Tektronix CURVE-TRACER 576 Instruction Manual page 87

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power supply is shorted, the defective circuit can be iso-
- lated by disconnecting the pin connectors at the boards
\..,.until the shorting condition is removed.
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6.
Check Voltages and Waveforms. Often the defective
component can be located by checking for the correct volt-
ages and waveforms as given on the circuit diagrams on
foldout pages in the back of this manual.
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NOTE
Voltages and waveforms given on the diagrams are
not absolute and may vary slightly between instru-
ments. To obtain operating conditions similar to
those used to take these readings, see the beginning of
the Diagrams section.
7.
Check Semiconductors. Most circuit failures result
from the failure of a transistor, FET, diode, or integrated
circuit due to normal aging and use. The following explains
various methods of checking semiconductor devices. Inser-
tion information is provided in Fig. 4-2.
TRANSISTORS. Transistor defects usually take the
form of the transistor opening, shorting, or developing
excessive leakage. The best method of checking transistors
is by direct substitution. Be sure the voltage conditions of
the circuit are not such that a replacement transistor might
also be damaged. If substitute transistors are not available,
use a dynamic tester (such as a Tektronix Type 576).
- Static-type testers are not recommended since they do
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not check the device under operating conditions. However,
\.,lit no other tester is immediately available, an ohmmeter
will usually indicate when a transistor is totally bad. As a
general rule, use the R X 1 k range where the current is
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usually limited to less than 2 rnA and the internal voltage is
usually 1 1 /2 volts. Check the current and voltage of the
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TABLE 4-2
Transistor and FET Resistance Checks
Ohmmeter
Connections
Emitter-Collector
Emitter-Base
Base-Collector
Drain-to-Source
Gate-to-Source and
\..,.Gate-to-Drain
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Resistance Readings That Can Be
Expected Using the R X 1 k Range
High readings both ways (about 60
kD. to around 500 kD.).
High reading one way (about 200
kD. or more). Low reading the
other way (about 400
n
to 2.5 kD..
High reading one way (about 500
kD. or more). Low reading the
other way (about 400
n
to 2.5
kD.).
Less than 500 Q
400 Q to 10 Q (approximately) in
one direction; more than 200 kQ
with leads reversed.
Maintenance-Type 576
ohmmeter by inserting a multimeter between the ohmmeter
leads and measuring the current and voltage of the various
ranges. After it has been determined which ohmmeter
ranges will not harm the transistor, use those ranges to
measure the transistor's resistance. Check the resistance in
both directions through the junctions as listed in Table 4-3.
FIELD EFFECT TRANSISTORS. The voltage and resis-
tance of field effect transistors can be checked in the same
manner as transistors, 1 1/2 V and less than 2 rnA should be
used for ohmmeter checks. See Table 4-2 for proper resis-
tance readings.
INTEGRATED CIRCUITS. Integrated circuits are best
checked in the circuit with a voltmeter, test oscilloscope, or
by direct substitution. A good understanding of the circuit
description is essential when troubleshooting a circuit using
integrated circuits. In addition, operating voltages and
waveforms, logic levels and other operating information for
the integrated circuits, which are provided in the Diagrams
section, are also helpful, Use care when checking voltages
and waveforms around the integrated circuits so that ad-
jacent leads are not shorted together. A convenient means
of clipping a test probe to the 14- and 16-pin integrated
circuits is with an integrated-circuit test clip. This device
also doubles as an integrated-circuit extraction tool.
DIODES. Diodes (except for tunnel diodes) can be
checked for an open or short-circuited condition by meas-
uring the resistance between the terminals after unsoldering
one end of the component. Use a resistance scale with an
internal voltage between 800 mV and 3 volts. The resist-
ance should measure very high (in megohm range) in one
direction and low in the other.
8.
Circuit Description. If the malfunction has not been
located after checking the voltages, waveforms and semi-
conductors, the circuit description should be consulted.
The circuit description describes the purpose of the circuit
and its components with emphasis on the semiconductors.
It will help in determining voltages and waveforms not
shown in the diagrams and thus help in further pin-pointing
the source of the malfunction.
9. Check ·other Components. If the semiconductors in
the circuit have been found to be good, the rest of the
components should be checked. Components which are
soldered in place are best checked by disconnecting one
end. This isolates the measurement from the effects of sur-
rounding circuitry.
10. Repair and Readjust the Circuit. If any defective
parts are located, follow the replacement procedures given
in this section. Be sure to check the performance of any
circuit that has been repaired or that has had any electrical
components replaced. If a component has been replaced,
recalibration is usually necessary.
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