5264 HYDROGRAPHIC MANUAL PaGE 484 



former. The increased voltage occurring across the secondary of the transformer flashes the neon 

 tube. A polarizing voltage of about 350 volts direct current helps to decrease the discharge voltage 

 required of the type 885 tube. 



5264. Keying Circuit 



A direct-coupled amplifier using type (jF5 and 2A3 tubes, which is actuated by the photoelectric 

 tubes, forms the nucleus of the keying circuit. A 5,000-ohm resistor in the plate circuit of the type 

 2vl3 tube acts as a grid resistor between the grid and cathode of the 17.5-kc oscillator. When the 

 2^13 keying tube is drawing current, as it normally does between keying periods, the voltage drop 

 across the 5,000-ohm resistor biases the oscillator sufficiently to prevent oscillations. When the 

 grid of the 2^3 tube is made sufficiently negative by the amplified photoelectric keying impulse the 

 voltage drop across the resistor is reduced to a sufficiently low value to permit the oscillator to operate. 

 The type 6F5 tube that precedes and is directly coupled to the type 2^13 tube, amplifies the output 

 of the photoelectric tube. Two additional circuits are associated with the direct-coupled amplifier: 

 One is a type 885 gas-discharge tube that can be switched into the circuit, between the type 6F5 

 and type 2A3 tubes, to shorten the transmitted signal for shoal soundings when the 20-fathom scale 

 is used. The other is a type 807 tube that is connected to the direct-coupled amplifier in such a way 

 that its grid operates from the type 2.43 tube output, and the plate circuit of the type 807 tube oper- 

 ates the keying relay for sounding with sonic signals, using the 312 Fathometer acoustic system. 



When the instrument is first put into service it is usually necessary to adjust the length of the 

 transmitted acoustic signal used in the 100-fathom circuit. This adjustment is made as follows: 

 With the synchronous driving motor stopped, set the depth-selector switch on position 3 and 

 the frequency-starting switch on position 4, Turn the motor slowly by hand in the normal direction 

 of rotation and note the position, on the 20-fathom dial, of the first and last flashes whicli mark the 

 beginning and end of the acoustic signal. The separation between the first and last flash should be 

 about 5 fathoms. The keying is governed by an arrangement comprising two shutters carried by 

 the shutter disk. During normal operation the position of the following edge of the leading shutter 

 determines the beginning of the transmitted signal, and the leading edge of the following shutter 

 determines the end of the signal. Hence, by adjusting the following shutter, the length of the signal 

 may be adjusted without changing the time at which the signal is transmitted. Each shutter is 

 slotted and held in position behind the disk by means of a screw through the slot. 



Exactly the same procedure is followed for the lOO-fathom cutout circuit with depth-selector 

 switch setting 4 and the starting-frequency switch still on position 4. 



And the same procedure is followed for the sonic circuit using depth-selector switch setting 5 and 

 starting-frequency switch setting 5 except that, in this case, the length of the keying period should be 

 equivalent to a 50-fathom movement of the index with respect to the 1,000-fathom depth scale. 



The shutter to be adjusted, in each case, can be easily identified, since it will be illuminated by 

 the associated exciter lamp. 



5265. Power Supply 



The power supply is housed in a metal rack, 51 inches high, 21^^ inches wide, and 21}^ inches 

 deep, whose weight is 475 pounds. The circuits of the power supply are contained in three compart- 

 ments arranged like drawers, to which access may be had by pulling out the subassemblies. In the 

 upper compartment is the 17.5-kc oscillator and amplifier whose purpose is to furnish power to the 

 transceiver (see 5261). The middle compartment contains the tuning fork and associated circuits, 

 which furnish 1025-cycle power to the synchronous motor. The lower compartment contains the 

 high-voltage, intermediate-voltage, and bias-voltage rectifiers and filters, and the transceiver polar- 

 izing circuits. 



The front of each compartment is a panel. On the top panel are two meters; a 0- to 2-ampere 

 a-c meter for measuring the 17.5-kc current to the transceiver, and a 0- to 1-ampere d-c meter for 

 measuring the plate current of the last 17.5-kc amplifier tubes. Between these two meters is a dial 

 for changing the frequency of the oscillator. On the left of the panel is a push button by which the 

 17.5-kc oscillator may be turned on for testing. 



On the middle panel are two meters; a 0- to 2-ampere a-c meter to indicate the 1025-cycle 

 current to the synchronous motor, and a 0- to 0.5-ampere d-c meter to indicate the plate current 

 to the 1025-cycle power-amplifier tubes. 



