TM 11-6625-486-14&P
often be located by sight, smell, and hearing. The
(4) Remove the frequency meter from its
majority of faults, however, must be isolated by a check
case.
of voltages and resistance.
(5) Looking down on the top of the meter
b. Localization. Frequency Meter FR149(*)/USM-
chassis, locate a white wire connected to the left switch
159 consists of seven major circuits: crystal oscillator,
terminal and a black wire connected to the right switch
modulator, low frequency vfo, high frequency vfo,
terminal of the panel interlock switch.
mixer, audio amplifier, and power supply. The first step
(6) Hold the negative test prod on the left
in tracing trouble is to locate the state or stages at fault
switch terminal and the positive test prod on the right
by the following methods:
switch terminal.
(1) Visual inspection. The purpose of visual
(7) To read dc current, turn the frequency
inspection is to locate fualts without testing or measuring
meter FUNCTION SW to OPER and push in on the
battery interlock switch plunger. The multimeter should
for indication of abnormally low power supply voltage.
indicate 50 mA 2. If the current drain differs, turn the
Midscale deflection of the frequency meter pointer
frequency meter FUNCTION SW to OFF and remove
indicates 6.5 volts. Normal operating voltage exists
the battery drawer. Make resistance measurements in
when the meter pointer swings to the right in three-
the battery wiring circuit. Be sure that the wiring is not
fourths of the green scale. Pay particular attention to
grounded and the battery interlock switch operates
filmstrip lighting. If the filmstrip is not lighted, either the
properly. An ohmmeter measurement from B- in the
dial lamps are defective or the modulator is inoperative.
battery circuit to ground should indicate at least 10,000
Observe all other visual signs, and attempt to localize
ohms.
the fault to a particular stage.
(2) Operational tests. Operational tests often
not on ac power and the trouble is not an open fuse,
indicate the general location of trouble. In some
instances, the tests will help in determining the exact
glows. If the lamp does not glow, measure its internal
nature of the fault. The procedures in table 4-4 provide
resistance. Resistance should be approximately 200
a good overall check of operation.
ohms (cold). The combined resistance of the primary of
c. Isolation. The tests and information listed below
power transformer T701 and ballast lamp DS703 can be
will help in isolating the trouble. First localize the
measured from the plug end of the power cable. With
trouble to a single stage or circuit; then, isolate the fault
the frequency meter FUNCTION SW set to OPER,
within that circuit by voltage, resistance, and continuity
measure the resistance between the two flat connectors
measurements.
of the plug. Resistance should be approximately 700
(1) Signal tracing. Signal tracing (para 6-9) will help
ohms. A measurement from either connector to ground
in localizing the trouble to the stage at fault.
should read infinity. If the trouble still persists, the most
(2) Signal substitution.
Signal substitution
common form of trouble is a shorted capacitor in the
procedures (para 6-12) enable the repairman to
distribution circuit. The quickest method of isolating a
defective capacitor is to make a resistance
isolate trouble in the audio circuits. An audio
measurement from either TP601 or TP603 (table 6-1) to
oscillator and headset are typical units of
ground. The voltage and resistance diagram of the
equipment that may be used in signal
power supply (fig. FO-2 (2) or FO-3 (2) shows the
substitution procedures.
proper values for all major points as measured from the
(3) Troubleshooting charts. The trouble symptoms
point indicated to ground.
and procedures listed in table 6-3 will help in
isolating trouble to a component part.
6-8. Troubleshooting Table
The troubleshooting table (table 6-3) is designed to
6-7. Checking B + Circuits for Shorts
supplement operational checks. Begin with sequence
a. If an operational failure occurs when the
number 1 and proceed until the malfunction is located.
frequency meter is operated on battery only, check the
CAUTION
amount of dc current required by the frequency meter.
Be sure that the power is on when
A convenient method of measuring this current is given
making voltage measurements and
below:
off for resistance or continuity
(1) Turn the frequency meter FUNCTION SW
checks.
to OFF and unplug the power cable.
WARNING
(2) Turn the multimeter FUNCTION switch to
Be extremely careful when working
DC CURRENT and the RANGE switch to 100 MA.
on the power supply circuit or the ac
(3) Plug a test lead (negative) into the OHMS
line connections. Serious injury or
-DC AC jack and another test lead (positive) into the
death may result from contact with
these points.
+DC CURRENT jack.
6-7
Previous Page
