I can even hear a small click from the rear of the SL-7 as if a swtich is responding but perhaps not making complete contact. It does seem to be working more often the more I use it, so maybe that will come back with use. I may also trace down the switch and try cleaning it.
The worse problem is that the tonearm will usually not advance at all when I play a record. Since it does advance occasionally, it does have all the symptoms of a dirty sensor or something out of spec. I just looked through the service manual and couldn't find a reference to the location of the offset sensor.
First try displacing the tonearm through it's full range of movement a few times with the power and the tonearm lock / safety off. Then exercise <<>>, in-out, in-out... etc....as it seems to be getting better. I also try to clean the related plug in connectors on the pcbs
I was looking at page 12 of the Service Manual "Adjustment" section and in particular "Offset adjustment of tonearm and servo gain" section and read through and follow the instructions on page 13 and 14, Steps 2, 3 and 4. Hopefully this will cure the skip problem while the tonearm tracks an LP.
Offset angle detect sensor is an LED / Sensor combination, @ PC 401 on the IC 301 block diagram. I believe this optical sensor is located in the tonearm support well opposite to where the hex wrench is inserted to adjust the centering voltage. You may want to gingerly "blast" some air in this well where the "T" goes into the tonearm support and dislodge any dust / dirt prior to centering adjustment voltage. (ie a turkey baster or something that can push some air quickly and semi forcefully, I think new canned air may be a little too strong unless you've got a "low" canned air bottle).
After displacing and moving the tonearm and application of air, try playing a record again. You may have cleaned the sensor to bring the fine tonearm movement back. If not continue with voltage measurements and adjustments according to steps 3 and 4. A 1 mm test LP will be difficult to find, however, I believe you can use a standard LP for adjustment of the voltages. You really want the voltages correct while moving across one of the vinyls in your collection.
Good luck. Hopefully this will get you spinnin', trackin' and postin' in Currently Playing....
Fred and refer to page 30 for the tonearm connections to clean leading into the pcb, 111 for audio signal and 110 for tonearm drive.....