Thanks to all for your feedback-
Reading the linear tracking description in the PDF at the top of Fred's post made sense and shed some light, but didn't really explain how the opto sensing worked.
I've just taken the lid off my Sony to have a closer look & tricked it into thinking a record was inserted. Now I can clearly see how it works.
The sliding head has a laterally pivoted part which the cartridge is mounted to.
If I put slight pressure on the cartridge so that it pivots towards the centre of the record, the motor pushes the head assembly along the rail.
I can see there's a thin metal strip attached behind the cartridge which bends around behind it, then runs parallel with it. This arm is about 60mm long, then has a right angle with a 5mm length at its end. That 5mm passes between the optosensor's send/receive. As soon as the beam is unbroken, the motor moves the head.
I'm going to draw a diagram and stick it on my website as my description alone doesn't really do it justice.
I was going to ask how an air bearing worked until I saw this image
. Is that the general idea behind all air bearings? Clever stuff. Both of the DIY ttables look incredible. Would love to see them in operation.
Olddude- do you have any images of your 1210 with its arm mounted? Would love to see that. I find it amazing that a microswitch can be as sensitive as on your Rabco. If someone described that as a concept I'd've said it would never work, picturing it just juddering itself along and wrecking the vinyl.
Thanks to all in this thread for demystifying something I've been wondering about since I saw my first linear tracking turntable some 20+ years ago!