A CARRIER TELEPHONE SYSTEM FOR TOLL CABLES 



103 



Provision is being made for the removal of an amplifier from an 

 active circuit without interruption of service. A spare amplifier at 

 each repeater station can be substituted for the active one by con- 

 necting it to jacks at the sealed terminal and operating associated 

 relays to make a quick transfer. 



Apparatus is also furnished which permits the substitution of a new 

 link between attended points for one which develops trouble. A 

 complete high-frequency circuit for each direction of transmission will 

 generally be reserved as a spare. It can be substituted for any working 



HS^* 



Fig. 20 — Transmission measuring set. 



high-frequency circuit without interfering with service by paralleling 

 the transmitting ends of the spare and working circuits and patching 

 the receiving ends through relays. The operation of a key controlling 

 these relays substitutes the spare circuit for the working one with a 

 transient disturbance of but 1 or 2 milliseconds. 



Three pilot frequencies, 15.9, 27.9 and 55.9 kilocycles, which are 

 produced at the transmitting terminals, may be used to check the 

 levels at the main repeater points and the receiving terminals. This 

 is done by means of a special testing circuit which can be bridged across 

 a pair to detect the level of the pilots without interference to service. 



