Marantz 105B Service Manual

The Marantz 105B is a professional audio amplifier featuring high-quality components, robust construction, and reliable performance for professional audio applications. This service manual provides comprehensive technical information including specifications, circuit diagrams, alignment procedures, troubleshooting guides, and maintenance instructions for qualified service technicians.

Technical Specifications

Complete technical specifications and performance characteristics of the Marantz 105B amplifier.

Parameter	Specification
Power Output	50 Watts RMS per channel (8Ω)
Frequency Response	20Hz - 20kHz ±0.5dB
Total Harmonic Distortion	<0.05% at rated output
Input Sensitivity	0.5V for rated output
Input Impedance	47kΩ
Signal-to-Noise Ratio	>90dB
Damping Factor	>100 at 8Ω
Power Supply	120V/240V AC, 50/60Hz
Dimensions	17" W × 4.5" H × 12" D
Weight	22 lbs (10 kg)

Block Diagram

System overview showing signal flow and major functional blocks.

1. Input Stage: Balanced and unbalanced inputs with impedance matching
2. Pre-amplifier Section: Voltage amplification and tone control
3. Power Amplifier: Complementary symmetry output stage
4. Power Supply: Transformer, rectification, and regulation
5. Protection Circuitry: Overload and thermal protection

WARNING! High voltages present - discharge capacitors before servicing.

Circuit Description

Detailed explanation of each circuit section and component function.

1. Input Circuit: Selector switch and impedance matching network
2. Pre-amplifier: Low-noise transistor stages with feedback
3. Tone Control: Bass and treble adjustment circuits
4. Driver Stage: Voltage amplification for output transistors
5. Output Stage: Complementary power transistors with bias adjustment

CAUTION! Use proper grounding techniques when testing live circuits.

Alignment Procedures

• Bias Current Adjustment: Set to 25mA using test points TP1/TP2
• DC Offset Adjustment: Adjust VR101 for 0V DC at output
• Gain Matching: Ensure both channels within 0.5dB
• Frequency Response: Verify flat response 20Hz-20kHz
• Protection Circuit: Test overload and thermal shutdown
• Power Supply: Verify ±45V DC rails
• Test Equipment Required: Oscilloscope, DMM, Audio Generator, Dummy Load

Parts List

Complete component listing with manufacturer part numbers and substitutions.

Semiconductors: 2N3055/MJ2955 output transistors, BC547/BC557 small signal, 1N4007 rectifiers

Capacitors: Electrolytic: 10,000μF/63V main filter, 100μF/50V coupling; Film: 0.1μF/100V

Resistors: Metal film 1% tolerance for critical circuits, carbon film for general use

Potentiometers: 100kΩ audio taper for volume, 50kΩ for tone controls

Schematic Diagrams

Complete circuit schematics for all sections.

1. Main Amplifier Circuit: Left and right channels
2. Power Supply: Transformer, rectifier, filter capacitors
3. Protection Circuit: Overcurrent and thermal protection
4. Input Selector: Source switching and impedance matching
5. Tone Control: Bass and treble networks

Tip: Refer to component designators on PCB for accurate identification.

PCB Layout

Printed Circuit Board component placement and trace routing.

1. Component Side: Shows all component locations and values
2. Solder Side: Displays trace patterns and via locations
3. Ground Planes: Identify ground return paths
4. Power Traces: Heavy traces for high current paths
5. Signal Routing: Low-level audio signal paths
6. Test Points: Designated measurement locations
7. Adjustment Points: Bias and offset adjustment locations

WARNING! Observe proper soldering techniques to prevent board damage.

Test Procedures

Step-by-step testing methodology for performance verification.

Power Output Test: Apply 1kHz signal, measure output at clipping

Frequency Response: Sweep 20Hz-20kHz at 1W output

Distortion Measurement: THD at various power levels

Noise Floor: Measure with inputs shorted

Protection Testing: Verify overload and thermal protection

Troubleshooting Guide

Systematic approach to identifying and resolving common issues.

Symptom	Possible Cause	Corrective Action
No output	Blown fuse, faulty power supply	Check fuses F101/F102; measure DC rails
Distorted audio	Bias misadjustment, faulty output transistors	Adjust bias current; test output devices
Hum/buzz	Ground loops, filter capacitor failure	Check ground connections; test C101/C102
One channel dead	Open input connection, failed component	Verify signal path; check transistors Q101-Q108
Overheating	Excessive bias current, poor ventilation	Reset bias; ensure adequate airflow

Service Notes

Important service considerations and component replacement guidelines.

Transistor Replacement: Match gain characteristics when replacing complementary pairs

Capacitor Replacement: Use same or higher voltage rating; observe polarity

Bias Adjustment: Allow 30 minutes warm-up before final adjustment

Thermal Compound: Apply fresh thermal compound when replacing output devices

Safety Precautions

Essential safety procedures for service personnel.

Electrical Safety: Always disconnect power before working on unit; discharge large capacitors

Test Equipment: Use isolated test equipment; ensure proper grounding

Component Handling: Observe ESD precautions with semiconductors

High Voltage: Exercise extreme caution around power supply and output stages

Service Support: For technical assistance contact Marantz service department