SOTA Sapphire speed issue

You've likely discovered your problem, rabbi.  The belt should remain in a fixed position on the pulley regardless of whether the motor is running.  I suspect you may have a loose/failed motor mount.  It's possible you may have a failed motor bearing, but that would almost certainly be audible.  The best course of action would be to call Donna and see what she might suggest.  I'm not familiar enough with the motor assembly to give you a good opinion about a DIY attempt.  BTW, the belt should be centered on the pulley when everything is level and adjusted correctly.

For what it's worth, if the belt is riding low on the pulley, and if the pulley is narrowest at its mid-point, then the speed of the platter would tend to be a bit faster than correct, assuming the pulley is rotating at its intended velocity.

@lewm Thank you very much for chiming in. Actually, the pulley in profile is convex – its midpoint is the fattest part. For whatever that's worth. ;-)

Donna suggested starting with a new belt, so one is on the way. I'll report back!

Hi, Y'all,

Well, got the new belt from Donna and it seems to have solved the speed stability issue. The SOTA manual suggests replacing the belt after three years of normal use, and mine was around 8 or 9 years old! Now I'd like to get my hands on a stroboscopic disk and see what the current speed actually is.
Thanks for all your advice, folks.