Troubleshooting Techniques

1. Preparation for Troubleshooting : • A labeled schematic with proper dc and signal voltages. • You should know the expected voltages in the circuit should be before you can identify an incorrect voltage • The Complete Troubleshooting Process : • Identify the symptom(s): Check the input and output voltages. • Perform a power check: Confirm dc supply is correct. • Perform a sensory check: Check for wrong connections or extremely hot components. • Component Replacement: Determine a certain symptom to indicate a component may be defective. • Apply a signal-tracing to isolate the fault to single circuit: Determine the faulty component which caused the • incorrect voltages. • Use replacement or repair to fix the problem COLLECTOR-FEEDBACK BIAS VOLTAGE-DIVIDER BIAS BASE BIAS EMITTER BIAS • Q-point values • Collector Current: • Collector-to-emitter voltage: • Q-point values • Collector Current : • Collector-to-emitter voltage: • Q-point values • Collector Current : • Collector-to-emitter voltage: • Q-point values • Collector Current : • Collector-to-emitter voltage : Troubleshooting Techniques Summary of Transistor Bias Circuits Troubleshooting Transistor You can view the transistor as two diodes connected as shown in the figures below Many digital multimeters(DMMs) have a diode test position that provides a convenient way to test a transistor. If the junction is good, you will get a reading of 0.7V being typical for forward bias. If it is reverse bias, you will see the reading of OL, indicates that the junction has an extremely high reverse resistance  Quick-Test on Base-Emitter Junction Voltage Quick-Test on Pull-Down Test for Transistor Operation The base-to-emitter junction voltage must be in the range from 0.5 to 0.8 volts if working properly. The test includes the measurement of the emitter-to-ground voltage and the base-to-ground voltage and subtract the emitter voltage from the base voltage. If there is problem, the junction voltage will not even be close to the correct (0.6-0.7V) value. The trick in this test is to force the transistor’s collector voltage to change, to see if the base actually has control over the collector current. Connecting a temporary jumper from base to emitter to turn the base current off. If the collector current turns off, as it should, the collector voltage will rise to approximately the power-supply voltage. If this doesn’t happen, the transistor is probably defective. Quick Test on Collector-to-Emitter Voltage The voltage (VCE), ideally is equal to roughly one-half the supply voltage. VCE voltage of 0.2 to 0.8 volt usually indicates saturation and an excessive base bias condition. Any VCE voltage of less than about 3 volts merits further investigation.