Specifications

LBI-33057
15
Squelch Problem
A squelch circuit problem (assuming good signal
and/or noise is present at 101/P1 pin 4) indicates a prob-
lem with Audio Processor U8 on the Control Board.
Troubleshooting should begin at the discriminator output
Signal trace through the discriminator amp to the Audio
Processor. Check the operation of the noise rectifier to
verify the noise is being converted into the proper dc
voltage. See LBI-39157 for details.
Excessive Distortion
If the measured distortion exceeds the rated specifi-
cation signal trace starting at the discriminator output
and through the audio stages until the fault is isolated.
Signal tracing with an oscilloscope proves very useful in
locating the trouble areas.
Synthesizer Troubleshooting
Will Not Lock Or Stay Locked
Each time the channel is changed, the PTT Button is
pressed or the PTT Button is released, Synthesizer IC U1
is serially loaded with new TX or RX data. If the syn-
thesizer does not lock or stay locked, the following will
be observed:
• the radio continuously or intermittently beeps
• "NO LOCK" will flash in the display
• LOCK DET (J102/P2 pin 8) will be low or
pulsing to flag the I/O Microcontroller of the
unlocked condition
If this condition occurs, the I/O Microcontroller will
continue to try to reload U1 with channel data until the
synthesizer locks.
1. Verify the 5.5 Vdc supply to the RF Board is
within ±0.1 Vdc, 7.5 VBATT is present, and TX
5.5V is low (receive mode only).
2. Monitor STROBE, DATA and ENABLE (on
J102 pins 11, 10 and 9 respectively) for pulse ac-
tivity when the channel is changed. See the serv-
ice information on the Control Board for specific
waveforms.
3. Check A5 pin 9 (output) for 5.5 Vdc to A4, U2,
and U6. If this power source is not present, check
A5 pins 7 and 13 (inputs) for 5.5 Vdc and check
A5 pin II (input) for 7.5 Vdc. Replace A5 if the
inputs are good and the output is not.
4. Using a frequency counter with a high impedance
probe or amp, check U1 pin 2 for the 13.2 MHz
(±100 Hz at 25°C) signal from Reference Oscilla-
tor U3. Replace U3 if this signal is not present
and U3 pin 3 is 5.5 Vdc. See ALIGNMENT
PROCEDURES for oscillator alignment details.
5. If LOCK DET is low and not pulsing, temporar-
ily pull it high by connecting J102/P2 pin 6 to
J102/P2 pin 8. If the radio now operates normally
on all test TX and RX channels, replace U1 - the
output on pin 9 is defective.
6. Check the VCO output with a frequency counter
at A4 pin 9 or by tuning a service monitor to the
radio's local oscillator (VCO) signal and loosely
coupling the VCO signal into the service moni-
tor. The VCO should be running 45 MHz below
the RX frequency. VCO output power level is
typically +8 dBm.
7. VCO tuning voltage can be monitored at A4 pin
5. See Table 3 for details. Also check A4 pin 6
for the -3.7 Vdc supply developed from the OSC
OUTput from U1, D3 and the associated compo-
nents.
8. If a synthesizer lock problem cannot be narrowed
to a problem in the Rear Cover Assembly, repro-
gram the inoperative channels and test the unit
again. Also see the service information on the
Control Board for further details.
Excessive Switching Time
The synthesizer should generally lock within 10
milliseconds after a frequency change.
1. Suspect leaky low-pass filter capacitors C25 or
C26 if slow switching time is a symptom. Replace
these components with original equipment parts
only. See the RF Board parts list for part num-
bers.
2. Module A5 also influences switching time. Verify
the ENABLE pulse is present at A5 pin 4.
FRONT COVER ASSEMBLY
See maintenance manual LBl-39157 for component-
level troubleshooting information on the Control Board.