Chapter 9 Basic Electrical Tests 185 Copyright by Goodheart-Willcox Co., Inc. To check a transistor with an ohmmeter, you must fi rst fi nd the base lead. To do this, connect an ohmmeter test lead to any of the transistor leads. You can tell which lead is the base because the ohmmeter reading will be the same when you measure resistance between the base lead and either of the other two transistor leads. Th e ohmmeter reading will vary when you connect the meter across the emitter- collector junction. Aft er fi nding the base, you must make four measure- ments. With one test lead on the base, measure resistance across the base-emitter junction and then the collector-base junction. Reverse the ohmmeter lead and check resistance across the emitter-collector junction again. In one direction, the resistance reading should be low (about 2 to 10 ohms). When the meter leads are reversed, the reading should be slightly higher (about 60 to 150 ohms). Th ese readings show that the transistor junctions are functioning normally. A transistor is bad and needs to be replaced if it shows infi - nite resistance or zero resistance across one or more of the junctions. Testing Other Components Methods for testing other electric and electronic components are discussed in later chapters. However, most testing requires the techniques discussed in this chapter. Figure 9-38 shows how an ohmmeter and voltmeter can be used to test various components. If you use common sense, your knowledge of component operation, and rec- ommended testing practices, you should have little trouble fi nding problems. Electrical Service Chemicals Several chemicals can be helpful when you are doing electrical tests and repairs. Electrical solvent can be used to clean oil and grease from components before soldering. It can also be used to remove soldering fl ux from a com- ponent. Contact cleaner is a chemical for cleaning deposits from contacts, switches, and other components. Freeze spray can be used to fi nd intermittent problems in electronic components. Electronic components are very sensitive to temperature. When you spray the freeze solu- tion on a component, the component’s temperature lowers instantly. Th is may make a change in the component’s out- put, helping you fi nd some internal problems. Figure 9-37. Before using an ohmmeter to check a capacitor, be sure to ground the capacitor lead to discharge any electricity stored in the capacitor. ~V OFF V Ω mA mA A ~ 1.262 Ω At First, Meter Shows Low Resistance Bar Graph Increases Toward Infinity as Capacitor Charges Capacitor