Marantz PM-42 Service Manual

The Marantz PM-42 is a professional stereo power amplifier designed for high-fidelity audio applications. This service manual provides comprehensive technical information for qualified service personnel, including schematics, alignment procedures, troubleshooting guides, and replacement parts lists. The PM-42 features robust construction, thermal protection, and delivers clean power output for professional audio systems.

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

Technical Specifications

Detailed electrical and mechanical specifications for the Marantz PM-42 amplifier.

ParameterSpecification
Power Output2 x 100W RMS @ 8Ω, 20Hz-20kHz
Total Harmonic Distortion<0.05% @ rated power
Frequency Response10Hz - 50kHz (+0/-3dB)
Input Sensitivity1V for rated output
Input Impedance47kΩ
Signal-to-Noise Ratio>100dB (A-weighted)
Damping Factor>200 @ 8Ω
Power Supply220-240V AC, 50/60Hz
Dimensions (WxHxD)430 x 150 x 380 mm
Weight12.5 kg

Block Diagram & System Overview

System architecture showing signal flow and major functional blocks.

  1. Input Stage: Balanced XLR and unbalanced RCA inputs with selector switch
  2. Pre-amplifier Section: Voltage amplification with tone control bypass
  3. Power Amplifier: Complementary symmetry output stage with thermal protection
  4. Power Supply: Toroidal transformer with dual rectification and filtering
  5. Protection Circuitry: DC offset protection, thermal shutdown, and soft-start

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

Circuit Description

Detailed analysis of each circuit section and component function.

  1. Input Selector: Relay-based input switching with muting circuit
  2. Volume Control: ALPS motorized potentiometer with remote capability
  3. Amplifier Stages: Differential input, voltage amplification, and complementary output
  4. Protection Circuits: VI limiter, DC detection, and thermal management
  5. Power Supply: Dual secondary transformer with bridge rectifiers

CAUTION! Use isolation transformer when testing live circuits.

Disassembly Instructions

Alignment & Calibration

Procedures for adjusting amplifier to factory specifications.

DC Offset Adjustment: VR101 for left channel, VR201 for right channel. Adjust for 0mV ±5mV at speaker terminals with no input.

Bias Current: VR102 (left) and VR202 (right). Set for 20mA across emitter resistors with no signal.

Protection Threshold: Test DC protection activates at ±1.5V output offset.

Troubleshooting Guide

Common symptoms and diagnostic procedures.

SymptomPossible CauseCorrective Action
No outputProtection circuit activeCheck output transistors for shorts; measure DC offset
Distorted soundBias current incorrectAdjust bias per section 6; check driver transistors
Hum in outputGround loop or filter capsCheck ground connections; test power supply capacitors
One channel deadOpen fuse or failed componentCheck fuses F101/F201; test output devices
OverheatingExcessive bias or poor ventilationReset bias current; check heat sink contact

Schematic Diagrams

Complete circuit schematics for all amplifier sections.

  1. Main Amplifier Board: Power amplifier and protection circuits
  2. Power Supply: Transformer, rectification, and filtering
  3. Input/Control Board: Selector switches and preamplifier
  4. Protection Circuit: Relay control and fault detection

Tip: Refer to component designators when ordering replacement parts.

Parts List

Complete component listing with manufacturer part numbers.

ReferenceDescriptionPart Number
Q101-Q104Output Transistor 2SC52002SC5200-N
Q105-Q108Output Transistor 2SA19432SA1943-P
IC101Protection IC TA7317PTA7317P
C101,C201Main Filter 10000μF/63VECA-1JM103
F101,F201Main Fuse 4A SBF4AL250V

PCB Layouts

Component placement and trace routing for all circuit boards.

  1. Main Amplifier Board: Top and bottom layer views
  2. Power Supply Board: Component side and solder side
  3. Input Board: Switch and connector locations
  4. Front Panel Board: Display and control interfaces

WARNING! Observe correct polarity when replacing electrolytic capacitors.

Safety Precautions

Essential safety procedures for service personnel.

CAUTION! Always disconnect from mains before servicing. High-voltage capacitors retain charge - discharge properly. Use approved replacement parts meeting original specifications.

Service Notes

Important service information and common repair procedures.

Factory service bulletins and updates. Common failure points: Output transistors (2SC5200/2SA1943), driver transistors (2SC2383/2SA1013), and power supply rectifiers. Thermal compound replacement recommended when servicing output stage. Always verify bias and offset after transistor replacement.

Service Support: Contact Marantz technical support for schematic updates and service bulletins.

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