5o The top p . tains five male Cannon receptacles labeled 



L, , etc Five hydrophone circuits are per- 



manently connected to these receptacles by the hydrophone terminal strip 

 located on the upper right-hand side of Rack 1 (Figo 2) A protective 

 cover which acts as a magnetic shield is provided for this terminal strip 

 On the front panel below the male Cannon receptacles are two female Cannon 

 receptacles labeled • and „ Jumpers connect these 



receptacles to the hydrophones , making it possible to connect any one of the 

 five hydrophones to either channel „ 



6„"~ Between the male and the female Cannon receptacles are two lever-action 

 switches, one for each channel,, These switches are spring-loaded to return 

 automatically to their central positions, When centered, each switch connects 

 the female Cannon receptacle below it to its channel? when pushed up, it lifts 

 the hydrophone from the channel input and replaces it with a 60Q~ohm terminating 

 resistor which is used for self -noise measurements! and when pushed down it 

 again lifts the hydrophone and connects the channel input to a set of binding 

 post terminals on the panel labeled . K„ Since these switches 

 automatically return to the center position, a hydrophone is always connected 

 to each channel unless one of the switches is actuated., 



Self -noise Measurements 



7» Since self-noise, generated within the amplifier channel, may alter the 

 signal characteristic or mask the signal and prevent clear reception, it 

 is necessary to measure each channel to determine that self-noise is at a 

 minimum. This should be done as part of the initial alignment and should 

 be repeated each day„ Sketch 1 illustrates the circuit to be used for the 

 first step in this test. Remove the Cannon output plug from the 121A 



flU 



itch or. 

 ont Panel 



120B 



0-40 db Attenua- 

 tor Set at "0" • 



121A 



Sketch 1 - First Step in Self -Noise Measurement 



amplifier (marked and at the back of Rack 1, top 



panel, or _ and at. the back of panel 2 from the top), 



In its place connect a vacuum tube voitmeter terminated in 500 ohms and 

 adjust the channel volume control, to 0, Push the channel input switch for 



- 2 - 



