Ericsson LBI-31511B, Maintenance Manual, Page: 9 / 18

TRANSMITTER FREQUENCY
ADJUSTMENT
First check the frequency to determine if any adjustment
is required. The frequency should be set with a frequency
meter or counter with an absolute accuracy that is 5 to 10 times
better than the tolerance to be maintained, and with the entire
radio as near as possible to an ambient temperature of 25
°
C
(77
°
F).
The oscillator frequency should be set at 25
°
C ambient
temperature. In the range of 15
°
C to 40
°
C, if the frequency
deviates more than
±
1 PPM, it may be reset to
±
1 PPM,
respectively.
Adjust L352 to set the transmit frequency while monitor-
ing RF output jack J2 through a 30 dB decoupler. If adjusting
L352 does not result in setting transmitter on frequency, re-
move synthesizer top cover, set L352 two turns from top of
coil form, then adjust course frequency control L354 on fre-
quency. Replace cover. This procedure would be necessary if
repair/replacing parts in oscillator circuit.
MODULATION LEVEL
ADJUSTMENT
The MOD ADJUST controls are adjusted to the proper
setting before shipment and normally do not require readjust-
ment. This setting permits approximately 75% modulation for
the average voice level. The audio peaks which would cause
overmodulation are clipped by the modulation limiter. The
limiter, in conjunction with the de-emphasis network, instan-
taneously limits the slope of the audio wave to the modulator,
thereby preventing overmodulation while preserving intelligi-
bility.
TEST EQUIPMENT
1. An audio oscillator (GE Model 4EX6A10)
2. Deviation Monitor
3. An output meter or a VTVM
4. GE Tes t Set Mo de l 4E X3A1 1 T est Cab le
19C850590G1
PROCEDURE
1. Select a center frequency channel. Disable Channel
Guard if present.
2. Preset R366 fully counterclockwise and R22 on
Channel Guard board (if present) to the center of its
range.
3. Apply a 1 kHz tone at 1.0 VRMS to mic input jack
J603-17. Connect deviation monitor to RF output
jack J2 through a 30 dB decoupler. Set DEVIATION
ADJUST R237 for rated deviation.
±
3.75 kHz with
Channel Guard or
±
4.5 kHz without Channel Guard.
4.
Apply a 400 Hz tone through a 100
µ
F capacitor to
J603-15. Set output level to obtain a deviation of
±
2.0
kHz. Note and maintain this voltage level while
switching the output frequency to 10 Hz. Adjust REF
OSC Deviation Control R366 starting from the fully
clockwise position for
±
2.0 kHz deviation. Remove
the modulation.
5. Select a channel with Channel Guard nearest the
center frequency and adjust R22 on the Channel
Guard option board to
±
0.65 kHz (tone or digital
Channel Guard).
ACCESSING CENTER TUNE FREQUENCY
When a radio is factory programmed for less than 16
channels, channel 16 is programmed for transmit and receive
center tune frequency.
To access center tune frequency use available control unit
and select channel 16.
TRANSMITTER ALIGNMENT
NOTE
Refer to Figures 3 and 4 Frequency Correction Fac-
tor.
NOTE
Under normal operation, in radios built prior to Janu-
ary 1989, the microcomputer will not access this
channel. The following tests accesses Channel 16 to
assure its availability for tuning purposes.
1. Apply +12 Vdc through a 10K resistor to J604-
3, then turn the radio on. This tells the micro-
computer to always access Channel 16.
2. To select the transmit center tune frequency,
press the PTT switch (J601-11). Release the
PTT switch to select the receiver center fre-
quency.
3. To exit this mode remove power from J604-3
and momentarily remove power from the radio.
AUDIO CHECK
TEST EQUIPMENT REQUIRED
•
Audio Oscillator
•
AC Voltmeter
•
Oscilloscope
•
Deviation Monitor
AUDIO AC VOLTAGES
1. Connect audio oscillator output across J603-10 (MIC HI) and J603-16 (MIC LO).
AUDIO SENSITIVITY
1. Connect audio oscillator output across J603-10 (MIC HI) and J603-16 (MIC LO). Adjust output for 1000 Hz at 1.0 VRMS.
2. Reduce generator output until deviation falls to 3.0 kHz for radios without Channel Guard or to 2.25 kHz for radios with
Channel Guard. Voltage should be less than 120 millivolts.
U301-7 C301-1
SCOPE
SETTING
HORIZONTAL
200
µ
SEC/DIV 200
µ
SEC/DIV
VERTICAL 2 VOLTS/DIV 2 VOLTS/DIV
SET AUDIO OSCILLATOR AT 1000 Hz WITH OUTPUT OF
1.0 VRMS. MODULATION ADJUSTED FOR 4.5 kHz
DEVIATION. NOTE: AN RMS OR PEAK READING
VOLTMETER WILL READ 1/2 TO 1/3 OF PEAK-TO-PEAK
READINGS.
Figure 4 - Correction Factor in Freq. Setting Figure 3 - Correction Factor in Freq. Setting
LBI-31511 LBI-31511
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