Kenwood TH 78A Service Manual

The Kenwood TH-78A is a dual-band amateur radio transceiver operating on 2-meter (144-148 MHz) and 70-centimeter (430-450 MHz) bands. This service manual provides comprehensive technical information for maintenance, repair, and troubleshooting of the TH-78A, including schematics, alignment procedures, parts lists, and technical specifications.

1. Technical Specifications 2. Block Diagrams 3. Circuit Descriptions 4. Alignment Procedures 5. Disassembly Instructions 6. Troubleshooting Guide 7. Parts List 8. PCB Layouts 9. Wiring Diagrams 10. Service Notes 11. Firmware Information

Technical Specifications

Complete technical specifications for the Kenwood TH-78A dual-band transceiver.

Parameter	Specification
Frequency Range	144-148 MHz (RX/TX), 430-450 MHz (RX/TX)
Mode	FM (16F3)
RF Power Output	5W (High), 0.5W (Low)
Power Supply	13.8V DC ±15%
Current Drain	TX: 1.3A max, RX: 150mA (squelched)
Sensitivity	0.16μV (12dB SINAD)
Selectivity	±12kHz (-6dB), ±24kHz (-60dB)
Audio Output	500mW at 8Ω
Dimensions	140(W) × 35(H) × 130(D) mm
Weight	Approx. 550g

Block Diagrams

System block diagrams showing signal flow and major components.

RF Section Block Diagram: Shows receiver and transmitter RF paths
IF Section Block Diagram: Intermediate frequency processing stages
Audio Section Block Diagram: Audio amplification and processing
Control Section Block Diagram: Microprocessor and control circuits
Power Supply Block Diagram: Voltage regulation and distribution

WARNING! High voltages present in transmitter section. Exercise caution when servicing.

Circuit Descriptions

Detailed circuit descriptions for each major section of the transceiver.

Receiver Circuit: Dual-conversion superheterodyne with 45.05 MHz first IF
Transmitter Circuit: Phase-locked loop synthesizer with power amplification
PLL Circuit: Frequency synthesis and control
Audio Circuit: Microphone preamp, audio power amplifier, and squelch
Power Supply Circuit: Voltage regulation and protection

CAUTION! Use proper ESD protection when handling circuit boards.

Alignment Procedures

Receiver Alignment: RF amplifier, mixer, IF stages
Transmitter Alignment: Power amplifier, modulation
PLL Alignment: VCO, reference oscillator
Squelch Adjustment: Threshold and hysteresis
Audio Alignment: Deviation and audio response
Required Test Equipment: Signal generator, oscilloscope, power meter, frequency counter
Alignment Points: Test points TP1-TP12 throughout circuit

Disassembly Instructions

Step-by-step disassembly procedure for accessing internal components.

Procedure: Remove battery, unscrew case, separate front/rear panels, access main board.

Remove battery pack and antenna
Remove six case screws (two on top, two on bottom, two on sides)
Carefully separate front and rear case halves
Disconnect ribbon cables and flex circuits
Remove shield covers as needed for component access

Tip: Document cable positions and take photos during disassembly.

Troubleshooting Guide

Systematic troubleshooting procedures for common problems.

Symptom	Possible Cause	Corrective Action
No power	Power supply, fuse, regulator	Check DC input, measure voltages at test points
No transmit	PA circuit, PLL, microphone	Verify PLL lock, check PA transistors
No receive	RF amp, mixer, IF stages	Inject signal at various stages, check DC voltages
Poor sensitivity	RF amplifier, alignment	Check front-end components, realign receiver
Frequency error	Reference oscillator, PLL	Check reference crystal, measure VCO control voltage

Reset: Factory reset procedure available in user manual.

Parts List

Complete parts list with component designations and specifications.

Semiconductors: Transistors, ICs, diodes with part numbers
Resistors: All fixed and variable resistors with values and tolerances
Capacitors: Ceramic, electrolytic, and tantalum capacitors
Inductors: RF coils, chokes, and transformers
Mechanical Parts: Case, knobs, connectors, hardware
Kenwood Part Numbers: Official replacement part numbers

WARNING! Use only specified replacement parts to maintain performance.

PCB Layouts

Printed circuit board layouts for main board and display board.

Main Board: Component placement and routing for RF, audio, and control sections. Display Board: LCD driver and front panel controls.

Main Board Top Side: Component placement diagram
Main Board Bottom Side: Solder side and ground plane
Display Board: Front panel controls and LCD interface
Power Supply Board: Regulator and filtering components

CAUTION! Handle boards by edges to avoid damage to components.

Wiring Diagrams

Complete wiring diagrams for internal and external connections.

Internal wiring: Front panel to main board, speaker, microphone. External connections: DC power, antenna, accessories.

Service Notes

Important service notes and common repair issues.

Common Failures: Power regulator IC, final PA transistors, display backlight. Modification Notes: Factory modifications and service bulletins.

Firmware Information

Microprocessor firmware version information and programming.

Component	Firmware Version	Function
Main CPU	Ver 1.2	System control and user interface
PLL Controller	Internal	Frequency synthesis
Display Driver	Integrated	LCD control and key scanning

Service: Authorized Kenwood service centers only for firmware updates.