Chanter 10 -SAFETY; TEST EQUIPMENT; TEST METHODS 



for Transistorized and Printed Circuits . NavShlps 



93394. 



Four main failures are associated with tran- 

 sistors: (1) opens, caused by a break in the leads 

 or a break in the emitter-base or collector-base 

 junction; (2) shorts, caused by a rupture of the 

 crystal through the base material; (3) high leakage 

 current, caused by contamination building up on 

 the emitter-base or the collector-base junction; 

 and (4) low gain, caused by excessive heat or mal- 

 treatment. 



Before testing a transistor, first determine the 

 type, whether PNP, NPN, or diode. Groand all test 

 equipment to the chassis under test. If removal of 

 the transistor from the circultisnecessary, use a 

 low-wattage soldering iron (35 watts or less), use 

 proper heat sinks, and ground the soldering iron 

 tip to the chassis under test. 



Two basic units of test equipment can be used to 

 test transistors. The TS-llOOAl transistor tester 

 is designed to test the beta (gain) of a transistor 

 while in the circuit, the ICO (collector leakage cur- 

 rent), and shorts with the transistor removed from 

 the circuit. 



If the TS-llOOAl test set is not available, any 

 good multimeter or vacuum tube voltmeter 

 (VTVM) may be used after first observing certain 

 precautions. To avoid loading the circuit, the 

 multimeter m-ist have a sensitivity of at least 

 20,000 ohms per volt on all voltage ranges; the 

 ohmmeter circuits must not pass a current 

 exceeding 1 milliampere. The VTVM jshould have 

 an Input resistance of 11 megohms or more, and 

 mast have an isolation transformer between the 

 mater and the powerline. 



Diodes 



Before testing a diode, its polarity must be 

 determined. The diode usually is marked with a 

 plus (+) or minus (-) sign. Connect the test leads 

 to the diode in a forward bias condition, that is, 

 the positive lead to the positive side of the diode 

 and the negative lead to the negative side. Cur- 

 rent will now flow easily, and if a reading of 1000 

 ohmr> or less is obtained, this part of the test is 

 good. If a higher reading is obtained, the diode is 

 open. 



Connect the test leads to the diode in a 

 reverse bias condition— the positive lead to the 

 negative side of the diode and the negative lead to 

 the positive side. Current will not flow readily, 

 and a reading in excess of 10,000 ohms should be 

 obtained. If a reading under 10,000 ohms is 

 obtained, the diode Is shorted. 



71.99 



Figure 10-22. — Transistor leakage current test. 



A general statement can thus be made: The 

 front-to-back ratio must always be at least 10 

 to 1. 



Transistors 



Three tests can be accomplished on the tran- 

 sistor by using the multimeter. The transistor 

 must be removed from the circuit for testing. 



Determine the type of transistor, whether PNP 

 or NPN. The sam?. resistance test can be per- 

 formed on the transistor as on the diode. The 

 transistor has two junctions, the emitter-base and 

 the collector-base. Because each junction can be 

 treated as a diode, the same readings hold true. 



When testing power transistors, the sam? ratio 

 (10 to 1) holds true, except that the reverse resist- 

 ance must be in excess of only 1000 ohma. 



To perform the leakage current test, the man- 

 ufacturer's specifications should he consulted to 

 ascertain the allowable lim'ts.Determlne the type 

 of transistor and set up one of the tests shown in 

 figure 10-22, using a 6-volt battery and microam- 

 meter. 



If the leakage current is twice as much as the 

 specification sheet calls for, replace the transis- 

 tor. If no specLflcatlons are available, use the 

 following rule of thumb: 



1. Silicon transistor — less than 1 mlcroamp. 



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