Electronics Principles & Applications Fifth Edition Chapter 10 Troubleshooting ©1999 Glencoe/McGraw-Hill Charles A. Schuler

Preliminary Checks No Output Reduced Output Intermittents Operational Amplifiers INTRODUCTION

GOAL Good troubleshooting Observe the symptoms Analyze the possible causes Limit the possibilities

A System Point of View Inputs Network Other hardware Software Power Controls Connectors Symptoms? Diagnostics * adjust framostat * synthesizer off * buy low * sell high Causes? Components

R LOAD SYMPTOMS V LOAD = 0 V LOAD < NORMAL V LOAD > NORMAL FAULTS ZENER OPEN ZENER SHORTED R 1 OPEN R 1 > NORMAL C SHORTED V IN < NORMAL R1R1 V IN V IN = 0 C I LOAD > NORMAL C OPEN V LOAD SHOWS NOISE

ESD device packaging minimize device handling minimize motion use a wrist strap stand on ESD work mat use ESD work surface touch ground first ionized air instrument grounding follow procedures PREVENTION

G.O.A.L. quiz The key words in troubleshooting are observe, analyze and ________. limit Symptoms must be analyzed from the __________ point of view. system With computer based systems, a fault can be caused by hardware or _________. software Some solid-state devices are easily damaged by ____________ discharge. electrostatic Technicians sometimes wear a wrist strap to prevent _________. ESD

SIGNAL INJECTION METHOD STAGE 1 STAGE 2 STAGE 3 STAGE 4 SIGNAL GENERATOR

SIGNAL TRACING METHOD STAGE 1 STAGE 2 STAGE 3 STAGE 4

0 V mA E B C R1R1 In-circuit ohmmeter testing can be misleading. Additional paths cause the reading to be low.

SIGNAL COMPARISON METHOD RIGHT STAGE 1 RIGHT STAGE 2 RIGHT STAGE 3 RIGHT STAGE 4 LEFT STAGE 1 LEFT STAGE 2 LEFT STAGE 3 LEFT STAGE 4

If any of these are open, there are no dc voltage errors. Open causes decrease in gain Open causes loss of signal Open might cause hum or noise

If any of these are shorted, there are dc errors. Short causes increase in transistor current Short causes loss of B-E bias and transistor is cutoff (depends on prior stage) Short causes amplifier voltages to be zero

If this is open, the amplifier voltages are 0.

If either of these is open, the transistor is in cutoff. Which of these two faults will produce a non-zero emitter voltage?

If this is open, the transistor approaches saturation. Will the collector voltage be high or low?

If this is open, the transistor collector goes to zero volts. Will the base voltage be off very far?

Component level troubleshooting quiz Ohmmeter troubleshooting can be misleading when performed _____ a circuit. in Dc voltage errors are usually not caused by _________ coupling capacitors. open A shorted coupling capacitor could cause the Q-point to move to cutoff or ______. saturation When a base bias resistor opens, the Q-point __________ changes. always Stage-by-stage verification with an oscilloscope is called signal ____________. tracing

VERIFYING POWER OUTPUT SIGNAL GENERATOR DUMMY LOAD AMPLIFIER

CHECKING FOR DISTORTION TRIANGLE GENERATOR AMPLIFIER Crossover Clipping

Making intermittents show up System view (Is any software involved?) Wiggle it Thump it (don’t get carried away) Heat it (don’t melt anything) Cool it Raise supply voltage (moderate amount) Lower supply voltage

100 k  1 k  1 V DC 1 k  RLRL Suppose a signal source has a dc component that must be eliminated but a coupling capacitor is not acceptable. - 5 V 5 k  Here’s a solution: add a negative voltage to cancel the positive dc offset of the source.

100 k  1 k  1 V DC 1 k  RLRL Suppose the output voltage is at negative saturation. - 5 V 5 k  - 12 V Could the 5 k rr esistor be open? Could the - 5 V supply be at fault? What else could be wrong?

Verification and intermittent quiz Amplifier power output is best measured using a ________ load. dummy The waveform that makes it easy to see amplifier distortion is the _______. triangle Intermittents caused by poor connections may be located by using __________. vibration Some intermittents can be made to appear by changing temperature or supply _____. voltage When an op amp shows dc output error, check first for dc _________ error. input

Preliminary Checks No Output Reduced Output Intermittents Operational Amplifiers REVIEW