AIR FORCE
T033A1
-13-496-1
NAVELEX
0969-LP-170-0010
Maintenance
Instructions
voltages
of
a
saturated
transistor is
about
0.2 volts. Because
these values
are small, the best way to check them is by
connecting
the voltmeter across the junction and using
a
sensitive voltmeter
setting, rather than
by comparing 2
voltages taken with
respect
to
ground
(both leads
of
the
voltmeter must
be isolated from ground if
this
method
is
used). If values less than these are obtained, either the de-
vice
is
short-circuited or no current
is
flowing in the circuit.
If values are in excess of the base emitter values given, the
junction is back biased or the device is defective. Values in
excess
of those given for emitter
collector could indicate
either
a
nonsaturated device operating normally,
or
a
de-
fective (open-circuited) transistor. If the device
is
con-
ducting,
voltage will be
developed across resistances
in
series with it; if it is open, no
voltage will be
developed
across
resistances in series with it unless
current
is being
supplied by a
parallel
path.
2 When
troubleshooting field effect
transistors,
the voltages
across its
elements
can
be
checked in the same
manner as
transistors.
However,
is
should
be remembered
that
normal depletion
mode operation has
the gate
to
source
junction
reverse biased, while
the enhanced mode
has the junction
forward
biased.
^
IC's
(integrated circuits) can
be checked
with
a
voltmeter, test
oscilloscope, or by
direct substitution. A
good
understanding
of circuit
operation is essential
to
troubleshooting circuits
using IC's. Use care
when checking
voltages and
waveforms around
the IC's
so
that adjacent
leads
are not shorted
together.
A
convenient
means of
clipping
a test
probe to the
14-and 16-pin IC's is with
an
1C
test clip. This device also
doubles
as
an extraction tool.
(b) Diodes. A diode can be checked
for an open or
for
a
short
circuit by measuring the resistance
between
terminals
with an ohmmeter set
to
the R
X 1 kilohm scale.
The
diode
resistance should be very high in one
direction
and very
low when the meter leads
are reversed.
Do not use an ohmmeter scale
that has
a
high
internal current. High currents
can
damage
diodes.
Check diodes in
the same manner as
transistor
emitter to base junctions. Silicon
diodes should have
0.6 to 0.8 volts
across the
junction
when conducting.
Higher readings
in-
dicate
that they are either
back biased or de-
fective, depending
on polarity.
(c) Resistors.
Check
the resistors with an ohm-
meter. Check
the parts list
for tolerance
of the resistors
used in this
instrument.
Resistors normally do
not need
to
be replaced
unless the
measured
value
varies
considerably
from the
specified value.
(d)
Inductors. Check
for open inductors by
checking
continuity with an
ohmmeter. Shorted or
parti-
ally shorted inductors
can usually
be
found by
checking
the waveform
response
when high-frequency signals are
passed
through the circuit.
(e) Capacitors.
A leaky or
shorted capacitor can
best be detected by
checking
resistance with an ohmmeter
on
the
highest scale. Do not
exceed the voltage rating of
the
capacitor. The resistance
reading should be high after
initial charge of the
capacitor. An open capacitor can be de-
tected with
a
capacitance meter or
by
checking whether
the
capacitor passes ac signals.
(f)
Attenuators.
The
thick
film
attenuators
are
best
checked
by substitution.
If
only
one
channel
is
not
operating
properly,
and
there
is reason
to believe
an
at-
tenuator
is defective,
replace
the
suspected
attenuator
with
the
same
attenuator
from
the
other
channel
and
check
instrument
operation.
If
proper
operation
results,
replace
or
repair
the
defective
attenuator.
5-4.
CALIBRATION
AND
CHECKOUT
AFTER
REPAIR.
Whenever
repairs
involve
the
power
supplies
or
instrument
disassembly.
Calibration
and
an
Operational
Checkout
should
be performed.
5-5.
COMPONENT
REMOVAL,
REPLACEMENT,
AND
DISASSEMBLY.
To prevent
electrical
shock or damage to the
instrument, always disconnect the instrument
from
the power source before removing
or re-
placing components.
Also,
review
the Safety
Summary page in the front of
this manual.
a. Cabinet Top
and
EMI Shield Removal
and Replace-
ment.
(1)
Using
a coin or large bladed screwdriver, rotate
the three
circular locks on each
side of
the
cabinet (see
Figure
5-5)
counterclockwise until the slots are vertical.
(2)
Lift
the cabinet
top
straight up.
5-32
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