affect the AR motor and not DEC or the HC.

Anyone experienced this recently? Any idea or comment would be very welcome.



#2 RTLR 12

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Posted 29 December 2012 - 06:09 PM

Remove the gear inspection plug and see if the gears are meshed or if the motor is moving at all.

Stan

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#3 grom

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Posted 30 December 2012 - 08:23 AM

I removed the plug and motor is not moving at all.

I also opened the front panel, and connected the DEC motor cable to the AR connector on the board, and the DEC still works when the AR buttons are operated. This seems to discard HC and board.

I keep looking.

#4 mich_al

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Posted 30 December 2012 - 09:49 AM

Sounds like an intermittant connection somewhere. Leave the Dec & RA cables swapped and test over a days time or so. Wiggle the motor cables near the connectors on each end with the arrow key pushed. Examine the gold connection pad where the cable connects to the PC board for contamination (clean lightly with a pencil eraser). Keep us informed.

Al

#5 grom

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Posted 30 December 2012 - 10:31 AM

In my opinión, the motor connector swapping at board level points to a mechanical problem, more likely than electrical. It could also be in the cable that goes to the motor, this I can not swap.

So, I decided to take out the AR motor and have a look. With the motor removed, I found that at first it didnt work, but after I nudged the cog a little it started working.

I am going to see if I can get to the gears (if the Little box behind the cog is indeed a gearbox) and see if they are dirty or something.

I attach a photo of the AR motor. Does anyone know what is the black wheel besides the cog and the little device at the end of the red cable?
It is not present in the DEC motor.

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#6 EFT

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Posted 30 December 2012 - 11:59 AM

The black wheel is the sensor for the periodic error correction.

You can open up the gearbox. However, this would only worth it if you can feel the motor running but the spur gear is not turning. If that is the case, then you could have a stripped gear in the gearbox.

If the problem is with the motor, then the most likely culprit is the encoder. You should take the cover off of the encoder and make sure that there is nothing in there blocking the sensor. A bad encoder is the most common problem with these motors.

You can determine if it is a mechanical problem with the mount by reaching into the housing with the motor removed and turning the worm spur gear with your finger to see if it moves.

#7 grom

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Posted 30 December 2012 - 12:24 PM

The black wheel is the sensor for the periodic error correction.

You can open up the gearbox. However, this would only worth it if you can feel the motor running but the spur gear is not turning. If that is the case, then you could have a stripped gear in the gearbox.

If the problem is with the motor, then the most likely culprit is the encoder. You should take the cover off of the encoder and make sure that there is nothing in there blocking the sensor. A bad encoder is the most common problem with these motors.

You can determine if it is a mechanical problem with the mount by reaching into the housing with the motor removed and turning the worm spur gear with your finger to see if it moves.


Thanks for the info.

I dont understand however the reference to the encoder. How could the encoder stop the motor from spinning? even if the encoder is broken, the motor should spin although the mount wouldnt know the position. Or I am missing something.

In any case, I put the motor in the freezer, gearbox included. After half an hour or so I checked it and it doesnt work, so it seems related to temperature.
I disassembled the gearbox, still not working. The only way to make it work is to nudge it manually with the arrow key pressed, then the motor suddenly starts to spin and continues until you reléase the key.
It seems like the motor has "dead spots".

I am considering disassembling the motor, out of curiosity, although it seems likely I will have to get a new one...

#8 EFT

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Posted 30 December 2012 - 12:37 PM

Thanks for the info.

I dont understand however the reference to the encoder. How could the encoder stop the motor from spinning? even if the encoder is broken, the motor should spin although the mount wouldnt know the position. Or I am missing something.

In any case, I put the motor in the freezer, gearbox included. After half an hour or so I checked it and it doesnt work, so it seems related to temperature.
I disassembled the gearbox, still not working. The only way to make it work is to nudge it manually with the arrow key pressed, then the motor suddenly starts to spin and continues until you reléase the key.
It seems like the motor has "dead spots".

I am considering disassembling the motor, out of curiosity, although it seems likely I will have to get a new one...


While a mechanical problem with the motor gearbox or a problem with the motor itself is possible, I find it to be a rare occurance. Small DC servos like this, while not high quality, are still relatively robust. However, if the axis were jammed and power continued to be supplied to the motor, something will give, either the gearbox, the motor windings or the motor board.

Bad encoders however, are a more common problem and can lead to a variety of strange behaviors from runaways to rough operation, to no operation at all. The mount's computer system depends on signals it gets from the encoders. How it handles things when it gets problematic signals is anyone's guess. You can easily test the motor operation itself by attaching a 12VDC power source directly to the two motor power leads.

One way or another, I pretty sure that you are going to end up needing a replacement motor.

#9 orlyandico

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Posted 30 December 2012 - 12:38 PM

this motor "supposedly" is almost a drop-in replacement..

http://www.mpja.com/...tinfo/18746 MD/

the gearhead ratio is almost exact (or is exact, I forget..) but it doesn't have the encoder and PEC index wheel.

one thing you can try - disconnect your existing motor from the motor board, and put 6V - 9V directly to the motor power wires. If it turns.. it's not the motor - and I would think it might be as Ed says the encoder.

#10 grom

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Posted 30 December 2012 - 01:18 PM

Thanks for the suggestion.
I put 12V from an external power supply and it shows the same behaviour. There are some spots in which the motor does not start spinning right away but it starts if you move slightly the axis.
So, I suppose the fault is not the encoder.

#11 EFT

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Posted 30 December 2012 - 01:33 PM

Thanks for the suggestion.
I put 12V from an external power supply and it shows the same behaviour. There are some spots in which the motor does not start spinning right away but it starts if you move slightly the axis.
So, I suppose the fault is not the encoder.


Sounds like it is in fact the motor. Unfortunately, the only source I know of for these motors is Celestron.

Now the question is why did it go bad? It may be that the motor just went bad (these are cheap "throw-aways" after all). But you need to be sure the worm turns freely. If the axis is jammed (completely unable to move) or the worm is jammed against the ring gear, then that could result in the motor burning out.

#12 grom

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Posted 30 December 2012 - 06:23 PM

Sounds like it is in fact the motor. Unfortunately, the only source I know of for these motors is Celestron.

Now the question is why did it go bad? It may be that the motor just went bad (these are cheap "throw-aways" after all). But you need to be sure the worm turns freely. If the axis is jammed (completely unable to move) or the worm is jammed against the ring gear, then that could result in the motor burning out.


How can I check the gear&axis inside the mount? where is this worm? all I can see after removing the motor is the cog where the motor gear connects

Also, regarding the spare parts, I assume the gear reduction must be exactly the same as the original motor in order for the mount to interpret properly the encoder readings? Or a moderate difference is somehow taken into account by the Software?

#13 EFT

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Posted 30 December 2012 - 06:29 PM

The spur gear ("cog") that you can see inside the housing with the motor removed is attached to the worm. You can test it out to some extent by reaching into the housing and turning the spur with your finger. You can also take the cap off of the end of the worm housing and attach something like a rubber hose to the worm shaft and turn it from there.

There is no way to change the gear ratios in the firmware so any replacement motor must have the exact same ratio since the encoder is located on the motor shaft rather than the gearbox shaft.

#14 orlyandico

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Posted 30 December 2012 - 09:24 PM

you could use the MPJA motor but put the original gearbox and encoder on it.. "esbarton" on the CelestronCGEM forum has done something similar... I don't know how much the Celestron motor is, I know it is not available for sale outside the US..

#15 EFT

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Posted 30 December 2012 - 09:26 PM

you could use the MPJA motor but put the original gearbox and encoder on it.. "esbarton" on the CelestronCGEM forum has done something similar... I don't know how much the Celestron motor is, I know it is not available for sale outside the US..


Actually, it is just the opposite. The MPJA motor will not work because it is too small and does not have a double shaft. The motor gearbox from the MPJA motor is the same as the Celestron.

#16 orlyandico

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Posted 30 December 2012 - 09:50 PM

oh. i got confused there. thanks Ed!

grom, since you're in Spain.. probably Maite (the NexSXD supplier) can help you find some replacement motor that you can transplant the existing gearbox and encoder to...

#17 grom

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Posted 31 December 2012 - 04:32 AM

The spur gear ("cog") that you can see inside the housing with the motor removed is attached to the worm. You can test it out to some extent by reaching into the housing and turning the spur with your finger. You can also take the cap off of the end of the worm housing and attach something like a rubber hose to the worm shaft and turn it from there.

There is no way to change the gear ratios in the firmware so any replacement motor must have the exact same ratio since the encoder is located on the motor shaft rather than the gearbox shaft.


Thank you for the info!




you could use the MPJA motor but put the original gearbox and encoder on it.. "esbarton" on the CelestronCGEM forum has done something similar... I don't know how much the Celestron motor is, I know it is not available for sale outside the US..

Really? OMG... why? :mad:

#18 mclewis1

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Posted 31 December 2012 - 09:09 AM

Celestron doesn't sell parts directly to end users outside of the US. They expect you to buy parts through your local dealer. So it's more correct to say that the motor is not available directly from Celestron outside of the US.

#19 grom

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Posted 27 January 2013 - 03:12 PM

I got the motor and installed it. I checked the result yesterday and everything seems to work fine, movement, alignment, goto....

I purchased the motor from the Celestron dealer, but they were reluctant to sell parts. I dont know why.
I asked two other dealers in Europe, one didnt have the motor and the other had it, but significantly more expensive than the local one.

I didnt know that getting parts for Celestron products was this difficult (and expensive too). I will keep this in mind for future purchases.






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