I am having an issue with the thrust washer (this would be the washer at the end of the worm, that threads into the motor housing assembly, it is flat with two holes 180 degrees apart) for the DEC worm on my 1200gto, it will loosen up for some odd reason resulting in DEC backlash. When you move the OTA in DEC, even in park three it goes "chunk chunk". I had this problem several years ago and adjusting this washer fixed the problem.
Many years ago either George or Howard at AP told me how to adjust it. Of course I have forgotten how to do this so asking if one of the AP wizards might chime in.
A few related questions...is this adjustment done with the motor housing off the mount?
I would assume done in park 3 (counterweight shaft pointed down to ground, OTA pointed towards north pole) if done on mount.