INTRODUCTION TO SONAR 



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resistor, and the like) prevents the flow of cur- 

 rent. Before conducting the test, secure power and 

 disconnect the circuit to be tested. 



An ohmmeter normally is ^jsed to test for con- 

 tinuity on those parts of a circuit wherein resist- 

 ance is low. An open circuit is Indicated when 

 the meter reads very high or infinite resistance. 



When testing for circuit continuity, the ohm- 

 meter should be set on the lowest scale. If a 

 medium or high scale is used, the needle may 

 indicate zero ohms, whereas the actual resistance 

 could be as high as 500 ohms. 



Occasionally, you m?iy have to make a contin- 

 uity test of a circuit whose ends are in different 

 spaces. Several methods can be used to test a 

 circuit from one compartment to another. (Always 

 make sure the power is secured before you dis- 

 connect the circuit.) The following means of testing 

 are accomplished more easily by two men. 



71.100 

 Figure 10-23. — Transistor gain test. 



2. Small germanium transistor — less than 10 

 mlcroamps. 



3. Medium germanium transistor — less than 

 100 m'croamps. 



4. Power transistors — less than 1 milliam- 

 pere. 



The beta test can be accomplished by inserting 

 a 10,000-ohm resistor between the coUector-base 

 junction and connecting the meter to the emitter 

 and collector as shown in figure 10-23. 



Once the resistance is obtained on the meter, 

 use the following formula to figure the gain; 

 B = 1200^R. The letter B represents gain, 1200 

 is a constant, and R is the resistance read on the 

 meter. 



Check the mimufacturer's specification for the 

 gain of each transistor. If the specifications are 

 not available, use the following rule of thumb; 



1. Silicon transistors — 5 times or more. 



2. High-frequency transistors — 10 times or 

 more. 



3. All other transistors — 15 times or more. 



1 . Attach a jumper from one end of the circuit 

 to ground. At the other end, connect the ohm - 

 meter in series between ground and the con- 

 ductor. A very high resistance reading 

 indicates an open circuit. 



2. If an ohmmeter is not available, a pair of 

 sound-powered telephone handsets with 

 alligator clips on their leads may be used. 

 Connect the handset leads between the cir- 

 cuit and ground at each end of the circuit. 

 If the circuit is open, communication is 

 impossible. 



3. A battery-powered test lamp also can be 

 used. Groxmd one endof the circuit. Connect 

 the lamp between the other end and ground. 

 If the lamp does not light, the circxdt is 

 open. 



Intermittently open circuits sometimes are 

 encountered. These breaks usually are found in 

 cables or wrapped multiple wiring. When the test- 

 ing equipment Indicates an open circuit, the cable 

 or wiring can be flexed. Any momentary contin- 

 uity of the circuit is apparent by an indication on 

 the test equipment, and the location of the break 

 is then known. 



Grounds 



CONTINUITY TESTS 



The purpose of the continuity test is to 

 ensure that a circuit is complete or continuous. 

 An open circuit is one in which a break in the cir- 

 cuit (a broken lead, defective switch, burned out 



Grounded circuits are caused by some part of 

 the circuit making contact, either directly or 

 indirectly, with the metallic structure of the ship. 



Grounds have several causes, the two most 

 common being frayed or worn insulation that 

 allows bare wire to come into contact with the 

 equipment's chassis or ship's structure, and 



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