Marantz Model 240 Service Manual

The Marantz Model 240 is a high-performance stereo power amplifier featuring 120 watts per channel RMS power output, advanced thermal protection circuitry, direct-coupled output, and professional-grade audio components. This service manual provides comprehensive technical information for qualified service technicians, including schematics, alignment procedures, troubleshooting guides, and replacement parts lists.

Technical Specifications

Complete electrical and mechanical specifications for the Marantz Model 240 stereo power amplifier.

Block Diagram Overview

Functional block diagram showing signal flow and major circuit sections.

1. Input Stage: Balanced differential amplifier with FET input
2. Voltage Amplification: High-speed driver stage with current mirror
3. Output Stage: Complementary Darlington power transistors
4. Protection Circuitry: DC offset, thermal, and short-circuit protection
5. Power Supply: Dual-mono design with toroidal transformer

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

Circuit Description

Detailed analysis of each circuit section and component function.

1. Power Supply: Dual 65V rails, 15,000μF filter capacitors per channel
2. Input Circuit: JFET differential pair with current source load
3. Driver Stage: Complementary voltage amplifier with Vbe multiplier
4. Output Stage: Triple Darlington configuration, 6 output transistors per channel
5. Protection: VI limiter, DC detection, thermal cutoff

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

Alignment Procedures

• Bias Current Adjustment: Set to 30mA using test points TP1/TP2
• DC Offset Adjustment: Adjust VR101 for 0V ±5mV at output
• Protection Threshold: Verify protection activates at 2V DC offset
• Meter Calibration: Adjust for accurate power level indication
• Thermal Protection: Verify cutoff at 85°C heatsink temperature

Test Points & Measurements

Critical test points and expected voltage measurements under normal operating conditions.

Power Supply: +65V and -65V rails (±2V tolerance)

Driver Stage: Q104 collector +45V, Q105 collector -45V

Output Stage: Emitter resistors 0.3V drop at idle

Protection Circuit: Relay driver voltage +24V when operational

Troubleshooting Guide

Systematic approach to diagnosing and repairing common issues.

1. Check all power supply voltages before proceeding
2. Verify bias current and DC offset settings
3. Inspect output transistors for shorts or opens
4. Test protection circuitry functionality
5. Check input and driver stages with signal tracer

Tip: Always use dim-bulb tester for initial power-up after repairs.

Replacement Parts List

Authorized replacement components with manufacturer part numbers.

Schematic Diagrams

Complete schematic diagrams for all circuit sections.

1. Main Amplifier Circuit (both channels)
2. Power Supply and Protection Circuit
3. Meter Driver and Control Circuit
4. Input Selector and Preamp Stage
5. Front Panel and Control Layout

WARNING! Schematics contain high voltage sections - exercise extreme caution.

Mechanical Assembly

Exploded view diagrams and assembly instructions.

Safety Precautions

Essential safety procedures for servicing high-power audio equipment.

CAUTION! Lethal voltages present - always discharge capacitors and use isolation transformer.

Service Notes

Factory Service: Contact authorized Marantz service center for complex issues.

Technical Support: Refer to Marantz technical documentation library