Let's determine if the slow motion drive is slipping.
Look under the turntable and see where the RA pinion engages the edges of the drive discs. You need to determine whether the pinion is ever slipping with respect to those disc edges. There should never be any slipping under any circumstances, no matter which is trying to push what. Usually, it will be the discs that stay still as the knob is turned. Try this while holding the fork stationary with the base. You will turn the knob dozens of times until the disc makes a full circle. A Sharpie mark at the edge will help tell when you have come full circle. Do this both directions.
If that operates perfectly (which would be a pleasant surprise given the age - and only a ~$200 factory fix if not - maybe half if only the RA needs to be done) then there may be something slipping in the "stack:
- Motor pinion turns the main gear.
- Main gear (resting on Teflon sheets to avoid friction with base housing) has a synthetic disc atop it. In later models this is pinned to the gear, but yours may rely on friction.
- You'll know the disc is turning because the RA ring is frictionally engaged to the disc. The Ring, fork, and RA indicator all turn together when the drive runs properly. The RA reading remains the same as the scope remains tracking an object. Be sure this is operating right. The RA ring should move with respect to everything under deliberate force.
- The metal drive discs stack on the synthetic disc, and are engaged only by friction.
- The turntable is engaged to the drive discs by the RA pinion, and the turntable slides with respect to the discs only when the knob is turned. I've never seen a failed one slide at the pinion interface.
So, if the RA ring turns when the fork doesn't, there is probably slipping between the synthetic disc and the drive discs. I've never seen this.
If the motor turns without ring or fork turning, then you'll want to pin the disc to the main gear. Easy factory fix. And might be avoidable by tightening the RA axis nut (under the red disc in the turntable). But if you had stiff slewing already that would be unappealing.
Note that if the RA slow motion is slipping, you might get a little temporary help by tightening the knob tension screw at the edge of the turntable under one of the arms.
If it were mine, I'd take off the red disc, take apart the stack, and clean what's dirty, remove all lubricant, and then reassemble and test. Consider pinning the disc to the gear if that is suspicious.
My videos #7,11, and 12 will help with all this.