LBI-33057
14
this point should produce
±
2.5 kHz deviation.
Improper modulation from 300 - 3000 Hz points
to a VCO modulation problem. Check the VCO
pin 4 for the TX MOD signal. Suspect the VCO
if the signal is present and incorrect modulation
exists.
2. If improper low-frequency modulation exists
(trunked mode low-speed data), first check J102
pin 1 for the proper tone. Next check U3 pin 4 for
the tone; suspect C30 if the tone is not present.
Replace U3 if the tone is present on pin 4 and in-
correct low-frequency modulation exists. See the
ALIGNMENT PROCEDURES for details.
3. Check Tracking and Channel Data. Reprogram if
necessary.
4. See the service information in LBI-39157 on
troubleshooting the Control Board.
Distorted Modulation
Check TX MOD for an undistorted signal to the RF
Board. The audio signal on J102/P2 pin 1 should appear
undistorted at maximum deviation and it should be rid-
ing on a 2.5 Vdc bias developed on the Control Board.
1. If TX MOD distortion is minimal and transmitter
distortion is excessive, suspect the VCO or the
Reference Oscillator on the RF Board.
2. If TX MOD is distorted, suspect a defective mi-
crophone, mic amps, modulation limiting, or pre-
emphasis circuitry.
If audio sensitivity is good, the microphone, amplifi-
ers and limiters are probably OK. Regeneration from an
open decoupling capacitor may make the unit appear to
be too sensitive. See the service information in LBI-
39157 on troubleshooting the Control Board.
Receiver Troubleshooting
Inoperative
1. If the synthesizer is not locked the radio should
be beeping and flashing "NO LOCK" in the LCD.
See Synthesizer Troubleshooting
2. Check RX DISC on J101/PI pin 4 for signal
and/or noise. With an RF input modulated at 1
kHz,
±
1.5 kHz deviation, RX DISC should be
100 - 150 mV rms (approximately 350 mV p-p).
If the 1 kHz audio is present, troubleshoot the
audio circuits in the Front Cover Assembly. Noise
levels on RX DISC with no RF input should be
approximately 700 mV p-p.
3. Check for 7.5 Vdc at RF Amp A6 pin 5 and IF
Amp A2 pin 1. Check U6 pin 4 for 5.5 Vdc from
A5. Verify TX 5.5V is low in receive mode.
4. Verify mixer injection from the VCO is present at
U5 pin 8. Check the VCO output with a fre-
quency counter. The VCO should be running 45
MHz below the RX frequency. VCO output
power level is typ8 dBm at U5 pin 8.
5. To test the back-end circuits, follow the below
procedure:
•
Verify 5.5 Vdc is present on U6 pin 4.
•
Connect a frequency counter to U6 pin 5 to
monitor the IF. Use a high-impedance probe (or
amp).
•
Couple a 45 MHz signal (no modulation) from
an HF signal generator to the emitter of Q3.
Adjust the signal generator level to achieve ac-
curate counting of the IF; the signal generator
should be set 10 dBm above the lowest level
which provides stable counting of the 455 kHz
IF. See ALIGNMENT PROCEDURES if the
2nd IF is in error by more than 90 Hz.
•
Modulate the HF signal generator and observe
the recovered audio on U6 pin 9 and J101/P1
pin 4.
Low Sensitivity
Low receiver sensitivity and/or failure of a modula-
tion acceptance bandwidth test indicates a receiver gain
or selectivity problem in the IF stages. An excessively
wide bandwidth will cause unnecessary noise, detracting
from the receiver quieting. If the bandwidth is too narrow
squelching could occur at the modulation peaks. Suspect
a defective IF filter if one of the above symptoms occurs.
A 12 dB SINAD sensitivity of around -90 dBm indi-
cates a problem with High-IF Amp A2. The module pro-
vides a nominal gain of around 17 dB.
A 12 dB SINAD sensitivity of around -100 dBm
could be caused by a problem with RF Amp A6. This
module provides 8 dB of gain.
