Tektronix 2213, Инструкция по эксплуатации

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Maintenance—2213 Service
8. Check Voltages and Waveforms
Often the defective component can be located by
checking the appropriate voltage o r waveform in the circuit.
Typical voltages are listed on the schematic diagrams.
Waveforms are shown adjacent to the schematics, and wave­
form test points are indicated on both the schematics and
circuit board illustrations by hexagonal-outlined numbers.
NOTE
Voltages and waveforms given on the schematic
diagrams are n o t absolute and may vary slightly
between instruments. To establish operating con­
ditions sim ilar to those used to obtain these readings,
see the " Voltage and Waveform Setup"conditions in
the " Diagrams" section fo r the prelim inary equip­
ment setup. Note the recommended test equipment,
in itia l front-panel control settings, and cable-
connection instructions The control-setting changes
(from in itia l setup) required to obtain the given
waveforms and voltages are located on the waveform-
diagram page.
J N A R N I N ^ J ^
To avoid electric shock, always disconnect the instru­
ment from the pow er in p u t source before removing
o r replacing components.
9. Check Individual Components
The following procedures describe methods o f checking
individual components. Two-lead components that are
soldered in place are most accurately checked by first
disconnecting one end from the circuit board. This isolates
the measurement from the effects o f surrounding circuitry.
See Figure 9-1 fo r value identification or Figure 9-2 fo r
typical semiconductor lead configuration.
C A U T I O N <
When checking semiconductors, observe the static-
sensitivity precautions located at the beginning o f
this section.
TRANSISTORS. A good check o f transistor operation
is actual performance under operating conditions. A tran­
sistor 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 recom­
mended, since they do not check operation under simulated
operating conditions.
When troubleshooting transistors in the circuit w ith a
voltmeter, measure both the emitter-to-base and emitter-
to-collector voltages to determine whether they are con­
sistent w ith normal circuit voltages. Voltages across a
transistor may vary w ith the type o f device and its circuit
function.
Some o f these voltages are predictable. The emitter-
to-base voltage fo r a conducting silicon transistor w ill
normally range from 0.6 to 0.8 V, and the emitter-to-
base voltage fo r a conducting germanium transistor ranges
from 0.2 to 0.4 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 com­
paring tw o voltages taken w ith respect to ground. If the
form er method is used, both leads o f 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 fo r 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 w ill be developed across the
resistors in series w ith it; if it is open, no voltage w ill be
developed across the resistors in series w ith it, unless
current is being supplied by a parallel path.
When checking emitter-to-base junctions, do n o t use
an ohmmeter range that has a high internal current.
High current can 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
fo r an open or shorted condition by measuring the
resistance between terminals w ith an ohmmeter set to a
range having a low internal source current, such as the
R X 1 kJ2 range. The junction resistance should be very
high in one direction and very low when the meter leads
are reversed.
When troubleshooting a field-effect transistor, the
voltage across its elements can be checked in the same
manner as previously described fo r 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.
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