|L CABLE CARRIER TELEPHONE TERMINALS 115 



H The 120-kc. carrier which feeds the group modulators of ten systems 

 ^r a total of one hundred and twenty talking channels must be very- 

 dependable. Therefore separate filters are used for the regular and 

 emergency supply and separate amplifiers for the large power required 

 by group modulators. Regular and emergency distributing buses are 

 provided. Each group modulator and each group demodulator is 

 wired through protective resistances to the regular bus and through 

 another set of protective resistances to the emergency bus. With this 

 arrangement an accidental short circuit even across one of the busses 

 or across one of the output coils of one of the 120-kc. amplifiers will not 

 stop the whole supply of 120 kilocycles. 



The 4-kc. oscillator of the emergency generator is in constant 

 operation so that when it is needed no time is required to start it, 

 but the grid bias on the second tube is held above its cutoff value by 

 the automatic transfer circuit. This prevents the 4 kc. from going 

 further until called for in an emergency. 



An emergency is indicated when there is no 120-kc. supply on either 

 the regular or emergency bus. When this happens, the copper-oxide 

 rectifier in the transfer circuit gets no 120 kc. and so loses its rectified 

 voltage. This triggers off one or both of the two gas-filled tubes 

 (multipled for safety) which increases the grid bias on the control tube 

 of the regular generator to stop its 4 kc. supply and at the same instant 

 restores the bias to normal on the control tube of the emergency to let 

 its 4 kc. pass through and put the whole emergency circuit into opera- 

 tion. The keys in the transfer circuit are provided for maintenance 

 purposes, and to return from emergency to regular operation, since the 

 gas tube circuit is arranged to transfer automatically in only one 

 direction. 



The pilot supply circuit is shown in Fig. 5. The 3.9-kc. tuning 

 fork oscillator at the left supplies that frequency, through the three 

 transformers, to the three copper-oxide modulators the carriers of 

 which are obtained from the regular channel carrier supply bus-bars as 

 shown. The three filters, which are identical with channel carrier 

 supply filters, select the lower sidebands to be used for pilot frequencies 

 at 64.1, 92.1 and 104.1 kc. The three pilot frequencies are distributed 

 to the different systems through protective resistances from a bus-bar as 

 shown. They are set 100 cycles off the carrier frequencies to obtain 

 locations of minimum interference from carrier leak and other sources. 



Signaling circuits do not form an integral part of the carrier terminal 

 equipment. Signaling equipment of a type already widely used in the 

 Bell System for toll circuits, is connected between the toll switchboard 

 and the four-wire terminating set of the individual channel. 



