Motorola Two Way Portable Radio Ht750ls Detailed 6881088c46 E Manual

Chapter 9 VHF Troubleshooting Charts
9.1 Troubleshooting Flow Chart for Controller
MCU Check
Power Up 
Alert Tone
 OK?
Speaker
OK?
U409 EXTAL= 
7.3728 MHz? 
U3701 Pin 19 
16.8 MHz
5V at U3711?
3.3V at U3201?
U409 Reset Pin 
94 High?
MCU is OK
Not able to pro-
gram RF Board 
ICsBefore replacing 
MCU, check SPI 
clock, SPI data, 
and RF IC select 
Replace 
Speaker
Read Radio 
OK? 
Check 
Setup
Reprogram the 
correct data. 
See FGU 
Troubleshooting
7.5V at 
Pin 3/5 U3711? 
7.5V at Pin 1 
U3201Check 
Q400...

December 26, 2003FMR-2045A-2
92VHF Troubleshooting Charts: Troubleshooting Flow Chart for Receiver (Sheet 1 of 2)
9.2Troubleshooting Flow Chart for Receiver (Sheet 1 of 2)
Bad SINAD
Bad 20dB Quieting
No Recovered AudioSTART
Audio 
at pin 8 of 
U3220?Check Controller
(in the case of no audio)
or else go to “B”Ye s
No
Spray or inject 44.85MHz 
into XTAL Filter Y3200
Audio heard?BYe s
No
Check 2nd LO
(44.395MHz) at C3274
LO Present?BYe s
Voltages 
OK?
Biasing ok?
Check circuitry 
around U3220. 
Replace...

FMR-2045A-2December 26, 2003
VHF Troubleshooting Charts: Troubleshooting Flow Chart for Receiver (Sheet 1 of 2) 93
Troubleshooting Flow Chart for Receiver (Sheet 2 of 2) 
IF 
Signal at 
L3200?
No
RF 
Signal at 
T3301?
RF 
Signal at 
R3313?
No
RF 
Signal at 
C3306?
No
RF 
Signal at 
C3302?
No or 
Check harmonic filter L3531 and L3532, 
C3532 and antenna switches 
D3521, D3551, L3551, R3551, C3551, 
C3552, and L3552
Check filter between 
C3302 & C3306; pro-
gram filter to schematic 
test freq and check...

December 26, 2003FMR-2045A-2
94VHF Troubleshooting Charts: Troubleshooting Flow Chart for Transmitter
9.3 Troubleshooting Flow Chart for Transmitter
START
No Power
Is There B+ 
Bias for Ant 
switchCheck 
Q3561
Is Current 
OK?Is Control 
Voltage high or low?Check PCIC
1. Check Pin Diodes
2. Check Harmonic Filter
Inspect/Repair Tx. 
Output Network
Is Power 
OK?
Done
Check Drive to 
Module
Is Drive 
OK?
Troubleshoot 
VCO
Inspect PA Network/
Check power out of 
U3501 at Cap C3512
Is Power 
OK?
Replace U3501...

FMR-2045A-2December 26, 2003
VHF Troubleshooting Charts: Troubleshooting Flow Chart for Synthesizer 95
9.4 Troubleshooting Flow Chart for Synthesizer
5V
 at pin 6 of 
D3701
Is information
from µP U409
correct?
Is U3701
Pin 18 AT
4.54 VDC?
Is U3701 Pin 47
AT = 13 VDC
Is U3701 Pin 19
4.3 VDC in TX?
Sta rt
Visual 
check of the 
Board OK?Correct
Problem
Check 5V
Regulator
+5V at 
U3701
Pin’s
13 & 30?
Is 16.8MHz
Signal at
U3701 Pin 
19?
Check Y3761, 
C3761, C3762, 
C3763, D3761 & 
R3761
Are signals
at Pin’s...

December 26, 2003FMR-2045A-2
96VHF Troubleshooting Charts: Troubleshooting Flow Chart for VCO
9.5Troubleshooting Flow Chart for VCO
START
No LO?
Tx Carrier?
VCO OK
Check 
R3829
TRB = 3.2V?Pin 10 
>1V?
L3831, 
L3832, 
L3833 
O/C?
Change 
L3831, L3832
Change 
U3801
AUX 3 
High?
Check U3701 
Pin 2 for 3.2V
Pin 19 
=0V
AUX 3 
Low?
Change 
U3801
V ctrl 0V 
or 13V?L3821, 
L3822, 
L3823 Open 
Circuit?
Change 
U3801
Change 
L3821, L3822, 
L3823, andL243
Change 
U3701
Check for faulty parts or dry 
joints of...