30 o The 110-volt AC input to the monitor amplifier is fused at the rear 

 of the amplifier chassis (Fig,, 3) I" case of power failure in the 

 amplifier, check this fusing.* The chassis is bolted to the tray by four 

 bolts at the four corners on the top of the chassis,, These bolts pass 

 through the chassis and screw into the tray, 



31 n Figc 45 is the circuit schematic of the 124A amplifier. A low- 

 frequency rise equalizer compensates for the falling off of the monitor 

 speaker response at low frequencies „ The amplifier's normal over~all 

 gain is 50 db| its maximum undistorted power output is approximately 12 

 wattsj and its output noise level should be about 35 db below one milliwatto 



32, The 124A amplifier uses the following tubes: 



2 type 6J7 (replaceable with Western Electric type 348A) 

 2 type 6L6 (replaceable with Western Electric type 350B) 

 1 type 5U4 (replaceable with type 5T4 or Western Electric type 274B) 



33„ -The monitor speaker, a 15-inch Altec Lansing permanent magnet speaker 

 housed in a bass reflex cabinet (Fig, 28), should be located to suit the con- 

 venience of the station operator,, The output of the 124A amplifier, avail- 

 able at the rear of Rack 1 at the right-hand side of the amplifier chassis 

 (Figo 3) # is connected to the speaker by a cable furnished with the speaker 

 The monitor provides an excellent and rapid means of checking for channel 

 noise, since any change in the background noise characteristic of the channel 

 being used will be immediately heard by the operator e 



AUTOMATIC SWITCHING UNIT 



34o When a signal arrives the automatic switching unit (FigSo 17 and 18), 

 located at the top of Rack 2 (Fig„ 6), automatically alerts the station 

 operator and switches on the motors and time index control relays of the 

 two graphic level recorders Fig. 46 is a schematic diagram of the Automatic 

 Switching Unit, The unit employs two stages of amplification, the second 

 of which works into a tuned circuit that is resonant at 175 cycles*" This 

 circuit controls the grid of a type 2050 thyratron tube*, Normally, the DC 

 grid voltage on the thyratron, which can be varied from -32 volts to 

 Nvolts by means of a control marked on the front panel (Fig„ 17), 



is set at such a value that about -1 to -2 volts DC are on the grid. This 

 voltage just suffices to block the tube and keep it from firing,, 



35o When a signal containing energy building up to 175 cycles is received 

 it is amplified and passes through the tuned circuit to the grid of the 

 thyratron. As soon as the magnitude of the amplified signal exceeds the 

 predetermined voltage, the thyratron fires and plate current flows, energizing 



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