1052 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER, 1953 



when the light is blocked. Therefore the transistor amplifier-gas tube 

 circuit is not appHcable. For the down check channels, the output of the 

 phototransistors is amplified by the first section of a conventional double 

 triode therminonic emission tube VI. The ac signal at the plate of VI 

 is rectified by a conventional full wave rectifier consisting of transformer 

 T3 and tube V2. The rectifier voltage is negative with respect to ground 

 and when it is impressed on the grid of the second section of tube VI, 

 that section is driven beyond cutoff. Therefore, as long as light falls on 

 the phototransistor, no current flows through the relay. When the 



ii 



PHOTO 

 TRANSISTOR 



4000 n 



tt 



isoon. 



i65n 



' WW— vw 



165A 



Fig. 9 — Card down check circuit. 



dropped card blocks the light, the negative voltage on the grid disap- 

 pears and the tube conducts, thus operating the associated relay. While 

 the prime purpose of this down check is to signal the associated equip- 

 ment that the card is in position for recording its output code, advantage 

 is taken of this circuit's ability to distinguish between a light and dark 

 phototransistor at any time to provide alarms in the event of a lamp or 

 modulating disc failure. 



After the card has been checked down and the associated equipment 

 is ready to accept the output of the card, that equipment connects a 

 positive 130-volt battery through its channel relays to the main anodes 

 of the gas tubes. All gas tubes associated with illuminated channels 

 will have their control gaps fired and will transfer to the main gaps. 

 This operates the corresponding channel relays in the associated equip- 



