Download Troubleshooting Techniques

Troubleshooting Techniques
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 :
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•
•
•
•
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
•
BASE BIAS
EMITTER BIAS
• Q-point values
I  I 
 C E


• Collector Current:
 V V 
I c   DC  CC BE 
 RB 
• Collector-to-emitter voltage:
VCE  VCC  I C RC


• Q-point values IICCI EI E 
VOLTAGE-DIVIDER BIAS
• Q-point values IC  I E 
• Collector Current :
• Collector Current :
IC 
COLLECTOR-FEEDBACK BIAS
 R2

 R R
1
2
IC  
VEE  VBE
RE  RB  DC
• Collector-to-emitter voltage:
VCE  VCC  VEE  IC RC  RE 

VCC  VBE


RE
• Collector-to-emitter voltage:
VCE  VCC  IC RC  RE 
• Q-point values IC  I E 
• Collector Current :
IC 
VCC  VBE
RC  RB  DC
• Collector-to-emitter voltage :
VCE  VCC  I C RC
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
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-toground voltage and the base-toground 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.
Quick-Test on Pull-Down Test for Transistor Operation
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 powersupply 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.