I own an SP-15, and the platter sits on a conical sleeve below the spindle, and the hole in the platter has a corresponding conical bore. It's fairly easy, with age and/or abuse, to get the platter cocked slightly, which will cause it to rub the case. Remove the platter by taking off the mat, grabbing the platter through the holes and pulling straight up. You can rock it a minute amount side to side to help loosen it, but be careful because too much side to side will damage the bearing. If it doesn't come off fairly easily (pretty common) then spray the joint between the platter and sleeve with something like WD-40 and let it sit overnight, then try it again. With some patience you should be able to get it off.
After the platter's been removed, clean up the sleeve and platter bore and reinstall the platter with about a quarter turn in relation to its old position relative to the spindle. This is frequently enough to true up the platter and make the table usable again.
While you have the platter off, clean all the dust out of the vents and holes in the case, and oil the bearing using something like Mobil One.
Good luck!
David
After the platter's been removed, clean up the sleeve and platter bore and reinstall the platter with about a quarter turn in relation to its old position relative to the spindle. This is frequently enough to true up the platter and make the table usable again.
While you have the platter off, clean all the dust out of the vents and holes in the case, and oil the bearing using something like Mobil One.
Good luck!
David