5214 HYDEOGRAPHIC MANUAL PaGE 458 



5214. Echo Amplifier 



Numerous changes have been made to the echo amphfiers to take advantage of 

 recent improvements in tubes and circuits, with the result that no two 312 Fathometer 

 echo amphfiers in use on vessels of the Coast and Geodetic Survey are the same. They 

 differ principally in the number and kinds of tubes and the source of power used to 

 operate the amplifier. The power source may be the ship's 110-volt d-c current, bat- 

 teries, or rectified alternating current. Iron-core transformers are normally used to 

 form the coupling between tubes. The gain control of the echo amplifier is generally 

 on or near the indicator, so that sensitivity adjustments can be made while viewing 

 the depth scale of the indicator. A band-pass filter, tuned to 1050 cycles, is sometimes 

 used in conjunction with the amplifier to aid in discriminating against spurious noises. 

 The output voltage of the echo amplifier is used to excite the grid of a gas discharge 

 tube, whose discharge flashes neon tubes in the indicator. 



5215. Indicator 



The visual indicating mechanism of the 312 Fathometer is housed in a metal cabi- 

 net whose outside dimensions are 13 by 15 by 11)^ inches. In the front of the cabinet 

 is a circular hole, 11 inches in diameter, fitted with a glass on which are two concentric 

 circular scales. The inner one is graduated in fathoms, from 5 to 100 fathoms; and the 

 outer one in 5-fathom intervals, from to 600 fathoms. A motor-speed indicator, a 

 start-and-stop switch, and two adjustment knobs are also installed in the front of the 

 cabinet. A reed tachometer is used to indicate the amount and direction of motor- 

 speed deviation (see 5234). The left-hand adjustment Ivnob is for manual control of 

 the motor speed for line-voltage changes beyond the limit of the governor. The 

 right-hand knob is the sensitivity adjustment and controls the current flowing in the 

 carbon button of the hydrophone. On the left side of the cabinet is a gearshift knob 

 for changing the disk speed to correspond to one of the two depth scales. On the right 

 side of the cabinet is a push button by which the transmission of the signals may be 

 stopped, as 'desired. The important parts contained in the cabinet are: the indicator 

 motor and governor, the reed tachometer, reduction gears, gearshift mechanism, rotating 

 disk that carries the neon tubes, and the keying cam and contactors. 



A 110-volt d-c, 34o-horsepower motor, operated at 1,800 r.p.m., is used to revolve 

 the index disk and operate the keying cam. It is important, for the sake of accuracy, 

 that the speed of this motor be maintained constant. A centrifugal governor is used to 

 control the speed for changes in torque and line-voltage variations between 90 and 115 

 volts. 



The motor speed is reduced by means of gears to the proper speed of the index 

 disk. The speed ratio may be changed by means of a gearshift mechanism, so that 

 the index disk will rotate at either 4.1 r.p.s. or 0.683 r.p.s. for soundings on the 100- or 

 600-fathom scale respectively. 



The index disk, of thin bakelite, is circular in shape, its diameter being a little less 

 than the circular opening in the face of the cabinet. Attached to the back of the disk, 

 and rotating with it, are two small U-shaped neon tubes. Electric connections are 

 made to these tubes by means of slip rings. The index, one for each tube, is a narrow 

 slit, cut in the disk, through which the light is seen when the tube flashes so that the 

 position of the index relative to the depth scale may be read. The neon tube that oper- 

 ates with the 100-fathom depth scale is installed so that it can be rotated with reference 

 to the disk through an arc equivalent to 7 fathoms, to adjust for the mean depth of the 



