the channel under test to The voltmeter should then read 



at least 3 056 volts, or 25 db below 1 volt. If this value is not obtained 

 noise is either being generated within the channel amplifier or is being 

 picked up from some external source, 



8. If noise is present remove the rear end plate from the 121A amplifier 

 chassis of the channel under test and disconnect the input connections, 

 which are the right-hand red and blue leads, Bridge a 500-ohm resistor 

 across the input terminals, as shown in Sketch 2, The voltmeter should be 



500 jx 



Sketch 2 ~ Second Step in Self -Noise Measurement 



left connected as in Sketch 1„ The meter will now read the noise level of 

 the 121A amplifier alone. This should be at least „008 volts, or 42 db 

 below 1 volt„ 



9o If the 121A amplifier is not the source of the noise the 120B preamplifier 

 and the 40-db Daven potentiometer should be checked,; Sketch 3 illustrates 



600jX 

 Term<> 



Switch on 

 Front Panel 



120B 



© — vwv|v~~v — vyvw 



■•> 



J 0~40 db attenuator 



bet, at "0" 



500 



Sketch 3 - Third Step in Self -Noise Measurement 



3 - 



