AUTOMATIC TESTING IN TELEPHONE MANUFACTURE 1153 



hydrogen atmosphere, high speed capabihty and high current capacity. 

 The magnetic arrangement reduces shock torque loads on the servo 

 motor, which might result from mechanical operation. The high speed 

 relay which operates at the 2,500-cycle points is a Western Electric 275-B, 

 chosen because of the speed required (about 10 operations per second). 



The time comparison circuit has a small amount of long time constant 

 positive feedback (shown at 1 in Fig. 10) to raise or lower the midvoltage 

 of the sawtooth wave in cases of extreme correction and prevent the 

 control point from slipping one or more teeth. In effect this supplies extra 

 acceleration to the master film when needed. 



There is also incorporated in the design an arrangement which permits 

 an important variation in the method of use. A magnetic tape is driven 

 by a sprocket which is geared to the main drive motor and moves with 

 the oscillator drive. The magnetic head for recording on the tape receives 

 its signal in the form of 2,500 c.p.s. bursts through an amplifier. These are 

 the same bursts that time the progress of the oscillator through its spec- 

 trum. Thus it is possible to separate the function of calibration from 

 that of printing the film scale. The calibration data on the oscillator is 

 stored on the tape and may be checked for absence of abrupt departures 

 from linearity before it is used to drive the servo and master film in an 

 actual printing run. This eliminates some wastage of raw film. Also a 

 recording (or calibrating) run is made without the servo linkage and can 

 be made at twice the speed of a printing run. A 56A oscillator can be 

 driven through its spectrum, 50 to 10,000 kc, in 100 seconds, allowing 

 very little opportunity for temperature effects to change the check points. 

 In fact no particular effort need be made to control the temperature 

 beyond an ordinary warm up interval. 



The control portion of the machine contains various circuits for con- 

 venience in setting up and starting the runs. For example one relay cir- 

 cuit under the control of a start button brings a fixed dc voltage into the 

 servo loop, and automatically disconnects after a period long enough for 

 the motor to reach approximately the right speed. A gear shift lever per- 

 mits changing the ratios between the speeds for the recording run and 

 the printing run. 



It is doubtful that a calibration of the 56A oscillator could ho per- 

 formed by manual means. It has been estimated that even if possible, 

 such a task would require more than a week of the most painstaking 

 effort, under very carefully controlled conditions. By comparison, the 

 calibrator requires one minute forty seconds to obtain the data, and 

 three minutes twenty seconds to reproduce it. Development and checking 

 of the exposed film takes about a day. Accuracy of the scales has always 

 been well within the ±2-kc limit. 



