1100GTO Reduction Spur Gear-set Play Adjustment

Oct 31, 2017



#59277 Oct 31, 2017

I have experienced greater declination backlash from day one in my 1100GTO purchased new in December of 2015 than I would have anticipated.  I have played around with adjusting the worm mesh using both the slewing and park 3 push on the gearbox methods, but have not seen any significant change in the behavior.  I do not feel any backlash when I try to wiggle the declination axis, so I believe the preload on the worm bearing is adequately tight.

The leaves the reduction spur gear-set as the next possible source of backlash.  I took the cover off of the gear box and indeed I found that there was about 3 teeth worth of play at the small gear on the motor shaft.  I removed the intermediate gear with the three holes spaced 120 degrees apart and discovered what the holes are for.  The position of the pivot point of this gear is adjustable and the hex head screw that locks the pivot position in place is accessed through these holes.  With the gear in place I rotated the gear to allow the hex wrench to loosen the screw.  Using rotation of the motor shaft gear in one direction or the other tightens or loosens the lash of the gear-set.  I was able to take out virtually all the lash without the gears binding.  I then readjusted the worm mesh and ran the axis through 360 degrees to verify there was no binding anywhere in the full range of motion.

For the heck of it I opened up the RA gearbox and found that it too had significant backlash, though less at about 2.5 teeth of motion.  I went ahead and adjusted the gearbox and then the worm mesh and verified full 360 operation.  Of course, in theory, this should have no effect on guiding as long as the guide sped is 1 or below.

The proof is in the pudding, but of course the weather here is going to be bad for at least the next week, so it will be a while before I will know if this has any positive effect on the declination issue.  I have not found anything about making this adjustment on the 1100GTO on this site or the AP site.  But I did find a link on this site to AP instructions on various adjustments on the 1200GTO, but not the reduction spur gear-set play.  Here is the link:  www.astro-physics.com/tech_support/mounts/1200gto/Gear%20Mesh%201200.pdf.  At least it defines the proper terminology to use for various aspects and does show some other possible sources of lash that could be seen in other models as well.

I will post with results when I finally to get suitable conditions.  I am interested if anyone out there has played with this adjustment.  It is not clear that this adjustment is possible on other models, and may be a refinement first incorporated on the 1100GTO.  I had been looking at retrofitting with the self-adjusting worm gear boxes, but if this results in the kind of improvement I think is possible, I may not need that.  It would be interesting to compare the performance in this regard to the original like mine and the latest version.