Motorola Two Way Portable Radio Ht750ls Detailed 6881088c46 E Manual

FMR-2045A-2December 26, 2003
UHF Troubleshooting Charts: Troubleshooting Flow Chart for Controller 31
Chapter 3 UHF Troubleshooting Charts
3.1 Troubleshooting Flow Chart for Controller
MCU Check
Power Up 
Alert Tone
 OK?
Speaker
OK?
U409 EXTAL= 
7.3728 MHz? 
U201 Pin 19 
16.8 MHz
5V at U247?
3.3V at U248?
U409 Reset Pin 
94 High?
MCU is OK
Not able to 
program RF 
Board ICsBefore replacing 
MCU, check SPI 
clock, SPI data, 
and RF IC select 
Replace 
Speaker
Read Radio 
OK? 
Check 
Setup
Reprogram the...

December 26, 2003FMR-2045A-2
32UHF Troubleshooting Charts: Troubleshooting Flow Chart for Receiver (Sheet 1 of 2)
3.2 Troubleshooting Flow Chart for Receiver (Sheet 1 of 2)
Bad SINAD
Bad 20dB Quieting
No Recovered AudioSTART
Audio at pin 8 
of U301?Check ControllerYe s
No
Spray or inject 1st IF 44.85 
MHz into 
XTAL Filter
Audio heard?BYe s
No
Check 2nd LO signal 
44.395 MHz at C367
Signal 
Present?BYe s
Voltages 
ok?
Biasing ok?
Check circuitry 
around U301. 
Replace U301 if 
defect
No
No
A
Ye sCheck...

FMR-2045A-2December 26, 2003
UHF Troubleshooting Charts: Troubleshooting Flow Chart for Receiver (Sheet 2 of 2) 33
3.3 Troubleshooting Flow Chart for Receiver (Sheet 2 of 2)
IF Signal at 
L311?
No
RF Signal
at T301?
RF Signal
at C310?
No
RF Signal
at C307?
No
RF Signal
at C301?
No or 
Check harmonic filters L101 & L102 and 
ant. switches CR101, CR102, L104
Check filter between 
C301 & C307; program 
filter to schematic test 
freq and check varactor 
voltages.
Inject RF into J101
Are varactor 
voltages...

December 26, 2003FMR-2045A-2
34UHF Troubleshooting Charts: Troubleshooting Flow Chart for Transmitter
3.4 Troubleshooting Flow Chart for Transmitter
START
No Power
Is There B+ 
Bias for Ant 
switchC h e c k  Q 111
Is Current 
OK?Is Control Volt-
age High or LowCheck 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 
U101 at Cap C160
Is Power 
OK?
Replace U101...

FMR-2045A-2December 26, 2003
UHF Troubleshooting Charts: Troubleshooting Flow Chart for Synthesizer 35
3.5 Troubleshooting Flow Chart for Synthesizer
5V
 at pin 6 of 
CR201
Is information
from µP U409
correct?
Is U201 Pin 
18 AT 4.54 
VDC?
Is U201 Pin 
47AT = 13 
VDC
Is U241 Pin 19
4.3 VDC in TX?
Sta rt
Visual 
check of the 
Board OK?Correct
Problem
Check 5V
Regulator
+5V at U201
Pin’s
13 & 30?
Is 16.8MHz
Signal at
U201 Pin 19?
Check FL201, C206, 
C207, C208, CR203 
& R204
Are signals
at Pin’s 14 &
15 of...

December 26, 2003FMR-2045A-2
36UHF Troubleshooting Charts: Troubleshooting Flow Chart for VCO
3.6Troubleshooting Flow Chart for VCO
START
No LO?
Tx Carrier?
VCO OK
Check 
R260
TRB = 5V?Pin 10 
>1V?
L253 O/C?Change 
L253
Change 
U241
AUX 3 
High?
Check U201 
Pin 2 for 3.2V
Pin 19 
=0V
AUX 4 
High?
Change 
Q261
V ctrl 0V 
or 13V?
L243 Open 
Circuit?
Change 
U241
Change 
L243
Change 
U201
Check for faulty parts or dry 
joints of L271, L273, C370, 
C386, R339 & L320 A
A
No
No Ye s
Ye s Ye s
No
NoYe sYe s
Ye...

FMR-2045A-2December 26, 2003
UHF Band 2 Theory of Operation: Transmitter 37
Chapter 4 UHF Band 2 Theory of Operation
4.1 Transmitter
Figure 4-1.  Transmitter Block Diagram
4.1.1 General
(Refer to Figure 4-1)
The UHF transmitter contains five basic circuits:
 power amplifier
 antenna switch
 harmonic filter
 antenna matching network
 power control integrated circuit (PCIC)
4.1.2 Power Amplifier
The power amplifier consists of two devices:
 9Z67 LDMOS driver IC (U101)
 PRF1507 LDMOS PA (Q110)
The...

December 26, 2003FMR-2045A-2
38UHF Band 2 Theory of Operation: Transmitter
4.1.3 Antenna Switch
The antenna switch circuit consists of two PIN diodes (CR101 and CR102), a pi network (C107, 
L104 and C106), and two current limiting resistors (R101, R170). In the transmit mode, B+ at PCIC 
(U102) pin 23 will go low and turn on Q111 where a B+ bias is applied to the antenna switch circuit to 
bias the diodes on. The shunt diode (CR102) shorts out the receiver port, and the pi network, which 
operates as a...

FMR-2045A-2December 26, 2003
UHF Band 2 Theory of Operation: Receiver39
4.2 Receiver
Figure 4-2.  UHF Receiver Block Diagram
4.2.1 Receiver Front-End
(Refer to “Figure 5-4. UHF Band 2 Receiver Front End Schematic Diagram” on page 50 and “Figure 
5-9. UHF Band 2 Transmitter Schematic Diagram” on page 55.
The RF signal is received by the antenna and applied to a low-pass filter. For UHF, the filter consists 
of L101, L102, C102, C103, C104. The filtered RF signal is passed through the antenna switch. The...

December 26, 2003FMR-2045A-2
40UHF Band 2 Theory of Operation: Frequency Generation Circuitry
The output of the post-RF amplifier filter which is connected to the passive double balanced mixer 
consists of T301, T302, and CR306. Matching of the filter to the mixer is provided by C381. After 
mixing with the first LO signal from the voltage controlled oscillator (VCO) using low side injection, 
the RF signal is down-converted to the 44.85 MHz IF signal.
The IF signal coming out of the mixer is transferred...