1168 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



and provide protection for the various power supplies. Two of these 

 supplies are mounted on the top of the set. 



The rows of vertical push button keys on the key shelf afford the 

 tester a means of determining (for trouble shooting purposes) the asso- 

 ciation (through lamp display signals) of the wired unit circuit terminals 

 with those of the test set and the corresponding test voltages which are 

 connected at that particular stage of the test. The lamp display panel 

 also indicates which test set circuits are in use and through fast or slow 

 (0.5 or 1 second) flashes whether the fault thus indicated is the result 

 of a failure to meet either an expected condition or the occurrence of an 

 unexpected condition. This feature is illustrated in Fig. 9 which shows 

 one link of the chain leads which extend through all pairs of signal and 

 watching relays for the check of satisfaction of all test conditions and 

 the application of steady or interrupted ground to the associated test 

 feature lamp. The operating condition of all test set key relays as pre- 

 viously established by the tape is also indicated by the display lamps. 

 xA.nother type of information obtained from the lamp display panel 

 which is valuable to the tester in trouble clearing is the indication of 

 the particular unit relays which should be operated at that part of the 

 test cycle. By checking the lamps against the operated or non-operated 

 position of the relays he can frequently localize the fault in a minimum 

 of time. 



As mentioned above an important part of the test set flexibility is 

 the ability of the tester to set up the test set to test only those optional 

 circuit arrangments which are provided in any particular unit ordered 



X 



FAST 



GROUND 



PULSES 



TO CKT. 

 UNDER _ 

 TEST 



ASSOCIATED FEATURE 

 SIGNAL LIGHT 



( 



cr 



TO PRECEDI 

 WATCH RELAY 



NG^ 



CHAIN 

 LEAD ' 



TO BATT OR 

 GRO.ASREQ'D. 



"j-t.^" 



SIG RELAY 



c-^ 



SLOW 

 GROUND 

 PULSES 



•- TO GROUND WHEN 

 REQUIRED FOR 

 EXPECTED 

 OPERATION 



HI 



^ 



CHAIN 



T»^^ 



SUCCEEDING 

 SIGNAL RELAY 



"watch" relay 



Fig. 9 — Chain circuit showing watching rehiy function. 



