Maintenance-2225 Service
When checking semiconductors, observe the
static-sensitivity precautions located at the
beginning of this section.
TRANSISTORS.
A good check of a transistor is
actual performance under operating conditions. A
transistor can most effectively be checked by
substituting a known-good component. However, be
sure that circuit conditions are not such that a
replacement transistor might also be damaged. If
substitute transistors are not available, use a
dynamic-type transistor checker for testing.
Static-
type transistor checkers are not recommended,
since they do not check operation under simulated
operating conditions.
When troubleshooting transistors in the circuit with a
voltmeter, measure both the emitter-to-base and
emitter-to-collector voltages to determine whether
they are consistent with normal circuit voltages.
Voltages across a transistor may vary with the type
of device and its circuit function.
Some of these voltages are predictable. The
emitter-to-base voltage for a conducting silicon
transistor will normally range from
0.6
V to
0.8
V.
The emitter-to-collector voltage for a saturated
transistor is about
0.2
V. Because these values are
small, the best way to check them is by connecting
a sensitive voltmeter across the junction rather than
comparing two voltages taken with respect to
ground. If the former method is used, both leads of
the voltmeter must be isolated from ground.
If voltage values measured are less that those just
given, either the device is shorted or no current is
flowing in the external circuit. If values exceed the
emitter-to-base values given, either the junction is
reverse biased or the device is defective. Voltages
exceeding those given for typical emitter-to-
collector values could indicate either a nonsaturated
device operating normally or a defective
(open-
circuited) transistor. If the device is conducting,
voltage will be developed across the resistors in
series with it; if open, no voltage will be developed
across the resistors unless current is being supplied
by a parallel path.
When checking emitter-to-base junctions, do
not use an ohmmeter range that has a high
internal current. High current may damage
the transistor. Reverse biasing the emitter-
to-base junction with a high current may
degrade the current-transfer ratio (Beta) of
the transistor.
A transistor emitter-to-base junction also can be
checked for an open or shorted condition by
measuring the resistance between terminals with an
ohmmeter set to a range having a low internal
source current, such as the
R
X
1-kil range. The
junction resistance should be very high in one
direction and much lower when the meter leads are
reversed.
When troubleshooting a field-effect transistor (FET)
,
the voltage across its elements can be checked in
the same manner as previously described for other
transistors. However, remember that in the normal
depletion mode of operation, the gate-to-source
junction is reverse biased; in the enhanced mode,
the junction is forward biased.
INTEGRATED CIRCUITS.
An integrated circuit (IC)
can be checked with a voltmeter, test oscilloscope,
or by direct substitution. A good understanding of
circuit operation is essential when troubleshooting a
circuit having
IC components. Use care when
checking voltages and waveforms around the
IC so
that adjacent leads are not shorted together. An
IC
test clip provides a convenient means of clipping a
test probe to an IC.
When checking a diode, do not use an ohm-
meter scale that has
a
high internal current.
High current may damage a diode. Checks
on diodes can be performed in much the
same manner as those on transistor emitter-
to-base junctions. Do not check tunnel
diodes or back diodes with an ohmmeter; use
a dynamic tester, such as the
TEKTRONIX
576
Curve Tracer.
DIODES.
A diode can be checked for either an open
or a shorted condition by measuring the resistance
between terminals with an ohmmeter set to a range
having a low internal source current, such as the
R
X
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