by semuenter » Mon May 18, 2015 7:58 pm
Hi Mhaznedhar:
Looking at the schematic, I think the motor driver works like this. The speed control pot adjusts the drive voltage to the gate of the MOSFET which drives the motor so that it spins the disk at approximately the speed you want. The NBTV frame rate of 12.5Hz means that you should adjust the pot so the disk spins at about 750 RPM. There is some inherent feedback to this speed control. As the motor spins faster, the motor's back EMF lowers the voltage on the speed control pot which lowers the gate voltage to slow the motor down. If something like frictional drag slows the motor down, the reduced back EMF raises the gate voltage to increase the motor speed.
The 4046 PLL uses the edge sensitive phase detector to add or subtract voltage from the MOSFET gate drive to speed up or slow down the motor around the operating point set by the speed control pot. When the opto fork pulses are aligned with the frame sync pulses, the output on pin 13 of the 4046 is high impedance which doesn't affect the motor speed. If the opto fork pulses are lagging behind the sync pulses, the 4046 outputs a train of positive pulses to raise the gate voltage of the MOSFET to drive the motor faster. Likewise, leading pulses from the opto fork cause the 4046 to output a train of negative pulses to slow the motor down.
To adjust the circuit, I would disconnect the 147K resistor from pin 13 of the 4046. You should then be able to use the speed control pot to adjust the free running motor speed anywhere between full blast and dead stop. Adjust the pot so the speed is set close to 750 RPM. Next, I would confirm that there is sufficient speed control from the 4046 to synchronize the disk. With the one lead of the 147K resistor still disconnected from pin 13 of the 4046, short this resistor lead to ground. This should slow the motor down a bit. Likewise, when you connect this lead to +12V the motor should speed up a bit. These slow and fast speeds should bracket the final desired speed of 750 RPM. If that works, reconnect the 147K resistor to pin 13 and you should be good to go. Even with no video input signal, the motor should be spinning.
I hope this helps.
Steve