Maintenance—2215A Service
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
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.
When checking emitter-to-base junctions, do not use
an ohmmeter range that has either a high internal cur
rent o r 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 k$J
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 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.
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.
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
nal source current, such as the R X 1
M l
range. The diode
resistance should be very high in one direction and very low
when the meter leads are reversed.
When conducting, silicon diodes should have 0.6 to 0.8 V
across their junctions, and schottky diodes should have 0.2
to 0.4 V across their junctions. 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 con
tinuity with an ohmmeter. Shorted or partially shorted induc
tors 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 high after
the capacitor is charged to the output voltage of the ohm
meter. An open capacitor can be detected with a capaci
tance meter or by checking whether the capacitor passes ac
signals.
10. Repair and Adjust the Circuit
If any defective parts are located, follow the replacement
procedures given under “Corrective Maintenance" in this
section. After any electrical component has been replaced,
the performance for that particular circuit should be
checked, as well as the performance of other closely related
circuits. Since the power supplies affect all circuits, perfor
mance of the entire instrument should be checked if work
has been done in any of the power supplies or if the power
transformer has been replaced. Readjustment of the af
fected circuitry may be necessary. Refer to the “Perfor
mance Check Procedure" and “Adjustment Procedure"
(Sections 4 and 5) and to Table 5-1 (Adjustment
Interactions).
6-9
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