Kenwood TM 731A Service Manual

The Kenwood TM 731A is a dual-band mobile transceiver operating on 144MHz and 430MHz bands with cross-band repeating capability, 50W output on VHF and 35W on UHF, built-in CTCSS/DCS encoder/decoder, wide frequency coverage, multiple scanning functions, and programmable memory channels. Below are key sections for technical specifications, circuit diagrams, alignment procedures, troubleshooting, and maintenance.

1. Technical Specifications 2. Block Diagrams 3. Circuit Descriptions 4. Alignment Procedures 5. Parts List 6. PCB Layouts 7. Troubleshooting Guide 8. Maintenance Procedures 9. Test Equipment Requirements 10. Safety Precautions 11. Service Notes

Technical Specifications

Key electrical and mechanical specifications for the Kenwood TM 731A transceiver.

Parameter	Specification
Frequency Range	144-148MHz (VHF), 430-450MHz (UHF)
Output Power	50W (VHF), 35W (UHF)
Power Supply	13.8V DC ±15%
Current Consumption	Receive: 0.5A (max 1.2A with audio), Transmit: 12A (VHF), 9A (UHF)
Modulation	16F3 (FM)
Antenna Impedance	50 ohms
Memory Channels	100 (including 10 scan edges)
Dimensions	140(W) × 40(H) × 180(D) mm
Weight	1.2 kg
Temperature Range	-10°C to +60°C

Block Diagrams

System block diagrams showing signal flow and major components.

RF Unit Block Diagram: Shows receiver and transmitter sections
Main Unit Block Diagram: Displays control and audio circuits
Power Supply Block Diagram: Illustrates voltage regulation
Display/Control Unit Block Diagram: Details front panel circuits

NOTE! Refer to actual service manual for detailed block diagrams with component references.

Circuit Descriptions

Detailed descriptions of major circuit sections and their operation.

Receiver Circuit: Dual conversion superheterodyne with 45.05MHz and 455kHz IF
Transmitter Circuit: Phase-locked loop synthesizer with power amplifier stages
Control Circuit: Microprocessor-based control system
Power Supply Circuit: Switching regulator and linear regulators
Audio Circuit: Pre-amplification and power amplification stages

CAUTION! High voltage present in transmitter section during operation.

Alignment Procedures

Receiver Alignment: RF amplifier, mixer, IF stages, squelch threshold
Transmitter Alignment: PLL adjustment, power output, modulation deviation
CTCSS/DCS Alignment: Tone frequency and deviation adjustment
VCO Alignment: Voltage controlled oscillator lock range
Power Supply Alignment: Voltage regulation and current limiting

Parts List

Access component references via PCB diagrams. Main components include:

ICs: MCU (IC1), PLL (IC2), RAM (IC3), EEPROM (IC4), Audio PA (IC5)

Transistors: RF amplifiers (Q1-Q4), Driver stages (Q5-Q8), Switching regulators (Q9-Q11)

Diodes: Switching (D1-D4), Regulator (D5-D7), Protection (D8-D10)

Transformers: RF (T1-T3), IF (T4-T6), Power (T7)

PCB Layouts

Printed circuit board layouts for main unit and control unit.

Main PCB Component Side: Component placement and identification
Main PCB Solder Side: Trace routing and test points
Control Unit PCB: Front panel switches and display
RF Unit PCB: RF circuitry and shielding

Tip: Use magnifying glass for component identification on densely populated boards.

Troubleshooting Guide

Common symptoms and recommended corrective actions.

Symptom	Possible Cause	Corrective Action
No power	Power supply/fuse	Check DC input; replace fuse F1 (10A); verify voltage regulator IC
No transmit	PA stage/VCO	Check final transistors Q3,Q4; verify PLL lock; test driver stages
No receive	RF circuit/IF stages	Test RF amplifier Q1; check mixer Q2; verify IF alignment
Display issues	Control circuit	Check display connector; verify microprocessor operation; test voltage
Memory loss	Backup battery	Replace battery BT1; check EEPROM IC4; verify backup circuit

Reset: Power cycle or factory reset procedure if available.

Maintenance Procedures

Routine maintenance and cleaning procedures.

Cleaning: Use isopropyl alcohol and soft brush for PCB cleaning
Lubrication: Apply minimal contact cleaner to potentiometers and switches
Inspection: Visual check for damaged components and cold solder joints
Testing: Performance verification after service

WARNING! Disconnect power before cleaning. Use proper ESD precautions.

Test Equipment Requirements

Essential test equipment for proper alignment and troubleshooting.

Power Supply: 13.8V DC, 15A capacity with current limiting
RF Wattmeter: 1-100W range, 50 ohm impedance
Frequency Counter: 500MHz minimum frequency range
Oscilloscope: 20MHz dual trace recommended
Multimeter: Digital with diode and continuity test
Signal Generator: RF generator with modulation capability
Dummy Load: 50 ohm, 50W minimum power handling

Safety Precautions

Important safety information for service personnel.

High voltage present in power supply and transmitter sections. Use isolation transformer when working on AC-powered test equipment. Follow ESD precautions when handling integrated circuits. Ensure proper ventilation during extended testing. Verify all covers and shields are reinstalled after service.

Service Notes

Additional service information and technical notes.

Factory service bulletins may contain updated procedures. Component substitutions should match original specifications. Alignment should be performed in proper sequence. Record alignment data for future reference. For technical support, contact authorized Kenwood service centers. Documentation revision: 1.0