Marantz 240 Service Manual

The Marantz 240 is a classic stereo power amplifier featuring 120 watts per channel RMS power output, high current capability, direct-coupled circuitry, professional-grade components, and Marantz's signature sound quality. This service manual covers technical specifications, circuit descriptions, alignment procedures, troubleshooting, and maintenance for qualified service technicians.

1. Technical Specifications 2. Block Diagram & Circuit Description 3. Disassembly Instructions 4. Alignment & Calibration 5. Schematic Diagrams 6. Parts List 7. Troubleshooting Guide 8. Test Procedures 9. Safety Precautions 10. Service Notes

Technical Specifications

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

Parameter	Specification
Power Output	120W RMS per channel (8Ω, 20Hz-20kHz)
Total Harmonic Distortion	<0.05% at rated power
Frequency Response	10Hz-50kHz (+0/-1dB)
Input Sensitivity	1.2V for rated output
Input Impedance	47kΩ
Damping Factor	>100 at 8Ω
Signal-to-Noise Ratio	>110dB (A-weighted)
Power Requirements	120V AC, 60Hz, 600W max
Dimensions	17" W × 6.5" H × 14.5" D
Weight	45 lbs (20.4 kg)

Block Diagram & Circuit Description

Detailed analysis of the amplifier's circuit topology and signal path.

Input Stage: Differential amplifier with current mirror load
Voltage Amplification: High-speed voltage gain stage
Driver Stage: Complementary driver transistors
Output Stage: Triple Darlington configuration with 6 output transistors per channel
Protection Circuits: DC offset protection, thermal protection, short-circuit protection
Power Supply: Massive toroidal transformer with dual rectifiers and large filter capacitors

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

Disassembly Instructions

Step-by-step guide for accessing internal components.

Remove top cover: Unscrew 6 Phillips-head screws from top panel
Remove bottom cover: Turn unit over and remove 8 screws
Front panel removal: Remove knobs and 4 front panel screws
Circuit board access: Remove mounting screws for main amplifier boards
Heat sink removal: Required for output transistor replacement

CAUTION! Handle circuit boards by edges to avoid electrostatic damage.

Alignment & Calibration

Bias Current: Adjust VR501/VR502 for 30mA per output pair
DC Offset: Adjust VR503/VR504 for <10mV at speaker terminals
Protection Circuit: Verify proper operation of relay timing
Meter Calibration: Adjust for accurate power level indication
Test Equipment Required: Oscilloscope, distortion analyzer, multimeter, audio generator, dummy loads

Schematic Diagrams

Complete schematics for all circuit sections.

Main Amplifier: Left and right channel power amplifier circuits

Power Supply: Transformer, rectification, and filtering circuits

Protection Circuit: Speaker protection relay and control logic

Meter Circuit: Power level meter drive circuitry

All schematics include component values, test points, and voltage readings.

Parts List

Complete component listing with manufacturer part numbers.

Transistors: 2SC2240, 2SA970, 2SC2705, 2SA1145, 2SD555, 2SB600
Diodes: 1S1555, 1S2473, ZD5.6V, ZD12V
Resistors: 1/4W carbon film, 1/2W metal oxide, 5W wirewound
Capacitors: Electrolytic, film, ceramic types with voltage ratings
Integrated Circuits: NJM4558, TA7317P
Mechanical Parts: Switches, potentiometers, connectors, hardware

Tip: Use only Marantz-approved replacement parts for optimal performance.

Troubleshooting Guide

Systematic approach to diagnosing and repairing common issues.

Symptom	Possible Cause	Corrective Action
No output	Protection circuit activated	Check for DC offset, shorted outputs, verify relay operation
Distortion	Bias current incorrect	Adjust bias potentiometers, check driver transistors
Intermittent operation	Cold solder joints	Reflow solder on power supply and output sections
Hum/buzz	Power supply issues	Check filter capacitors, ground connections, rectifiers
Overheating	Bias current too high	Readjust bias, check thermal compound, verify fan operation

Test Procedures

Comprehensive testing methodology for verifying amplifier performance.

Power-up Test: Verify no DC at output, protection relay engagement
Bias Current Measurement: Check and adjust idle current
Power Output Test: Measure maximum clean power into 8Ω load
Frequency Response: Verify flat response 20Hz-20kHz
Distortion Analysis: Measure THD at various power levels
Square Wave Testing: Check transient response and stability

WARNING! Use proper dummy loads - do not test without load connected.

Safety Precautions

Essential safety guidelines for service technicians.

Always disconnect AC power before servicing. Discharge large capacitors. Use isolation transformer when working on live circuits. Wear safety glasses. Keep work area clean and organized. Follow electrostatic discharge precautions.

CAUTION! Lethal voltages present - experienced technicians only.

Service Notes

Important service tips and common modifications.

Common failure points: Output transistors, driver transistors, power supply rectifiers. Recommended upgrades: Higher quality filter capacitors, improved thermal compound. Factory service bulletins: Updated bias procedure, protection circuit modifications. Warranty information: 90 days on service repairs.

For technical support: Contact Marantz service department or authorized service centers.