Further information:
The tag (what PE calls it) is a metal fitting on the outside edge of the groove track on the change cycle cam. Adjusting it changes the width of the track at that point. It sets the difference between the position of the arm when the feeler touches the record and the position of the arm when it sets down in the record.
The first drawing of a change cycle cam on the following website has a similar device. It is a Collaro changer, not a PE, but it also senses the record size by touching the record with the arm tip:
http://geocities.com/midimagic@sbcglobal.net/collaro.htmLook for where it says "Eccentric setdown adjustment screw" on the drawing. The white object around that screw is moved by turning the screw, changing the width of the test track. The groove track on the Collaro has a different shape, because the Collaro moves the arm away from the record after touching it, so the record will fall flat when it is dropped. PE does not do this, so the groove track is closer to egg shaped.
Another reason the groove track is different is that the PE disengages the friction clutch when he arm sets down. Collaro does not disengage the clutch in the pickup positioning assembly, so the groove track becomes wide when the changer is out of cycle, where the arm must be free to track the record.
But the setdown adjustment works the same. Adjusting the width of the test track changes the distance between the position of the arm when the tip touches the record and the setdown position. On both changers, make the test track narrower to move the setdown away from the spindle, and make it wider to move the setdown point closer to the spindle.
Note that if the tag is loose, the setdown will be erratic. A loose or lubricated friction clutch in the arm drive can cause the same thing. Also note that the friction disc in the arm clutch may have deteriorated.