Kenwood TR 7850 Service Manual

The Kenwood TR 7850 is a professional VHF/UHF transceiver designed for amateur radio communications, featuring dual-band operation, cross-band repeating capability, multiple scanning functions, CTCSS/DCS encode/decode, and robust construction for reliable performance. Below are key sections covering technical specifications, maintenance procedures, alignment instructions, troubleshooting, and service information.

1. Technical Specifications 2. Disassembly Procedures 3. Circuit Description 4. Alignment Instructions 5. Block Diagrams 6. Voltage Measurements 7. Component Layout 8. Parts List 9. Troubleshooting Guide 10. Service Notes

Technical Specifications

Key technical parameters and operating characteristics of the Kenwood TR 7850 transceiver.

Parameter	Specification
Frequency Range	VHF: 144-148 MHz, UHF: 430-450 MHz
Output Power	VHF: 50W, UHF: 35W (adjustable)
Power Supply	13.8V DC ±15%
Current Drain	Receive: 0.5A, Transmit: 8A max
Modulation	16K0F3E (FM)
Sensitivity	Better than 0.16μV for 12dB SINAD
Selectivity	±7kHz at -6dB, ±20kHz at -60dB
Antenna Connector	SO-239 (UHF type)
Dimensions	140(W) × 40(H) × 180(D) mm
Weight	1.2 kg

Disassembly Procedures

Step-by-step instructions for safely disassembling the transceiver for service and repair.

Remove top and bottom covers: Unscrew 6 Phillips screws from bottom case.
Remove front panel: Disconnect ribbon cables and remove 4 screws securing front assembly.
Access main board: Remove shield covers and disconnect all interconnecting cables.
Remove RF modules: Unscrew mounting hardware and carefully lift modules from main board.

WARNING! Always disconnect power before disassembly. Use proper anti-static precautions when handling circuit boards.

Circuit Description

Detailed explanation of the transceiver's major circuit blocks and their functions.

RF Front End: Dual-band receiver with preamplifier and image rejection filters.
IF System: 45.05 MHz IF with ceramic filters and dedicated IC for signal processing.
PLL Synthesizer: Dual PLL system with TCXO reference for stable frequency generation.
Audio Circuits: Separate transmit and receive audio paths with compression and filtering.
Power Amplifier: Class AB linear amplifiers with thermal protection circuits.

CAUTION! High voltages present in power amplifier section. Exercise extreme caution during testing.

Alignment Instructions

Receiver Alignment: Adjust RF amplifier, mixer, and IF stages for optimal sensitivity.
Transmitter Alignment: Set carrier balance, modulation level, and power output.
PLL Alignment: Adjust VCO coils and reference oscillator for proper lock range.
Squelch Adjustment: Set threshold levels for carrier and tone squelch operation.
Power Output: Adjust driver and final amplifier bias for specified output power.
Frequency Calibration: Verify transmit and receive frequency accuracy.
Test Equipment Required: Signal generator, oscilloscope, power meter, frequency counter.

Block Diagrams

Functional block diagrams showing signal flow through the transceiver system.

Receiver Section: Antenna → Bandpass Filter → RF Amp → Mixer → IF Filter → Demodulator → Audio Amp.

Transmitter Section: Microphone → Audio Processor → Modulator → Driver → Power Amp → Antenna.

Control System: CPU → Display → Keypad → PLL Synthesizer → Various Control Lines.

Voltage Measurements

Reference voltage measurements at key test points throughout the circuit.

Power Supply Input: 13.8V DC nominal.
Regulated +8V Line: 8.0V ±0.2V for receiver circuits.
Regulated +5V Line: 5.0V ±0.1V for digital logic.
PLL VCO Control Voltage: 2-10V depending on frequency.
Audio Output: 1-2V RMS at speaker terminals.

Tip: All measurements should be made with transceiver in receive mode unless otherwise specified.

Component Layout

Identification and location of major components on the main circuit board.

IC1: Main microprocessor (HD64F3343F16).
IC2: PLL IC (MB15E03SL).
IC3: IF Processor (TA31136FN).
Q1-Q3: VHF RF amplifier transistors.
Q4-Q6: UHF RF amplifier transistors.
Q7-Q9: Power amplifier driver and final transistors.
FL1-FL3: Ceramic IF filters (45.05 MHz).
X1: Reference crystal (16.8 MHz).
L1-L12: Various tuning coils and inductors.

WARNING! Handle surface-mount components with care. Use proper soldering techniques.

Parts List

Critical replacement parts and components for service and repair.

Part Number	Description	Notes
2SC3356	RF Transistor	VHF front end
2SC3357	RF Transistor	UHF front end
2SC1971	Power Transistor	VHF final
2SC2634	Power Transistor	UHF final
TA31136FN	IF Processor IC	45.05 MHz IF
MB15E03SL	PLL IC	Frequency synthesizer
HD64F3343F16	Microprocessor	Main controller
SFE10.7MS45	Ceramic Filter	45.05 MHz IF

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
No power	Power supply/fuse	Check 13.8V input; replace 8A fuse if blown.
No transmit	PTT circuit/PA	Verify PTT switch operation; check power amplifier bias.
Poor reception	RF front end	Check antenna connection; test RF amplifier stages.
Frequency error	PLL/TCXO	Adjust reference oscillator; verify PLL lock.
Low power output	PA stage	Check driver/final transistors; adjust bias current.
No audio	Audio amp/speaker	Test audio IC; check speaker and connections.
Display problems	LCD/controller	Verify ribbon cables; check display voltage.

Reset Procedure: Hold [F] + [V/M] while turning power on to reset microprocessor.

Service Contact: Authorized Kenwood service centers only.