IMPROVED CIRCUIT FOR TELEPHONE SET 



615 



trolled by the value of direct current. This is of great significance in the 

 control of sidetone and transmission equalization, as will be apparent 

 later. 



Silicon carbide varistors have been used as circuit elements for various 

 purposes for a number of years. In fact, the Vi varistor was used in the 

 original design of 500-type set to protect the tungsten filament against 

 abnormal voltages. In this instance it was shunted directly across the 

 filament, so that its inherent property of decreasing in resistance with 

 applied voltage would serve to bypass current from the filament and 

 thereby prevent damage when the applied voltage became too high. The 

 requirements imposed by the transmission equalization and sidetone 

 balance functions for the 500-type set are quite severe. A new varistor, 

 the V2, had to be developed which not only was much lower in resistance, 

 but also maintained the n of the voltage-current characteristic at a 

 high value. The successful outcome of this development and of the 

 accompanying problems of design and manufacture, were of critical 

 importance in making possible the simplification and economies offered 

 by the 500D set. 



TRANSMISSION EQUALIZATION 



Fig. 2 is a circuit schematic of the 500D telephone set. This circuit is 

 a modification of the circuit used in the 500B telephone set, which in 

 turn is one of the Campbell anti-sidetone circuits^ that has been standard 

 in the Bell System for a number of years. 



A simplified diagram of the transmission portion of the circuit is 

 shown in Fig. 3, and it can be seen that there are three branches in the 



RECEIVER 



INDUCTION COIL 

 C 



vMftj 



A I B 



22n 



XE 



0.4 /aF 



AAAr-f 

 e&n. 



2yU.F 



BALANCING NETWORK 



Fig. 3 — Diagram of transmission circuit of 500D telephone set illustrating 

 side tone balance. 



