AUTOMATIC TESTING IN TELEPHONE MANUFACTURE 1151 



the fixed harmonics of a standard 10-kc signal, the first order difference 

 frequency in the modulator output varies back and forth between and 5 

 kc. It passes through the 2500 c.p.s. point twice per period of variation, 

 or twice per 10-kc interval of the oscillator frequency. 

 The output of the modulator is sent through a narrow band amplifier 

 I which peaks at 2500 c.p.s. A burst of signal, therefore, leaves this ampli- 

 i fier twice per 10-kc interval. The bursts are further amplified and recti- 

 fied and become pulses which time the progress of the oscillator through 

 its spectrum. The pulses are impressed across the winding of a high speed 

 relay, causing its contacts to close momentarily twice per 10-kc interval. 

 During the instant when the contacts are closed they connect a particular 

 value from a sawtooth voltage wave to a 0.1 -mf capacitor. 



The voltage of the capacitor biases the grid of a cathode follower tube, 

 and the output voltage from this tube is fed to a servo system and con- 

 trols the speed of its motor. Thus the motor runs at a speed determined 

 by the voltage of the sawtooth at the instant when the relay contacts 

 close. As the sawtooth itself is timed by the rotation of the servo motor, 

 its voltage-time relationship is the device for pacing the master film. The 

 sawtooth wave originates in the alternate shorting and charging of a 

 1-mf capacitor. Each tooth begins when a pair of shorting contacts is 

 closed momentarily by a cam geared to the servo motor. After a dis- 

 charge, the voltage on the 1-mf capacitor increases negatively as a prac- 

 tically linear function of time, with charging current flowing through a 

 one megohm resistor. Thus the value of voltage transmitted to the 0.1-mf 

 capacitor at the instant of closure of the relay contacts depends on the 

 time elapsed since the most recent shorting of the 1-mf capacitor. Twenty 

 volts at the input to the servo system corresponds to midvoltage of the 

 sawtooth and to 3,600 rpm of the motor, which is the same as the con- 

 stant speed of the motor driving the oscillator and undeveloped film. 



If the characteristic of the oscillator causes a given 2,500-cycle point 

 to occur early, the contacts of the relay will close at a higher positive 

 voltage point on the corresponding sawtooth. The servo motor will start 

 to speed up to make subsequent sawteeth start earlier than they other- 

 wise would have. The motor will slow down if the 2,500-cycle points fall 

 later and lower on the teeth. 



Several design features in the system are of interest. The servo system 

 was supplied by Industrial Control Company (SL-1035). It has a ta- 

 chometer feedback in inverse sense to enhance system stability. The cam 

 used to operate the shorting contactor and start the sawtooth is a small 

 permanent magnet mounted on a wheel. The moving field causes the 

 contactor to operate very briefly as the magnet swings past. The con- 

 tactor itself is a Western Electric 222- A mercury switch, which has a 



