TM 11-6625-486-14&P
1 volt. Use a 33-microfarad capacitor in series with the
c. Turn the frequency meter front panel
oscillator output jack and connect the output signal to
FUNCTION SW to MOD and the AUDIO GAIN control
the base of audio output transistor Q403.
fully clockwise.
NOTE
With no input signal, transistor Q403
frequencies up to 10 MHz, RF Signal Generator
is
in
saturation
(conducting
AN/USM-44 for test frequencies of 10 through 400 MHz,
maximum current).
It requires
and Signal Generator AN/URM-49 to generate
approximately 1 volt ac of signal to
frequencies of 400 to 1,000 MHz.
bring the transistor out of saturation
e. Connect the output of the appropriate signal
and produce an output signal.
generator (depending on the test frequency specified
below) to the frequency meter ANT. jack. For each test
d. Through the headset, note the volume obtained.
frequency, adjust the signal generator rf output
If no signal can be heard in the headset, make voltage
attenuator for a signal of 0.1 volt. Tune the signal
and resistance measurements to isolate the trouble in
generator and frequency meter to the frequencies listed
this stage. If a signal can be heard, proceed to e below.
below and record the peak-to-peak audio output signals
e. With the ATTENUATOR, set the AN/URM127
as shown on the oscilloscope.
output signal to 15 millivolts. Inject a 1,000-hertz ac
(1) RANGE SW in range A.
signal (in series with the 33-microfarad capacitor) at the
Signal generator
Frequency meter
base of transistor Q402. Note that the audio volume
frequency (MHz)
dial (kHz)
increases.
1
8th
125
f. Adjust the AN/URM-127 for a 5-millivolt output
1.52
8th
190
signal and inject the test signal at the base of transistor
2.5
10th
250
Q401. If this stage of the audio amplifier is operating
(2) RANGE SW in range B.
properly, the headset volume will be excessive.
Signal generator
Frequency meter
frequency (MHz)
dial (kHz)
6-13. Audio Power Output Test
20
8th
2,500
30
8th
3,750
The audio power output test will indicate the power
65
13th
6,000
output of the meter with various external rf input signals.
(3) RANGE SW in range C.
For the test equipment required and associated
Signal generator
Frequency meter
technical manuals, refer to appenidx C of this manual.
frequency (MHz)
dial (kHz)
Set up the equipment according to figure 6-23 and
260
4th
65
proceed as follows:
520
8th
65
a. Construct a 600-ohm dummy load in
520
4th
120
accordance with paragraph 6-10 and insert the phone
1,000
8th
125
plug into one of the frequency meter phone jacks.
f. As indicated on the oscilloscope, the minimum
output signal on all harmonic frequencies should be not
less than 0.48 volt peak-to-peak (0.05 milliwatt). If
the phone plug terminals of the 600-ohm dummy load.
audio output power is not specified, troubleshooting is
Calibrate the oscilloscope for an input signal of 3 volts
required.
peak-to-peak.
Section IV. MAINTENANCE OF FREQUENCY METER
Except for two factory-calibrated subassemblies (the
following repair techniques and precautions apply
specifically to the frequency meter.
the majority of component parts in the frequency meter
can be easily reached and, if found to be defective, can
completely tested and accurately calibrated during
be replaced with new parts; however, the parts mounted
manufacture. In the high frequency vfo, the position
on printed circuit boards cannot be replaced without
and lead length of all component parts were taken into
removal of the circuit board. In certain cases, removal
consideration during the calibration procedure when the
of the circuit board involves the unsoldering of one or m
vfo was manufactured; therefore, if the trouble has been
ore wires. In other cases, the length of wiring is enough
traced to this unit and it is absolutely necessary to
to permit removal of the circuit board to the extent
replace a defective part, use exactly the same length
required for repairs without unsoldering any wires. The
and dress of part leads as used originally.
Previous Page
