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 kn 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
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
IC
components.
Use
care when checking voltages
and
waveforms around the
IC
so that adjacent leads
are
not
shorted together.
An
IC
test clip provides a convenient
means
of clipping a test probe to
an
IC.
HYBRIDS. Hybrid components
can
best
be
checked by
observing voltages
and
waveforms
on
the circuit board.
Measurements should not
be
made
on
any
hybrid
component while out of the circuit
as
they may easily
be
damaged. Direct substitution
is
the best troubleshooting
method when a hybrid failure
is
suspected.
The
CH
1
and
CH
2 hybrids are matched,
and
should
be
replaced
as
a
matched pair.
~
When
checking a diode, do not use an ohmmeter
scale that has a high internal
cu"ent. High current
may damage a
diode.
Checks on diodes can be per-
formed
in
much the same manner as those on
transistor emitter-to-base junctions. Do not check
tunnel diodes
or
back diodes with
an
ohmmeter; 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 kn
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
Junctions
when conducting. Higher readings indicate that
they
are
either reverse biased or defective, depending
on
polarity.
Maintenance-2445A/2455A
Service
Light Emitting Diodes (LEDs) should have 1.5 to 2.2
V,
depending on their current
and
color, across their junctions
when conducting. Higher readings usually indicate the
diodes
are
open, especially if they
are
not illuminated
(ON).
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
high
after the capacitor
is
charged to the output voltage
of
the ohmmeter.
An
open capacitor can
be
detected with a
capacitance 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 recommended troubleshooting method.
11. Repair and
Adjust
the
Circuit.
If
any
defective parts are located, follow the replace-
ment 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 instru-
ment 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 "Performance Check"
and
• Adjustment Procedure", Sections 4
and
5 of this manual.
DIAGNOSTIC ROUTINES
The diagnostic routines contained
in
the instrument
operating firmware consist
of
the various power-up tests,
automatically performed when power
is
first applied,
and
several circuit exerciser routines. The test or exerciser rou-
tines
are
selected
by
scrolling through a
menu
of available
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.
6-9
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