AUTOMATIC TESTING OF RELAY SWITCHING CIRCNITS 1163 



source from which are derived the 24-volt dc, 90-volt 20-cycle ringing 

 current and 600-cycle audio tone supplies that are required. The test 

 circuit features mchide tone transmission checking, dial pulsing, 90-volt 

 20-cycle ringing and ground and batter}^ supplied either directly or under 

 relay control. Other battery and ground relays are available for checking 

 the response of the circuit under test. 



These test features have been sufficient to perform operation tests on 

 most relay units associated with key telephone systems. The test cycle 

 is fast and the twenty test steps can be performed in approximately ten 

 seconds. The lamp indications given when the test is interrupted by an 

 open-circuited chain lead, convey information to the tester as to which 

 test step is involved and when any pairs of signal and watch relays fail 



Fig. 6 — Insertion of cross-connection plug board into Card-0-Matic test set. 



