Tektronix 2236, Instruction Manual

Page: 141 / 350

Maintenance— 2236 Service
W A R M I N G
I
To avoid electric shock, always disconnect the instru­
ment from the power input source before removing or
replacing components.
ages 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 de­
vice is conducting, voltage will be developed across the re­
sistors in series with it; if it is open, no voltage will be
developed across the resistors in series with it, unless cur­
rent is being supplied by a parallel path.
9. Check Individual Components
The following procedures describe methods of checking
individual components. Two-lead components that are sol­
dered in place are most accurately checked by first discon­
necting one end from the circuit board. This isolates the
measurement from the effects of surrounding circuitry. See
Figure 9-1 for value identification or Figure 9-2 for typical
semiconductor lead configuration.
When checking semiconductors, observe the static-
sensitive precautions located at the beginning of this
section.
When checking emitter-to-base junctions, do not use
an ohmmeter range that has either a high internal cur­
rent or voltage. High current or high voltage can dam­
age
the transistor. Reverse biasing the
emitter-to-base junction with a high current may de­
grade the transistor’s current-transfer ratio (Beta).
A transistor emitter-to-base junction also can be checked
for an open or shorted condition by measuring the resis­
tance between terminals with an ohmmeter set to a range
having a low internal source current, such as the R X 1 kfi
range. The junction resistance should be very high in one
direction and very low when the meter leads are reversed.
TRANSISTORS. A good check of transistor operation 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 tester. Static-type testers are not recommended,
since they do not check operation under simulated operat­
ing 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.
When troubleshooting a field-effect transistor, the volt­
age across its elements can be checked in the same manner
as previously described for other transistors. However, re­
member 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 (1C) can
be checked with a voltmeter, test oscilloscope, or by direct
substitution. A good understanding of circuit operation is
essential to troubleshooting a circuit having an 1C. Use care
when checking voltages and waveforms around the 1C so
that adjacent leads are not shorted together. The grabber
tip or an 1C test clip provides a convenient means of clipping
a test probe to an 1C.
Some of these voltage are predictable. The emitter-to-
base voltage for a conducting silicon transistor will normally
range from 0.6 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 compar­
ing two voltages taken with respect to ground. If the former
method is used, both leads of the voltmeter must be isolated
from ground.
When checking a diode, do not use an ohmmeter
range that has a high internal current. High current
can damage the diode. Checks on diodes can be per­
formed in much the same manner as on transistor
emitter-to-base junctions; use a dynamic tester, such
as the TEKTRONIX 576 Curve Tracer.
If values less than these are obtained, 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. Volt-
DIODES. A diode can be checked for either an open or a
shorted condition by measuring the resistance between ter­
minals with an ohmmeter set to a range having a low inter-
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