Kenwood TH-79A Service Manual

The Kenwood TH-79A is a dual-band amateur radio transceiver covering 144MHz and 430MHz bands with cross-band repeating capability, 5W output power, 200 memory channels, CTCSS/DCS encode/decode, wideband receiver coverage, and compact design. This service manual provides technical specifications, disassembly procedures, alignment instructions, circuit diagrams, and troubleshooting guidance for qualified service technicians.

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

Technical Specifications

Complete technical specifications for the Kenwood TH-79A dual-band transceiver.

Parameter	Specification
Frequency Range	RX: 118-174MHz, 430-470MHz; TX: 144-148MHz, 430-450MHz
Output Power	5W (High), 1W (Low) on both bands
Power Supply	13.8V DC ±15%
Current Drain	RX: 150mA, TX: 1.2A (5W)
Modulation System	16F3 (FM)
Antenna Impedance	50 ohms
Memory Channels	200 (100 per band)
Dimensions	140(W) × 35(H) × 160(D) mm
Weight	Approx. 650g
Operating Temperature	-10°C to +60°C

Disassembly Procedures

Step-by-step instructions for disassembling the TH-79A transceiver.

Remove battery pack and external connectors.
Remove six case screws (two on top, two on bottom, two on sides).
Carefully separate front and rear case halves.
Disconnect internal ribbon cables and flex circuits.
Remove main PCB mounting screws.
Lift PCB assembly from case for component access.

WARNING! Use proper anti-static precautions. Document cable positions for reassembly.

Circuit Description

Detailed description of major circuit blocks and their functions.

RF Front End: Dual-band receive/transmit with bandpass filtering
IF Stages: 45.05MHz first IF with ceramic filters
PLL Circuit: TC9122P synthesizer with 12.8MHz reference
Audio Section: MC34119P audio amplifier with CTCSS/DCS processing
Power Supply: 5V and 8V regulated supplies from 13.8V input
Control System: HD64F3644F20 microcontroller with EEPROM

Alignment Procedures

Required test equipment and step-by-step alignment procedures.

Receiver Sensitivity: Adjust L1, L2 for maximum sensitivity at 146MHz
Transmitter Power: Adjust VR1 for 5W output on high power setting
Frequency Deviation: Set to ±5kHz using modulation meter
CTCSS Tone: Adjust for proper 67.0Hz to 250.3Hz tone frequencies
Squelch Threshold: Set squelch to open at 0.15μV signal level

CAUTION! Use calibrated test equipment. Follow RF safety procedures.

Block Diagrams

System block diagrams showing signal flow and major components.

Complete block diagrams included for:

Overall system architecture
Receiver section (VHF/UHF)
Transmitter section (VHF/UHF)
PLL synthesizer circuit
Audio processing chain
Power supply distribution

PCB Layouts

Printed circuit board layouts with component identification.

Main PCB component side (top view)
Main PCB solder side (bottom view)
Front panel PCB layout
Display module PCB
RF module layouts

Note: All layouts include reference designators matching parts list.

Parts List

Complete component listing with manufacturer part numbers.

Reference	Description	Part Number
IC1	Microcontroller HD64F3644F20	K123456
IC2	PLL Synthesizer TC9122P	K123457
Q1-Q3	2SC3356 RF Transistor	K123458
L1-L4	VHF Coil Assembly	K123459
VR1	Power Adjustment Potentiometer	K123460

Troubleshooting Guide

Symptom-based troubleshooting for common problems.

Symptom	Possible Cause	Corrective Action
No power	Fuse, power connector, regulator	Check F1 fuse, DC jack, measure 5V/8V rails
No transmit	PA transistor, PLL unlock	Check Q3, measure PLL lock voltage
Poor sensitivity	RF front end, alignment	Check LNA transistors, realign receiver
Display issues	Ribbon cable, LCD module	Reseat display cable, check contrast voltage
Audio distortion	Audio IC, speaker	Check IC5, replace speaker if damaged

Test Equipment Requirements

Recommended test equipment for service and alignment.

RF Signal Generator (100-500MHz)
Spectrum Analyzer
Oscilloscope (20MHz minimum)
Frequency Counter
Modulation Meter
DC Power Supply (0-15V, 2A)
Digital Multimeter
Dummy Load (50 ohm, 10W)

Safety Precautions

Important safety information for service personnel.

Always disconnect power before servicing. Use anti-static wrist strap when handling PCBs. RF circuits may contain hazardous voltages when transmitting. Ensure proper grounding of test equipment. Follow local regulations for RF exposure. Replace components only with specified parts to maintain compliance.

WARNING! High voltages present in power supply and RF sections. Qualified technicians only.