Turn Table Control Amp. ターンテーブル制御アンプ

This is Pioneer's model PL-1100, purchased about 40 years ago, direct drive turn table, still possible to work.

I want to modify the motor drive circuit using DC power amp.

Searching on the Web, I found a copy of service manual of Pioneer's direct drive turn table, which might be made in similar year of the model PL-1100.  The schematic diagram in the manual shows all the detail about inside motor drive circuit. Rotor position is detected by 3 hall sensors. Motor coil drive transistors are switched by the position signals. Since the motor drive power is fed from +18V, the motor drive current should be single polarity signal or half wave signal, not by full wave signal. The each drive current signal is rectified by a diode (1N60) which detect a signal proportional to the switching frequency or speed of the rotation. This speed signal is fed back to power circuit of the motor coil drive transistors.

The motor coils equip 3 hall elements. The set of coil consists of totally 24 wound (8 wound per phase).

Here is a study to make motor rotational drive signals from detection signals from hall sensors.
Below is the experimental circuit, which consists of sensor amp, gain controller and motor amp.
The gain controller is LM13700N (trans-conductance amplifier IC).  The motor amp is half signal because of experimental purpose.

Below is simulation result which shows plot of current signal flows load resistor. 4 different level of half sine curves are obtained by stepping control signal for LM13700N. This circuit should control the motor torque and result in controlling the rotation of the speed.

Blow is the another simulation plot made by the same 3 circuits. 120 degree phase different signals are given to the sensors. This set of 3 motor drive current should generate continuous  rotational magnetic field for the motor.

Below is circuit board made for position sensor signal amp and gain controller. OPA134 is used for each sensor signal amp.

Below is measured signals of position sensor output when the motor is turned by hand. The shape of the waves are far from sin curves, but this is the signal from the hall sensor. The power supply to the hall sensor is -5V/+5V instead of originally supplied with 0/+18V.

Below is circuit board for power amp to drive the motor coil. (This power amp is for experimental purpose)

The sensor amp and gain controller are installed with three power amp circuit boards. Original circuit board (round one) is left but not functioning any more.

The experimental trial resulted in successful by confirming that the new motor drive circuit with gain controller manage to rotate the motor. As changing resistance at control pin of the gain controller (LM3700N), the speed of rotation changes.

The next step is to make speed control circuit. The reference is Motor Control Amplifier for SP-10MK1 designed by A. Kaneta. Reference pulse signal (or clock signal) at 88.88Hz is obtained from 2.4579MHz X'tal oscillator with two digital divider; 512 and 54.
Below is circuit board made for the reference pulse generator and rotation speed signal feedback control.

Below is speed sensor part made with photo reflector (RPR220). There are 160 notches around the edge of the platter. The sensor detects 160 pulses per rotation. When the platter rotates at 33.33rpm, the sensor detects pulse at 88.88Hz (33.33[rpm] /60[sec] x 160 [pulse] = 88.88 [Hz]). Therefore, if the motor speed is controlled so that the speed sensor pulse is synchronized with reference clock at 88.88Hz, the platter rotates with speed at 33.33rpm.

Below is speed control unit built in a casing with power supply and +/- 5V regulators.

It was emotional experience for me to see the platter rotation with speed control for the first time. The rotational speed is controlled by feedback signal which is inverse proportional to detected speed by the sensor. But the speed feedback control can not maintain the speed very constant because the deviation increases as time goes. Phase lock loop control enables the speed control perfect. By gradually increasing the phase feed back gain, find the point where the phase is lock or synchronized with the clock by the quarts oscillator.

Below time chart plots speed sensor signal pulse (blue) and control output (orange) during start up of the rotation. The control output shows tendency of hunting before reaching rated speed. Driving power of the power amp may be insufficient.

The sound from this experimental turn table was clear which is apparently different from original drive circuit. It is my first experience to know the fact that the turn table motor is the first source of the power to replay the sound from records, and precise driving is important.

Next is to replace the power amp circuit with DC power amps, which should make the sound even greater.

Below is the power amp circuit. Final stage transistors are 2N3055 which consist of SEPP. Driver transistors are 2SA606 with simple connection to the 2N3055. This is typical DC power amp circuit known as 'Full symmetric circuit' named by Mr. Kaneta)

3 sets of the power amp circuit boards were made to built in the turn table box. The power source fot the power amp is battery.

Driving torque with DC power amp is powerful. The turn table accelerates much faster. The platter starts and reaches at its rated speed within one forth of a rotation. The speed feed back gain was a bit relaxed to obtain smoother control output as shown the plot below.

In order to check the speed of the rotation, strobe scope light was made with clock pulse.

Neon lamp bulb was replaced with red LED.

The battery for the power amp source is supplied by two Li-po batteris (2100mAh 3cell at 11.1V/each) 

The sound from the record is remarkable. Solo play tone in violin concert is very smooth and impressible. Piano tone is also vivid as if it performes live.

This turn table should not be ideal because rotor position signal with hall sensor is far from sine wave. 

(2017)