Tektronix 2445, Инструкция по эксплуатации

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Maintenance— 2445 Service
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 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. How­
ever, remember that in the normal depletion mode of opera­
tion, 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 when troubleshooting a circuit having 1C compo­
nents. Use care when checking voltages and waveforms
around the 1C so that adjacent leads are not shorted togeth­
er. An 1C test clip provides a convenient means of clipping a
test probe to an 1C.
HYBRIDS.
Hybrid components can best be checked by
observing voltages and waveforms on the circuit board.
Measurements should not be made on any hybrid compo­
nent while out of the circuit as they may easily be damaged.
Direct substitution is the best troubleshooting method when
a hybrid failure is suspected.
When checking a diode, do not use an ohmmeter
scale that has a high internal current. High current
may damage a diode. Checks on diodes can be per­
formed in much the same manner as those on transis­
tor emitter-to-base junctions. Do not check tunnel
diodes or back diodes with an ohmmeter; use a dy­
namic 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 kfi 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 to 0.8 V across their junc­
tions when conducting. 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 check 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.
ATTENUATORS. The Attenuators are built as complete
assemblies and should not be taken apart. If an Attenuator
is suspected as having failed, direct substitution is the rec­
ommended troubleshooting method.
11. 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 of that circuit and any other closely related
circuit should be checked. Since the power supplies affect
all circuits, performance of the entire instrument should be
checked if work has been done on the power supplies or if
the power transformer has been replaced. Readjustment of
the affected circuitry may be necessary . Refer to the “Per­
formance Check” and “Adjustment Procedure”, Sections 4
and 5 of this manual.
DIAGNOSTIC ROUTINES
The diagnostic routines contained in the 2445 operating
firmware consist of the various power-up tests that are
automatically performed when power is first applied and
several circuit exerciser routines. The test or exerciser
routines are selected by “scrolling” through a menu of avail­
able routines when the firmware is under control of the Diag­
nostic Monitor. Monitor control is indicated by the message
“DIAGNSTIC. PUSH A/B TRIG TO EXIT” displayed in the
top crt graticule division.
Entry into the monitor is automatic if a power-up test
fails. The user may also force entry into the Diagnostic Mon-
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