MAGNETIC DRUM TRANSLATOR FOR TOLL SWITCHING OFFICES 739 



however, overcome the bias and will be transmitted to the transistor 

 monostable circuit. 



When triggered at the base, the transistor delivers a pulse of about one 

 millisecond duration to the load represented by the input transformer 

 and the channel detector gas tube and thus provides the drive required 

 to initiate ionization in the control gap of the gas tube. When brought 

 into action, the transistor serves as a switch to connect capacitor c to 

 collector supply resistor r6. The voltage change, occasioned by the re- 

 sultant flow of current in r6, is communicated to the transformer primary 

 through a blocking capacitor and a current limiting resistor. As capacitor 

 c charges, the voltage at the transistor emitter will approach the collector 

 supply potential at an approximately exponential rate. When the di- 

 minishing flow of emitter current can no longer maintain the transistor 

 in its low-impedance mode, it reverts to its pre-triggered condition, and 

 the timing capacitor c is then discharged, primarily through forward- 

 conducting varistor vr2 and resistors r5 and r4. 



Owing to the necessity of using early-production samples of the type 

 of point-contact transistor chosen for this application, the associated 

 circuitry for biasing the emitter into the normal non-conducting state 

 is somewhat more elaborate than that which might have sufficed with 

 later samples whose characteristics were more closely controlled. 



The principal components of the channel detector are a step-up trans- 



PULSE STRETCHER 



CHANNEL DETECTOR ASSOCIATED 

 EQUIPMENT 

 IN DECODER 

 OR MARKER 



INPUT 



Fig. 13 — Pulse stretcher and channel detector circuit, 



