Posted on Feb 7, 2014

Despite the high impedance of its inputs, Darlington Q10 develops enough current to drive an LED. The inverted output of Q10 drives opto U5`s internal LED. Opto U5 couples to motor-control triac CR1. Motor Voltage Control Triac CR1. Triac CR1 is a pulse-width modulator. CR1 connects in series with the color wheel motor. The triac circuit replaces V6, a 6BL7 dual triode tube

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in the original Col-R-Tel circuit. The triac also replaces T3, the Col-R-Tel motor control transformer. The triac gate voltage varies with signals from Q10. The triac output voltage varies with the gate voltage and the line AC. Like the tube in the original Col-R-Tel circuit, the triac has two halves. Each half of the triac is an SCR that interacts with one alternation of the power AC waveform. As triac CR1`s input voltage varies, the triac`s resistance changes. This resistance change increases or decreases the voltage drop across the motor. If the disc wedge is early, then the motor speed is too fast. The CR1 input voltage is low. Triac CR1 increases its resistance and reduces current flow to the motor. The motor slows down. If disc wedge is late, then the motor speed is too slow. The CR1 input voltage is high. Triac CR1 decreases its resistance and increases current flow to the motor. The motor speeds up.

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