Motorola Cm Radio Midband 66 88mhz Information Manual

Chapter 3
TROUBLESHOOTING CHARTS
This section contains detailed troubleshooting flowcharts. These charts should be used as a guide in 
determining the problem areas. They are not a substitute for knowledge of circuit operation and 
astute troubleshooting techniques. It is advisable to refer to the related detailed circuit descriptions 
in the theory of operation sections prior to troubleshooting a radio.
Most troubleshooting charts end up by pointing to an IC to replace. It is not always noted, but it is...

3-2TROUBLESHOOTING CHARTS
1.0 Troubleshooting Flow Chart for Out-of-Lock Receiver (Sheet 1 of 2)
Monitor TP200 at turn-on
See figure for typical trace
TP200
Is Voltage 
correct for freq?
Typ. 4V for 66MHz
to 11v for 88MHz
(Rx freq.)
No
Check C2078, R217, 
R224, C2074, R200, 
R234
Check U200-4 
(lock detect line)
Check U200-45, 43 lines.
If all ok, replace U200
Ye s
TP200
Is ramp-up 
slope correct?
Typ.14V in 500msNo
Ye s
START
Ye sNo
Go to ‘A’
sheet 2 of 2 
Go to 
Rx VCO Section
Ye s
No
TP200
Does...

Troubleshooting Flow Chart for Out-of-Lock Receiver (Sheet 1 of 2) 3-3
1.1Troubleshooting Flow Chart for Out-of-Lock Receiver (Sheet 2 of 2)
Ye s
Ye s
No
From ‘A’
sheet 1 of 2 
TP200
Does Voltage
stay 

3-4TROUBLESHOOTING CHARTS
2.0 Troubleshooting Flow Chart for 25W Transmitter (Sheet 1 of 4)
START
No
Check components between Q100 and 
RF output, Antenna Switch D104, 
D103, VR102 and Q106, Q103 and 
associated bias circuitry.
Check DC volts present at anode of 
D103.
No or too low power when keyed
Current
increase when 
keyed?
Check power settings, tuning, R142, 
R143, R156 & components between 
U103 Pin 3 and ASFIC Pin 6 before 
replacing ASFIC
Control 
Voltage at 
TP150
>1.6V?
Check Harmonic Filter...

Troubleshooting Flow Chart for 25W Transmitter (Sheet 1 of 4) 3-5
2.1Troubleshooting Flow Chart for 25W Transmitter (Sheet 2 of 4)
Check PA 
Stages
Quick check of PA devices.
Connect power supply.
Key radio.
Replace Q100.
Re-tune PA Bias 
with Tuner.
Check 13.8v present at drain Q100. Disconnect 
power supply, measure R131 to Ground.
10kOhm?
Ye s
No
Measure R147 to Ground.
Replace Q105. 22kOhm?
Ye s
No
Go to ‘A’
sheet 3 of 4  

3-6TROUBLESHOOTING CHARTS
2.2Troubleshooting Flow Chart for 25W Transmitter (Sheet 3 of 4)
Ye s
>3V
Check Q102, R139, 
R166, R126, R169, 
R138, R175, R147
DC
Voltage
at Q101 & Q102 
base=0?
DC
Voltage
at U103 
Pin 8
Check U403
Check resistive network at 
Pin 9 and 10 of U103 
before replacing U101
Measure DC Voltage at U102 Pin 1
Pin 1 
Voltage Check resistive network at 
Pin 2 & 3 of U102 before 
replacing Q105
Check Final PA Stages
DC
Voltage at
U103 Pin 3 = 
8.8V
Check U102 before 
replacing Q105
DC...

Troubleshooting Flow Chart for 25W Transmitter (Sheet 1 of 4) 3-7
2.3Troubleshooting Flow Chart for 25W Transmitter (Sheet 4 of 4)
No
Ye s
Ye sNo
Check Final PA Stage
PA_Bias 
Voltage at 
R134
RF Voltage 
after C1044 
>100mV?
Voltage 
across R122 
>50mV?
RF Voltage 
Q105 gate 
>250mV?
RF Voltage 
Q100 gate 
>1.0V?
Check components 
between Q100 & 
antenna connector
Replace Q100
Check FGU 
Check components 
between C1044 & 
C182
Check components 
between C182 & 
Q105
Check components 
between Q105 &...

3-8TROUBLESHOOTING CHARTS
3.0 Troubleshooting Flow Chart for Synthesizer (Reference Oscillator)
Incorrect or no µP
clock signal
TP403
NOYES
NO
YES
Power-up default freq.
Fault with ASFIC 
or Controller section
Check U504-28
should be 16.8MHz
YES
NOCheck Y201 and 
associated 
circuitry
Check U504 and 
C504, C5025, R5033
Reference Oscillator 
working Freq. = 14.7MHz
or 7.4MHz ?
TP403
Freq. = 3.68MHz
0.5Vpp
Check U504-28
Check Y202-1
should be 16.8MHz
0.5VppNO
YES
Faulty U200, 
or R229 

Troubleshooting Flow Chart for VCO3-9
4.0 Troubleshooting Flow Chart for VCO
No Receive VCO Signal
at TP1
>7V at Q2751D ?NO
YES
NO
YESCheck Q2742, R2743
U200-2 should be 0V
Check Rx - Tx switching 
Q2742, Q2752 (should be turned on)
Check Q2741 and associated circuitry.
Use Hi - impedance
probe to check frequency
YES
NO
Check Rx VCO parts 

3-10TROUBLESHOOTING CHARTS
5.0 Troubleshooting Flow Chart for Receiver (Sheet 1 of 2)
Bad SINAD
Bad 20dB Quieting
No Recovered AudioSTART
Audio at 
pin 8 of 
U2 ?Check Controller
(in the case of no audio)
OR ELSE go to “B” Ye s
No
Spray or inject 21.4MHz 
into XTAL Filter FL3101
Audio heard ?BYe s
No
Check 2nd LO 
(20.945MHz) at C53 
LO present ?BYe s
 Check voltages on U2Biasing OK ?
No
No
A
Ye s
Check Q6 bias 
for faults
Replace Q6
Go to B
Ye s
No
Check circuitry 
around U2.
Replace 
U2 if defectCheck...