ELECTRONIC RELAY TESTER 933 



associated C tube is made to conduct by shifting the grid voltage. All 

 of the C tube cathodes are connected together to a common cathode 

 resistor which couples the \'oltages from the contacts to the vertical 

 amplifier of the scope. The plate current that flows when a C tube is fired 

 causes a voltage drop through the common cathode resistor which 

 deflects the scope beam to the proper vertical level. The level is set to 

 the desired value by adjusting the plate circuit resistance. Since the 

 contact under test is connected to the grid circuit, the grid voltage and 

 the cathode voltage are shifted a small amount by opening or closing the 

 contact. That is, when a C tube is fired the vertical plate voltage jumps 

 to one value when the contact is open and it jumps to another slightly 

 different value when the contact is closed. The f 6 C tubes corresponding 

 to the 16 contacts under test are fired one at a time in succession by 16 

 associated multivibrator stages. The multivibrators are connected in a 

 ring so that each stage is fired by the preceding stage. When a stage is 

 fired it holds its C tube in a conducting condition for two microseconds. 



Fig. 6 is a detailed schematic of the single stage multivibrator (tubes 

 A and B) with an associated modulator (C) tube. The multivibrators 

 are normally in a stable waiting state and go into a temporary unstable 

 state onlj^ when a transient is applied. Normally the A tube is cut off 

 and the B tube is conducting. When a pulse is applied to the A tube grid 

 the A tube conducts and the B tube is cut off. After two microseconds 

 the A tube reverts back to its waiting state and sends a pulse to the 

 next stage. When the B tube is cut off it provides a flat two microsecond 

 pulse through the coupling circuit to the associated C tube. 



Each multivibrator stage consisting of the A and B tubes with other 

 circuit elements is mounted on a plug-in turret which may easily be 

 changed in case of trouble. 



BRIGHTNESS CONTROL CIRCUIT 



When pulsing a relay the armature remains on the operated and re- 

 leased positions for a relatively long time interval. If the intensity of 

 the scope beam is allowed to remain constant the spots at the ends of the 

 traces are bright enough to fog the entire scope face. This makes it 

 difficult to see the relatively weak lines that are caused by the motion 

 of the armature. Therefore a control circuit is pro^'ided to brighten 

 the scope trace only when the relay armature is in motion. The circuit 

 shown on Fig. 8 provides the voltage required to intensify the cathode 

 beam of the scope. This voltage is obtained by differentiating the output 

 voltage of the electrostatic gauge. As the latter is proportional to the 



