Tektronix 2235A Instruction Manual page 111

Malntenance
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2235A Instruction
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 testers 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 rarlge from 0.6 V to 0.8 V . The emitter-to-
collector voltage for a saturated transistor is about 0.2V.
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 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.
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 either a
high internal current or voltage. High current or
high voltage may damage the transistor.
Reverse biasing the emitter-to-base junction
with a high current may degrade 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 resistance between terminals with an ohmmeter set
to a range having a low internal source current, such as
the R X 1 ki2 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 an IC. Use care when checking voltages and
waveforms around the IC so that adjacent leads are not
shorted together. The grabber tip or 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 range 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; 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 1 ki2 range. The
diode resistance should be very high in one direction
and much lower when the meter leads are reversed.
Silicon diodes should have 0.6 V to 0.8 V across their
junctions when conducting; Schottky diodes about 0.2V
to 0.4 V . Higher readings indicate that they are either
reverse biased or defective, depending on polarity.
RESISTORS. Check resistors with an ohmmeter. Refer
to the Replaceable Electrical Parts list for the tolerances
of resistors used in this instrument. A resistor normally
does not require replacement unless its measured value
varies widely from its specified value and tolerance.
INDUCTORS. Check for open inductors by checking
continuity with an ohmmeter. Shorted or partially shorted
inductors can usually be found by checking the
waveform response when high-frequency signals are
passed through the circuit.
CAPACITORS. A leaky or shorted capacitor can best be
detected by checking resistance with an ohmmeter set
to one of the highest ranges. Do not exceed the voltage
rating of the capacitor. The resistance reading should be
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