| 摘要 |
In an illustrated embodiment scanner signals produced by marks of varying intensity on a moving web are effectively transmitted by self-biased capacitively coupled field effect transistors without overloading. The scanner circuit reliably responds to scanner pulses of variable duration and frequency over a wide range without readjustment and includes an inverted current restorer for driving an output transistor stage while providing substantial noise rejection. A servo motor is operable to adjust web errors as signified by the phase of the scanner signal relative to a reference signal, and a thyristor controlled bidirectional drive for the servo motor responds to excessive back voltage due to overrunning of the motor to automatically reverse energize the motor. Preferably the timing circuit for the thyristor is coordinated with the input alternating current power to insure that initial turn on of the thyristor in each cycle will accomodate a maximum error signal to the motor.
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