generator should be terminated with 600 ohms and a shielded cable should 

 be used between the generator and Rack 1, A constant signal voltage at 

 least as low as ,0001 volts should be fed into the channel for frequencies 

 between 30 cps and 500 cps, 



19 „ The wave shape of each frequency should be observed on an oscilloscope 

 connected across the system bus at any convenient point, such as the signal 

 input terminal strip to the automatic switching unit, the signal input to 

 either graphic level recorder, etc For this measurement both the channel 

 volume control and the control should be set at 0, The over-all 



gain for each channel should then be at least 105 db above the input voltage 

 and no distortion should be evident, 



Channel Amplifier Power Supplies 



20, Each channel has its own plate and filament supply. The AC mains for 

 these supplies are separately fused on the fifth .panel from the top of 

 Rack lo All the fuses on this panel except those labeled 1 

 and PPLI are 3-ampere Little Fuses The AC to Racks 2 and 3 

 is supplied through the and the fuses. 

 These fuses are therefore 5-ampere Little Fuses. 



21, The channel power supplies are identified from the back by the labels 



? f 5 and 



, The Western Electric 18A power supply (Fig, 41) is the plate 

 supply, designed to furnish 250 volts DC to both amplifiers of one channel D 

 The plate supply terminal strip is behind the chassis end plate while the 

 110-volt AC input is located at the front end of the 18 A power supply. The 

 18A uses a 274A rectifier which can be replaced by a type 80 tube. Note 

 that the X8A amplifier is shock-mounted. The negative (B-) side of the 

 high voltage supply should be grounded at the 120E and the 121A terminal 

 strips o 



22„ The 6„3 volt AC filament supply from the 18A is not used, and instead 

 a 7-volt DC power supply (Fig, 42) supplies the 120B and 121A filaments. 

 This power supply uses a selenium rectifier. An "on-off" switch is mounted 

 on the rear end plate of each of the 7-volt chassis so that the supply which 

 is not in use may be turned off, prolonging the life of the rectifier If 

 constant use over a long period of time considerably decreases the output 

 of the rectifier it can be increased by changing the 110-volt AC connection 

 from tap 4 to tap 3 on the rear terminal strip , With the AC connected to 

 tap 4 the normal filament voltage is 6,3 volts under load; if this voltage 

 is slightly high it can be reduced by adjusting the variable resistor in 

 the heater supply, mounted on the rear terminal strip. This resistor normally 

 has a value of one ohm. The negative terminal of each 7-volt supply is 

 grounded to the chassis. Note, in later units the one ohm resistor is located 

 on the under side of the chassis, 



23° IX the selenium rectifier develops mechanical noise as a result of 



7 fc . 



