830 THE BELL SYSTEM TECHNICA.L JOURNAL, JULY 1953 



from regular to standby equipment, and the condition of electron tubes 

 in the terminal equipment amplifiers. 



Provisions for connection to special alarm systems are made at main 

 repeaters which are not fully attended. These systems extend the alarms 

 to the nearest attended repeater and enable the attendant to determine 

 in considerable detail the condition at the remote repeater. The attendant 

 may also perform certain operations such as switching a working line 

 to a spare line at the remote repeater. 



3.8 MAINTENANCE 



Maintenance of the L3 system requires equipment and methods for 

 routine checking of the system and trouble location. Normally the 

 auxiliary repeaters will be visited at intervals of about three months, 

 when checks will be made of the power voltages and currents, the 

 electron tube bias and change in bias with a fixed change in heater 

 voltage (activity), and the pilot magnitudes. At these times amplifiers 

 and regulators which fail to meet prescribed limits will be replaced, 

 the 7,266-kc pilot will be brought to its normal value by adjusting the 

 regulator gain, and the amplifier gain control in the output beta circuit 

 will be adjusted by observing the 3,096-kc pilot. 



For these routine tests and adjustments two portable test sets are 

 provided. The power test set plugs into the repeater, amplifier, or 

 regulator and provides for measuring power supply voltage and currents 

 and electron tube cathode-grid voltages to an accuracy of ±1 per cent. 

 The pilot indicator makes it possible to measure the 7,26^ and 3,096-kc 

 pilots to an accuracy of ±0.1 db. 



For trouble locations at auxiliary repeaters a portable transmission 

 measuring set has been designed. It is capable of measuring the power 

 in a 500-cycle band at any place in the frequency spectrum from 50 to 

 11,000 kc to an accuracy of ±0.5 to ±0.02 db depending on its specific 

 use and the care used in calibration. 



At main repeaters, line sections will be checked for noise and modula- 

 tion performance and equalizers will be adjusted at intervals of one week 

 to several months. In addition, loss and gain measurements on sections 

 of the office suspected of being in trouble will be made. For all general 

 tests except equalization line up, point by point measuring equipment is 

 provided, consisting of a 50 to 10,000-kc oscillator, the tuned transmis- 

 sion measuring set referred to above, a milliwatt power meter accurate 

 to ±0.035 db and a complement of attenuators, pads and comparison 

 switches. 



