Replaced the turntable motor.
The APP video was very well done and instructive.
The old motor was held in place by a screw and a tab which fits into a corresponding slot.
Unfortunately, the replacement motor (while mechanically and electrically equivalent) has a flange around it that prevented the tab from sliding into the slot. The flange had to be ground away for about 1/2'' along its length near the tab in order for the tab to be insertable into the slot.
This may be a new design allowing it to fit a greater number of microwave models, and some may not need any modification to the flange. The JVM1870 appeared to.