AUTOMATIC TESTING OF INTERTOLL TRUNKS 941 



Accuracy Checks 



If the receiving pad adjustment has been successful, the power level 

 at the pad output will be very close to one milliwatt and the measuring 

 relay (m) will be just on the verge of moving from its front to its back 

 contact, or vice versa. Errors may creep in, however, to prevent these 

 things from being true. Some such sources of error are: 



(1) One or more of the pad control relays might fail to lock in the 

 operated position when they should, or fail to release when they should. 

 It would then be impossible to adjust the total receiving pad loss to the 

 correct value. 



(2) The trunk loss might change suddenly while the pad adjustment 

 is in progress and make it impossible, with the pads remaining to be 

 tried, to bring the power level at the pad output to one milliwatt. 



(3) The amplifier or amplifier-rectifier gains might increase or de- 

 crease due to a defective component. 



(4) The milliwatt test power supply might deviate from the standard 

 value. 



(5) Defective components or faulty control relay contacts might cause 

 the individual pad losses to be incorrect. 



To detect errors of the type in items (1) and (2) a "trunk check" is 

 made immediately after the pad adjustment is finished. Referring to 

 Fig. 4, two 0.5 db pads, a and b, are provided in the input circuit to the 

 amplifier-rectifier, pad a being normally out. Before the sending end re- 

 moves the test power, pad a is inserted, momentarily. The resulting 

 decrease in input power to the amplifier-rectifier should cause relay (m) 

 to release. Both pads a and b are then cut out. The resulting increase in 

 input power should cause relay (m) to operate. If relay (m) fails to pass 

 either of these checks the receiving pad loss is in error by 0.5 db or more 

 and another trial is needed to secure a more accurate adjustment. Pre- 

 mature removal of the test power at the sending end would, of course, 

 cause relay (m) to fail on the second check and result in another trial. 



Immediately after the trunk check and while pad b is still cut out, the 

 receiving end rearranges its circuit locally as shown in Fig. 5 for a "loop 

 check" to guard against errors of the types mentioned in items (3), (4) 

 and (5) above. This rearrangement inserts a 0.3 db pad in place of the 

 0.5 db pad b, which is cut out. The local milliwatt supply then applies 

 power to the amplifier-rectifier at a level about 0.2 db higher than 

 necessary to operate relay (m) . Relay (m) will fail to operate and pass 

 this check if the combined effect of any decrease in the value of the milli- 

 watt test power supply, any decrease in the amplifier and the amplifier- 

 rectifier gains and cumulative errors in the receiving pads and check 

 pads adds more than 0.2 db loss. After the above check, a 0.5 db loss is 



