Marantz PM 300 Service Manual

The Marantz PM 300 is a high-quality stereo integrated amplifier featuring 30 watts per channel RMS, MM phono input, multiple line inputs, tape monitoring, loudness control, and headphone output. This service manual provides comprehensive technical information for service technicians, including schematics, alignment procedures, parts lists, and troubleshooting guidelines for proper maintenance and repair.

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

Technical Specifications

Complete electrical and mechanical specifications for the Marantz PM 300 integrated amplifier.

ParameterSpecification
Power Output30W + 30W RMS (8Ω, 20Hz-20kHz)
Total Harmonic Distortion<0.05% (at rated output)
Frequency Response10Hz - 50kHz (+0/-3dB)
Input SensitivityPhono: 2.5mV, Line: 150mV
Signal-to-Noise RatioPhono: 76dB, Line: 90dB
Tone ControlsBass: ±10dB at 100Hz, Treble: ±10dB at 10kHz
Power RequirementsAC 120V/230V, 50/60Hz
Dimensions440mm (W) x 125mm (H) x 310mm (D)
Weight6.5 kg

Block Diagram

System block diagram showing signal flow and major functional blocks.

  1. Input Selector: Phono, CD, Tuner, Aux, Tape
  2. Phono Equalizer: RIAA equalization circuit
  3. Tone Control: Bass and treble adjustment
  4. Volume Control: ALPS potentiometer
  5. Power Amplifier: Complementary symmetry output stage
  6. Power Supply: Transformer, rectifier, filter capacitors
  7. Protection Circuit: DC detection and relay protection

NOTE: Refer to schematic diagram for detailed component connections.

Circuit Description

Detailed explanation of each circuit section and operation principles.

  1. Power Supply: EI transformer with dual secondary windings, bridge rectifier, 4700μF filter capacitors
  2. Phono Stage: Low-noise transistors with RIAA equalization network
  3. Pre-amplifier: High gain stage with tone control circuits
  4. Power Amplifier: Darlington output transistors with current mirror bias
  5. Protection Circuit: Transistor-based DC detection with relay delay

CAUTION! High voltages present in power supply section. Discharge capacitors before servicing.

Disassembly Instructions

Step-by-step procedure for accessing internal components.

WARNING! Label all connections before disassembly for proper reassembly.

Alignment Procedures

Factory adjustment procedures for optimal performance.

AdjustmentProcedureTest Equipment
DC OffsetAdjust VR401/402 for 0mV ±10mV at speaker terminalsDigital multimeter
Bias CurrentAdjust VR403/404 for 20mA across emitter resistorsMillivolt meter
Phono EQVerify RIAA response with audio analyzerAudio generator, analyzer
Input LevelSet input sensitivity with reference signalsOscilloscope

Schematic Diagrams

Complete schematic diagrams for all circuit sections.

  1. Main Amplifier Circuit: Power output stage and driver circuits
  2. Pre-amplifier Section: Input selector, phono stage, tone control
  3. Power Supply: Transformer, rectification, filtering
  4. Protection Circuit: DC detection and relay control
  5. Front Panel: Switches, potentiometers, indicators

Note: All component values and designations shown on schematics.

Parts List

Complete component listing with manufacturer part numbers.

ReferenceDescriptionPart Number
Q401-Q404Output Transistor 2SC2581/2SA1106MAR-2581-1106
C401,C402Filter Capacitor 4700μF/63VMAR-4700-63V
IC101Phono Equalizer ICMAR-PEQ-01
F401,F402Fuse 3.15A Slow BlowMAR-F315
RY401Protection Relay 24VDCMAR-REL-24V

PCB Layouts

Printed circuit board layouts and component placement diagrams.

Tip: Use exact replacement components to maintain original performance.

Troubleshooting Guide

Systematic approach to diagnosing and repairing common issues.

SymptomPossible CauseCorrective Action
No powerBlown fuse, faulty power switchCheck F401/F402, test power switch
One channel deadOutput transistor failure, protection relayTest output transistors, check relay operation
DistortionBias misadjustment, faulty capacitorAdjust bias current, test coupling capacitors
Hum/buzzGround loop, filter capacitor failureCheck ground connections, test filter capacitors
No phono signalPhono stage component failureTest phono equalizer transistors and capacitors

Service Notes

Important service tips and modification information.

Common issues: Output transistors 2SC2581/2SA1106 prone to failure under short circuit conditions. Replace in matched pairs. Power supply capacitors may need replacement after 15+ years of service. Use high-quality audio grade replacements.

Modification: Upgrading filter capacitors to higher values may improve bass response. Use same or higher voltage rating.

Safety Precautions

Essential safety procedures for service technicians.

  1. Always disconnect from AC power before servicing
  2. Discharge large filter capacitors before working on power supply
  3. Use isolation transformer when testing live circuits
  4. Replace safety critical components with exact specifications
  5. Verify proper grounding and insulation after repair

WARNING! Lethal voltages present in power supply section. Exercise extreme caution.

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