Kenwood A901 Service Manual

The Kenwood A901 is a high-performance stereo amplifier featuring 100W per channel output, MM/MC phono input, multiple line inputs, tone controls, loudness compensation, and direct speaker switching. This service manual provides comprehensive technical information for qualified service technicians, including schematics, alignment procedures, parts lists, and troubleshooting guidelines.

1. Technical Specifications 2. Block Diagram Overview 3. Circuit Description 4. Disassembly Instructions 5. Alignment Procedures 6. Voltage Measurements 7. Parts List 8. Schematic Diagrams 9. PCB Layouts 10. Troubleshooting Guide 11. Safety Precautions

Technical Specifications

Complete electrical and mechanical specifications for the Kenwood A901 amplifier.

Parameter	Specification
Power Output	100W + 100W (8Ω, 20Hz-20kHz, 0.08% THD)
Total Harmonic Distortion	0.008% (1kHz, rated power)
Frequency Response	5Hz-100kHz (+0, -3dB)
Input Sensitivity	Phono MM: 2.5mV, Phono MC: 0.25mV, Line: 150mV
Signal-to-Noise Ratio	Phono MM: 86dB, Line: 106dB
Tone Control	Bass: ±10dB at 100Hz, Treble: ±10dB at 10kHz
Power Requirements	AC 120V/230V, 50/60Hz, 350W max
Dimensions	440(W) x 150(H) x 380(D) mm
Weight	12.5 kg

Block Diagram Overview

System block diagram showing signal flow and major circuit sections.

Input Selector: Phono, CD, Tuner, Aux, Tape
Phono Equalizer: RIAA equalization for MM/MC cartridges
Tone Control: Bass and treble adjustment circuits
Volume Control: ALPS motorized potentiometer
Power Amplifier: Complementary symmetry push-pull output stage
Protection Circuit: DC detection, thermal protection, relay control
Power Supply: Toroidal transformer, rectification, filtering

WARNING! High voltages present in power supply section. Exercise extreme caution when servicing.

Circuit Description

Detailed explanation of each circuit section and component function.

Input Stage: JFET input for phono, op-amp buffers for line inputs
Phono Equalizer: Passive RIAA network with low-noise transistors
Tone Control: Baxandall-type active tone control circuit
Power Amplifier: Three-stage voltage amplification with complementary output
Protection Circuit: μPC1237 IC for speaker protection
Power Supply: Dual secondary transformer with bridge rectifiers

Disassembly Instructions

Step-by-step procedure for accessing internal components.

Remove top cover: Unscrew 6 Phillips screws from top panel
Remove bottom cover: Unscrew 8 Phillips screws from bottom
Front panel removal: Remove knobs, unscrew 4 front panel screws
PCB access: Remove relevant screws and carefully lift boards
Heat sink removal: Unscrew output transistors after disconnecting wires

CAUTION! Discharge power supply capacitors before working on amplifier circuits.

Alignment Procedures

Factory adjustment procedures for optimal performance.

Adjustment	Procedure	Test Equipment
DC Offset	Adjust VR501/VR502 for 0mV ±5mV at speaker terminals	Digital multimeter
Bias Current	Adjust VR503/VR504 for 20mA across emitter resistors	Millivolt meter
Phono Equalization	Verify RIAA response with test record	Audio analyzer
Protection Delay	Measure 3-5 second relay engagement delay	Stopwatch

Voltage Measurements

Reference voltage measurements at key test points.

Power Supply: +48V, -48V (main rails), +15V, -15V (preamp)
Output Stage: 0V DC at speaker terminals
Phono Stage: Q101 collector: +12V, Q102 collector: -12V
Tone Control: IC301 pin 7: +15V, pin 4: -15V

Note: All voltages measured with respect to chassis ground.

Parts List

Complete component listing with manufacturer part numbers.

Reference	Description	Part Number
Q101-Q104	Transistor 2SC2240BL	2SC2240BL-TL-E
Q201-Q204	Transistor 2SA970BL	2SA970BL-TL-E
Q501-Q504	Output Transistor 2SC3281	2SC3281-TL-E
Q505-Q508	Output Transistor 2SA1302	2SA1302-TL-E
IC301	Op-amp NJM4558	NJM4558D-TE1
IC501	Protection IC μPC1237	UPC1237HA-TE1
T001	Power Transformer	TP-258S

Schematic Diagrams

Complete circuit schematics for all amplifier sections.

Main Amplifier Circuit: Power amplifier and driver stages
Preamp Section: Phono equalizer and tone control circuits
Power Supply: Transformer, rectification, and regulation
Protection Circuit: Speaker relay and protection logic
Control Circuit: Input selection and volume control

Tip: Use full-size schematics for accurate component identification.

PCB Layouts

Printed circuit board patterns and component placement guides.

Main Board: Component side and solder side views
Power Supply Board: High-current traces highlighted
Control Board: Switch and potentiometer connections
Grounding Scheme: Star grounding implementation

Troubleshooting Guide

Systematic approach to diagnosing common problems.

Symptom	Possible Cause	Corrective Action
No sound, protection relay not engaging	DC offset, short circuit	Check output transistors, measure DC offset
Distortion in one channel	Bias current incorrect, faulty transistor	Adjust bias, check driver transistors
Hum in phono input	Grounding issue, faulty capacitors	Check ground connections, replace electrolytics
Intermittent operation	Cold solder joints, failing components	Resolder joints, check for thermal issues
No power	Blown fuse, power supply failure	Check fuses, measure transformer output

Safety Precautions

Essential safety guidelines for service technicians.

WARNING! Lethal voltages present even when unplugged. Always discharge power supply capacitors. Use isolation transformer when servicing live circuits. Wear safety glasses when testing under power. Ensure proper grounding of test equipment.

Service Support: Kenwood Technical Support: 1-800-536-9663