Jump to content


Photo

CGEM Advice

  • Please log in to reply
16 replies to this topic

#1 Loswarren

Loswarren

    Lift Off

  • -----
  • Posts: 4
  • Joined: 29 Jun 2008
  • Loc: Austin, Texas

Posted 18 February 2013 - 02:47 PM

I had an unfortunate event last weekend with my CGEM in the middle of the night while I was imaging and now I have a problem. My OTA got snagged on a brace on the east side of my mount in my observatory for 2.5 hours while the mount was not parked. :bawling: My OTA reached a point where it would not move west any more and so the mount applied torque without there being any westard movement possible. The OTA appears undamaged but my CGEM is not acting right any more.

Here's what I mean that my CGEM is not working correctly:

If I shut down and cycle power on everything (so I have to rebuild the CGEM sky model), I can follow the built-in alignment and calibration process - and this starts to show signs of weirdness. Pointing to calibration stars is grossly inaccurate even after adding as many as 6 calibration stars. My mount used to be spot on after the first 2 cal stars. In addition, after the 2-star alignment and 3 cal stars the mount can't move accurately to a specific RA or DEC. It's off by a lot (5-15 degrees of sky). I can't tell if the issue is RA or DEC.

I ran the encoder test as outlined on the Celestron website and the results indicate that the encoders are working correctly. The motors don't grind (any more than normal) and when slewing, there are not unusual sounds or pops. The mount actually appears to track DEC correctly but I haven't tried guiding.

I could send the mount back to Celestron but first I want to take a look at it myself because my mount is out of warranty. The tech support guy at Celestron recommended I check the gear train to see if the gears have gotten out of alignment. He guessed that the motors didn't burn up - I hope he's right. I may even hypertune the thing while I'm in there.

Is there a CGEM guru out there that has seen this behavior before? Any ideas where I could look for the problem or how to go about troubleshooting?

#2 darbyvet

darbyvet

    Viking 1

  • -----
  • Posts: 835
  • Joined: 19 Oct 2010
  • Loc: Seneca Falls, New York

Posted 18 February 2013 - 02:52 PM

If you are going to hypertune it why not contact Ed Thomas and see what he thinks? He was very helpful when I had a cgem issue.

#3 ghataa

ghataa

    Viking 1

  • -----
  • Posts: 625
  • Joined: 20 Jun 2011
  • Loc: Central, NJ

Posted 18 February 2013 - 04:41 PM

I'd talk to Ed Thomas. He fixed my out of warranty CG5 and did the hypertune at the same time. Send him a note.

Best,

George

#4 DaveJ

DaveJ

    Surveyor 1

  • *****
  • Posts: 1726
  • Joined: 07 Jan 2005
  • Loc: NE Ohio

Posted 18 February 2013 - 06:57 PM

Before you do anything, try a reset to factory settings. It's a long shot, but it just might solve your problems. Oh, and remember to input your Lat, Long, time, time zone, etc.

#5 Stew57

Stew57

    Mercury-Atlas

  • *****
  • Posts: 2545
  • Joined: 03 May 2009
  • Loc: Silsbee Texas

Posted 18 February 2013 - 07:51 PM

A reset would be my first try. I had one that I never could get to point correctly Celestron replaced it under warranty.

#6 mich_al

mich_al

    Vanguard

  • *****
  • Posts: 2401
  • Joined: 10 May 2009
  • Loc: Rural central lower Michigan Yellow Skies

Posted 18 February 2013 - 08:51 PM

Make sure the clutches aren't slipping.

Al

#7 Raginar

Raginar

    Fly Me to the Moon

  • *****
  • Posts: 6138
  • Joined: 19 Oct 2010
  • Loc: Rapid CIty, SD

Posted 18 February 2013 - 09:40 PM

Ed will respond, I'm sure. Also, try posting in the Yahoo! group. The one time I did this, I didn't find any lasting damage to the mount, it worked just fine after I did a factory reset.

Good luck!

#8 John O'Grady

John O'Grady

    Sputnik

  • *****
  • Posts: 45
  • Joined: 15 Nov 2009
  • Loc: Waterloo, ON CAN

Posted 18 February 2013 - 10:30 PM

As I understand, mounts such as the CGEM derive their position from an encoder that is mounted on the shaft of the motor. Thus, if a clutch or gear slips, or the mount binds during a slew than the motor controller believes it is at the right position even though the mount didn't move the full distance required. This is why Al suggests checking the clutches.

What could be a problem is if the gear box, which is part of the motor assembly, has been damaged. It sounds like the RA motor would be suspect. You could remove the RA motor from the mount and check it. You can turn the motor by hand and if it turns reasonably smoothly (takes some force) in either direction then you should be ok. If you think its damaged and you're not sure, you could remove the DEC motor for comparison.

This might be bit more of a goose chase but - "The motors don't grind (any more than normal)". If the gears grind at all, then it suggests to me that the worm gear or the motor gear mesh is wrong and needs to be adjusted.

On the CGEM yahoo group, there is a photo titled "Worm Gear inspection AB". This photo shows two screws that can be removed to inspect the gear from the motor. If the gear appears to bind or slip when slewing at a high slew rate then you need to adjust the motor gear mesh. You'd want to check this before removing the motor. Likewise, if you do remove the motors then you'll want to use these port holes when you re-install the motors in order to re-mesh the motor gear.
http://groups.yahoo....64/pic/80667...

Here's a link to a great CN post on worm gear (not motor gear) mesh adjustment:
(warning: when I first attempted this, I stripped the threads of one the bolts)
http://www.cloudynig...3918213/Main...

As others have suggested, I would recommend you try contacting Ed Thomas at Deep Space Products.

Hope this helps.

#9 orlyandico

orlyandico

    Fly Me to the Moon

  • *****
  • Posts: 5511
  • Joined: 10 Aug 2009
  • Loc: Singapore

Posted 19 February 2013 - 12:22 AM

one possibility is that the first pinion gear in the gearbox, which is made of plastic, has stripped its teeth.

that is exactly the purpose of this plastic gear - to destroy itself when the mount jams against something.

your encoder test would still work because the encoder is on the motor shaft, not on the gearbox output shaft. so the motor turns merrily but the gearbox is losing counts due to the stripped gear.

other than replacing the motor, there is a way to take the metal gear out of the $12 MPJA motor and replace the stripped gear. Not a trivial task though.

#10 Stew57

Stew57

    Mercury-Atlas

  • *****
  • Posts: 2545
  • Joined: 03 May 2009
  • Loc: Silsbee Texas

Posted 19 February 2013 - 07:11 AM

I have always questioned the wisdom of a sacrificial gear in a nonrepairable gearbox. If the motor needs to be replaced anyway let the motor fry. Why not add current overload protection?

Anyway one could check for gear damage easy enough through the inspection port.

The mount I had that had pointing accuracy problems would seem to do the 2 alignment fine. Adding the calibration stars never helped any. The mounts that worked correctly the calibration stars get successivly closer. Celestron could not find the problem so they replaced it. The thoughts were the motor control board had a fault. Perhaps the over current situation caused some contol board damage.

#11 Stew57

Stew57

    Mercury-Atlas

  • *****
  • Posts: 2545
  • Joined: 03 May 2009
  • Loc: Silsbee Texas

Posted 19 February 2013 - 07:33 AM

I have always questioned the wisdom of a sacrificial gear in a nonrepairable gearbox. If the motor needs to be replaced anyway let the motor fry. Why not add current overload protection?

Anyway one could check for gear damage easy enough through the inspection port.

The mount I had that had pointing accuracy problems would seem to do the 2 alignment fine. Adding the calibration stars never helped any. The mounts that worked correctly the calibration stars get successivly closer. Celestron could not find the problem so they replaced it.

#12 mich_al

mich_al

    Vanguard

  • *****
  • Posts: 2401
  • Joined: 10 May 2009
  • Loc: Rural central lower Michigan Yellow Skies

Posted 19 February 2013 - 09:14 AM

If you remove the motor the gear assembly can be exposed and inspected, just pay attention to what does where specifically washers.
The mentioned Yahoo group is a good resource but be aware that there are a couple of guys over there that will always say that any problem is 'operator error'.

Al

#13 rmollise

rmollise

    Hubble

  • *****
  • Posts: 15582
  • Joined: 06 Jul 2007

Posted 19 February 2013 - 10:48 AM

Does it slew smoothly? If so, the problem probably isn't a stripped gear. I'd start by checking cables and doing a Factory Reset.

#14 Stew57

Stew57

    Mercury-Atlas

  • *****
  • Posts: 2545
  • Joined: 03 May 2009
  • Loc: Silsbee Texas

Posted 19 February 2013 - 11:37 AM

They will say operator error or power supply!

#15 rmollise

rmollise

    Hubble

  • *****
  • Posts: 15582
  • Joined: 06 Jul 2007

Posted 19 February 2013 - 12:12 PM

They will say operator error or power supply!


Why will they say that? Because those are _always_ the first things to check. ;)

#16 orlyandico

orlyandico

    Fly Me to the Moon

  • *****
  • Posts: 5511
  • Joined: 10 Aug 2009
  • Loc: Singapore

Posted 19 February 2013 - 08:38 PM

yeah Rod but in my case they kept beating the operator error drum long after it ceased being funny.

turns out i was losing ticks on the encoder. so no, I was not going insane, the mount really had a problem.

but i digress. You cannot inspect the sacrificial gear through the inspection hole. You'd have to take apart the motor gearbox, which isn't trivial.

also, Mark - the reason for the sacrificial gear is that the CGEM motor board has no overload protection. So it's not the motor that will fry, but the motor board. I don't know if they've fixed that in the DX, or if the DX board won't fry due to the beefier motor controller chips.

#17 mich_al

mich_al

    Vanguard

  • *****
  • Posts: 2401
  • Joined: 10 May 2009
  • Loc: Rural central lower Michigan Yellow Skies

Posted 19 February 2013 - 11:07 PM

yeah Rod but in my case they kept beating the operator error drum long after it ceased being funny.

Yea, yours and alot more.

but i digress. You cannot inspect the sacrificial gear through the inspection hole. You'd have to take apart the motor gearbox, which isn't trivial.


Not trivial but anyone with any mechanical ability shouldn't shy away from it IMO.

Al






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics