I stumbled on this thread by searching, and even though it's old, since there's already quite a bit of information about various problems I figured I'd add one of my own problems I've found in the past just for reference in case someone in the future happens across the same issue.

If the automatic arm return at the end of the record either doesn't activate, or activates too late, check the position of the plastic optical ruler inside to make sure it isn't tilted away from the sensor.  If the angle is skewed, then by the time the arm reaches the end of the record the sensor is no longer close enough to the ruler to see the markings.

I'm not sure I understand - it's a plastic ruler, with printed markings, and 3 screws to hold it down.

How can it be skewed, other than it not being secured properly with the screws?

In the pdf which covers all the details, see section 9 for the ruler issues.

Hmm, it was awhile ago I fixed it, so I can't remember exactly, but I can see why it'd be confusing from that picture you posted.  I remember when I ran the carriage though, by the time it got to the end the sensor was too far from the ruler.  I had thought on my turntable one of the plastic screw slots on the top was perpendicular to the ruler, allowing you to adjust the angle before tightening the screw.  There was definitely something on mine that allowed me to adjust how far the sensor was from the ruler though.  Sorry I can't remember.

Yes, it is adjustable. Not by those three screws (which allow adjusting the ruler longitudinally, moving the touchdown point), but two other screws holding the bracket where the ruler is attached. Service manual page 3-2.

The plastic ruler is mounted on a metal bracket fixed to the moving carriage by two set screws which allow adjustment of the distance of the ruler from the phototransistor at both ends of the travel.

I normally use a folded piece of paper as a feeler gauge to set the ruler up prior to tightening the two screws.

That was it!  Thanks guys.  By the way, has anyone ever tried replacing the sensor arm-light with an led?

What a timely resurrection of this thread.  My dear 4002 just started having speed stability issues last night.  At 33rpm it will slow down momentarily and then return to 33.  I can only detect a slowing of speed and not any speeding up.  The warmer it gets the worse it fluctuates.  After eliminating belt slippage as the issue, it seems I need to start replacing caps.  Can anyone tell me which ones I should start with?  I can solder, but I am not the best at it so I don't want to replace everything whole hog (is that a yankee term?).  I read (and saved) Menahem's excellent paper on resoration, but if someone could tell me where I should start that would be great.  There are no other issues with the deck, and from the appearances it looks all original inside except for the belts and bulbs that I have replaced.

Kevin