Hi Bob, and welcome to Beoworld !
The platter lifts straight off. The black subplatter likewise but note the three
prongs that fits into similar cutouts in the hub to get it back correctly.
The center pin is attached to the hub and does not come off with the platter.
If you have a humming sound for each platter revolution (or the speed display
is not steady but jitters in light intensity, the tachodisc is bad.
You can see the tachodisc, a circular sheet with a pattern, apprx 10cm diameter,
sitting around the hub under the subplatter.
If yours is of the photographic type it will be falling apart and should be replaced. If it's of the metal type, it will last forever.
New stainless steel tachodiscs are available.
Martin