New Technics SP10 motor controller specification

[Steerpike]

I'm currently setting up a jig to hold the SP10 motor and rotate it via a brushed DC motor, so I can measure the characteristics at various speeds.
I cannot find any published data on the motor.

[Steerpike]

Just a very rudimentary setup to measure things like the tacho waveform, phase detector output waveforms, etc. at actual playing speeds.
The 'other' motor is stolen from a 5-1/4 inch floppydisc drive.

[rich121]

Great!
I'm glad your working on it, as I'm so very anxious to get my SP-10 in a new plinth, with motor only... and have a customized controller/power supply to run it..

You have my total attention...


Rick

[Steerpike]

I'm interested to know: what are your 'problems' or dislikes with the SP10(ii) as it is from the factory?

As I said, I need to create a drive system since I have nought but the motor, and it's simpler & cheaper to create my own rather than clone the original circuit boards from the service manual.

But IF I had originals, I don't think I'd bother changing them.

[BTW. someone somewhere ( I cannot now find where) asked what crystal the SP10(ii) used. If that person is reading this, the crystal is 3.5795MHz, which is more commonly used as the colour phase reference in NTSC (USA/Japan) TV sets and video recorders. VERY commonly available.]

[Steerpike]

This is the basic system / configuration I've decided upon. I'm working on a breadboard version of parts of it. The actual electronics will use no esoteric or unusual ICs - except for the microprocessor part, but since its only function - other than control logic - will be to generate the quartz-derived timing signal, it could be substituted with a discrete logic divider.

[Mark Kelly]

I believe that circuit will give performance comparable with the old voltage servos which were used before the FG servo circuit was developed. This is a distinct retrograde step.

The problem lies in the available accuracy and precision of the frequency to voltage conversion being nowhere close to the accuracy and precision available by staying in the frequency domain and using the PLL to generate the lead / lag signal.

[sek]

Feeding the split magnitude and phase components of the tacho frequency signal back into an error amplifer via an active filter network is how they do it until today (SL-12x0MK5)...

The 'vintage servos' were implemented without a tacho frequency coil signal at all. I have both an SL-2000 (servo control) and an SL-1200 (MK2, servo control with quartz referenced PLL using the tacho speed in a multi-loop) and the difference is like night and day.

Cheers,
Sebastian.

[Shaun]

Originally posted by sek
and the difference is like day and night

Hi Sebastian

Could you please expand on your observations? It is not clear what you mean.

[sek]

With the regular speed control via position detector coil feedback and servo control, speed regulation is slower and less damped. I attribute this to slower rise time and overshoot in the control loop.

This control scheme works best with heavy platters (flywheel effect) and is susceptible to speed slowdown through stylus drag (or any stronger force).

With the PLL and a stronger motor (like in the Technics MK2 players) higher forces can be applied, which enables the implementation of quicker regulation in the control loop. Speed is stable very quickly, and it behaves well damped with only slight overshoot (if any). Startup and full stop take place in less than a second, respectively, with just one (barely visible) ringing period under heavy load (like added weight or manual slowing).

This control scheme works best with light platters (reduced inertia) and is not susceptible to even tight clamping pressure at the spindle using thumb and index finger (or anything less forceful, like stylus drag).

Criticism of this "high speed, high torque" direct drive scheme (as it's advertised) adresses the noise and muddiness the quick corrections might presumably introduce into the pickup sound. Imagine the stylus dragging inside the groove, and the control loop in turn constantly accelerating and deccelerating the platter...

My personal opinion? I can hear only the improvements and never recognized any audible negative effects. They might exist, but this Technics direct drive scheme is reported to fare very well in comparison to good and very good record players in the sub-1500$ class.

Cheers,
Sebastian.

PS: I cannot provide you with figures and numbers, as I don't have test records available. My experience is based on visible (strobe) and audible (music) effects.

[Mark Kelly]

Umm...

I just noticed that I misread the schematic and didn't see the connection between the FG circuit and the PLL.

My previous comments are therefore moot.