Celestron CPC 800 issue with azimuth clutch.

[Loki1978]

Hi all, 

Just thought I would post this up here. I've been using my CPC 800 for a little while now (bought it second hand). Today I noticed if I did the azimuth clutch up. I could move the scope azimuth by about 5-10mm. I never really noticed it before and my gotos were ok but it didn't seem right. 

Not being afraid to take things apart I took the base apart to have a look. The setup is very simple and the problem was also a simple fix. But misleading making you think you have gear backlash or something is very wrong. I thought I would post this up here. It might help someone out with an out or warranty scope willing to take it to bits.  

Remove the six screws around the lid of the base. The clutch has a grub screw in the side of the knob. Back this grub screw off and unscrew the knob (this grub screw shouldn't be tight, it just stops the knob unscrewing too far). The base lid lifts off with some shimmying.

This is what it looks like inside(ignore the victoria biscuits, I was doing it on the kitchen counter). The motor worm gear assembly is sprung for close to zero backlash. You can see the motor assembly and encoder. My encoder wheel actually has a crack in it beside the IR cutout, but it seems to work ok. If it breaks I will 3d print a new one. You can see the teflon washer that the clutch tightens onto. I put the scope back on my tripod. Tightened the clutch knob back up to check where the movement was. 

The movement wasn't in any of the motor or gear assembly so next part came off (if it was the worm gear moving back and forth, this can actually be adjusted by a brass bushing at the encoder end) This pic is the hub that the pinion gear clamps down onto. The rubbery material is the clutch which transmits the drive to the base.  It also has a decent sized taper bearing under this hub. The clutch hub is actually where my problem was. It has two grub screws which attach it to the shaft connected to the base. Well they were both loose. Disappointingly the first one I tightened up stripped quite easily, or I was being a bit heavy handed. In my opinion the grub screws are too short and only engaged by about 3 threads in aluminium. I swapped them out with longer grub screws which allowed me to save the slightly stripped one, I also added a bit of thread lock. Worse case they could be tapped out a size up.  Or drilled and tapped further around the hub if you're really in trouble with stripped threads.

 

I cleaned all of the old grease off of the pinion. Re-greased with some decent moly. I filed the grub screw holes very slightly as they had some burs which made the pinion stiff to rotate. All back together and the azimuth movement has gone. Its now rock solid. I also use to get a bumping or notching feel when I loosened the clutch and rotated the scope. This has now gone. Initially I thought it much be flat spots in the ball bearings or grit. Now I know it has a taper bearing it was likely the burs on the grub screw holes. 

 

Edited February 28, 2021 by Loki1978