Marantz 120B Service Manual

The Marantz 120B is a professional stereo power amplifier designed for high-fidelity audio applications. This service manual covers technical specifications, circuit diagrams, troubleshooting procedures, alignment instructions, and maintenance guidelines for qualified service technicians. The amplifier features robust construction, thermal protection, and professional-grade audio performance.

1. Technical Specifications 2. Block Diagram Overview 3. Circuit Description 4. Alignment Procedures 5. Troubleshooting Guide 6. Parts List 7. PCB Layouts 8. Wiring Diagrams 9. Safety Precautions 10. Service Notes

Technical Specifications

Complete electrical and mechanical specifications for the Marantz 120B power amplifier.

Parameter	Specification
Power Output	60 Watts per channel (8Ω, 20Hz-20kHz)
Total Harmonic Distortion	<0.05% at rated power
Frequency Response	10Hz - 50kHz (+0/-1dB)
Input Sensitivity	1.0V for rated output
Input Impedance	47kΩ
Damping Factor	>100 at 8Ω
Signal-to-Noise Ratio	>105dB (A-weighted)
Power Requirements	120V AC, 60Hz, 350W max
Dimensions	17" W × 5.25" H × 13.5" D
Weight	28 lbs (12.7 kg)

Block Diagram Overview

System block diagram showing signal flow and major functional blocks.

Input Stage: Balanced and unbalanced input circuits with impedance matching
Pre-amplifier Section: Voltage amplification and tone control circuits
Driver Stage: Current amplification for power output section
Output Stage: Complementary symmetry power transistors with thermal protection
Power Supply: Dual rail power supply with regulation and filtering
Protection Circuits: DC offset protection, thermal shutdown, and current limiting

WARNING! High voltages present - discharge power supply capacitors before servicing.

Circuit Description

Detailed explanation of major circuit sections and their operation.

Input Circuit: Differential amplifier with DC servo for offset cancellation
Voltage Amplifier: High-speed op-amp with frequency compensation
Driver Stage: Class A driver with Vbe multiplier for bias stabilization
Output Stage: Triple Darlington configuration with emitter resistors
Protection: VI limiter and thermal cutoff circuits
Power Supply: Toroidal transformer with bridge rectifier and large filter capacitors

CAUTION! Use only specified replacement components to maintain performance and safety.

Alignment Procedures

Bias Current Adjustment: Set to 25mA using test points TP1/TP2
DC Offset Adjustment: Adjust VR101 for 0V DC at output terminals
Gain Matching: Ensure both channels within 0.5dB of each other
Protection Circuit Test: Verify proper operation of all protection systems
Thermal Protection: Confirm thermal cutoff at 85°C heatsink temperature
Frequency Response Verification: Check flatness across audio band

Troubleshooting Guide

Systematic approach to diagnosing and repairing common issues.

Test Equipment Required: Digital multimeter, oscilloscope, audio generator, dummy loads.

No Output: Check fuse, power supply voltages, protection relay operation. Distortion: Verify bias current, check output transistors, examine driver stages. Channel Imbalance: Test input circuits, verify gain settings, check potentiometers. Hum/Buzz: Inspect ground connections, filter capacitors, shielding. Protection Activation: Measure DC offset, check for shorted outputs, verify thermal sensor.

Parts List

Complete listing of all components with manufacturer part numbers.

Semiconductors: Transistors, diodes, ICs with cross-reference numbers
Passive Components: Resistors, capacitors, inductors with values and tolerances
Electromechanical: Switches, relays, connectors, potentiometers
Hardware: Screws, nuts, washers, brackets, heatsinks

Tip: Always verify component values before replacement.

PCB Layouts

Detailed printed circuit board layouts for main amplifier and power supply boards.

Main Amplifier Board: Component placement and trace routing
Power Supply Board: Rectifier and filter component layout
Input Board: RCA and XLR input connector arrangements
Protection Board: Relay and sensing circuit layout
Grounding Scheme: Star ground configuration details
Test Points: Location and purpose of all test points

WARNING! Observe proper ESD precautions when handling PCBs.

Wiring Diagrams

Complete interconnecting wiring diagrams and cable routing.

Connection	Wire Gauge	Color Code	Termination
AC Power	16 AWG	Black/White/Green	Spade connectors
Speaker Output	14 AWG	Red/Black	Binding posts
Signal Input	22 AWG shielded	Various	RCA/XLR
Internal Power	18 AWG	Red/Black/Yellow	PCB headers
Protection Circuit	20 AWG	Various	Quick connects

Safety Precautions

Essential safety procedures for service technicians.

CAUTION! Always disconnect AC power before servicing. Use isolation transformer when testing live circuits. Discharge large capacitors before handling. Wear safety glasses when working with high voltages.

Service Notes

Important service information and common repair procedures.

Issue	Common Cause	Recommended Action
Intermittent operation	Cold solder joints	Re-flow all power device solder joints
Thermal shutdown	Dried thermal compound	Replace thermal compound on output devices
DC offset drift	Aging input transistors	Replace matched pair input devices
Power supply hum	Failed filter capacitors	Replace main filter capacitors
Protection relay failure	Contact oxidation	Clean or replace protection relay

Factory Service: Contact authorized Marantz service centers for complex repairs.

Technical Support: Refer to Marantz technical documentation for latest updates.