The technology of the Technics direct drive turntables is quite interesting (as a geek!). I've never worked on the SL1200 etc. but I spent a summer fixing a misbehaving Technics SP10 (this was a turntable platter without a tonearm - it was up to you to mount the platter and a tonearm of your choice into a suitable box).
[geek on] The stability of the speed control results from phase-locking the turntable rotation to the quartz timer - the crystal drives a ramp generator that produces a sawtoth waveform, and the tacho pulses from (I think) a hall-effect sensor triggered by toothing on the platter side of the motor (i.e. derived from the actual rotation) sample the sawtooth waveform through a sample-and-hold circuit. If the pulse arrives "late" a higher voltage is sampled and the motor is driven harder, if it arives earlier the motor gain will be lower.
Because the turntable needs to be turning to generate the three-phase motor drive signals (from pickup coils out of phase with the motor coils) and the tacho pulses, there is an open-loop startup mode. If the closed-loop operation doesn't work, you have to turn the platter by hand to diagnose (or use a homebrew device -the "SP10 Electric Finger" which was a rubber wheel on a motor which you could clamp onto the unit and it would wind the platter for you while you poked at the electronics...
The manual start mode just drove the motor at full power - on the model I was working with at one point it got stuck in startup mode and the turntable kept accelerating and finally topped out at about 400rpm.
I suspect that as the SP10 was a 1970 design, the SL1200 etc. incorporates many of the features.... [geek off] (I'm sorry - wave of nostalgia!)
no subject
[geek on]
The stability of the speed control results from phase-locking the turntable rotation to the quartz timer - the crystal drives a ramp generator that produces a sawtoth waveform, and the tacho pulses from (I think) a hall-effect sensor triggered by toothing on the platter side of the motor (i.e. derived from the actual rotation) sample the sawtooth waveform through a sample-and-hold circuit. If the pulse arrives "late" a higher voltage is sampled and the motor is driven harder, if it arives earlier the motor gain will be lower.
Because the turntable needs to be turning to generate the three-phase motor drive signals (from pickup coils out of phase with the motor coils) and the tacho pulses, there is an open-loop startup mode. If the closed-loop operation doesn't work, you have to turn the platter by hand to diagnose (or use a homebrew device -the "SP10 Electric Finger" which was a rubber wheel on a motor which you could clamp onto the unit and it would wind the platter for you while you poked at the electronics...
The manual start mode just drove the motor at full power - on the model I was working with at one point it got stuck in startup mode and the turntable kept accelerating and finally topped out at about 400rpm.
I suspect that as the SP10 was a 1970 design, the SL1200 etc. incorporates many of the features....
[geek off]
(I'm sorry - wave of nostalgia!)