Marantz 27 Service Manual

The Marantz 27 is a high-performance stereo receiver featuring advanced audio circuitry, multiple input/output options, and professional-grade tuning capabilities. This service manual provides comprehensive technical documentation for service technicians, including schematics, alignment procedures, troubleshooting guides, and replacement part information for proper maintenance and repair of the unit.

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

Technical Specifications

Complete electrical and mechanical specifications for the Marantz 27 stereo receiver.

Parameter	Specification
Power Output	70 Watts per channel (8Ω, 20Hz-20kHz)
Total Harmonic Distortion	<0.05% at rated output
Frequency Response	10Hz - 50kHz (+0, -3dB)
Input Sensitivity	Phono: 2.5mV; Aux: 150mV
Signal-to-Noise Ratio	Phono: 75dB; Aux: 90dB
Damping Factor	>50 at 8Ω
Tuner Section	FM/AM with quartz PLL synthesis
Dimensions	440mm W × 130mm H × 330mm D
Weight	9.5 kg
Power Requirements	120V AC, 60Hz, 250W

Block Diagram Overview

System-level block diagram showing signal flow and major functional blocks.

Input Selector: Phono, Aux, Tuner, Tape inputs
Preamplifier Stage: RIAA equalization for phono
Tone Control: Bass, Treble, and Balance controls
Power Amplifier: Complementary symmetry output stage
Protection Circuitry: DC detection and thermal protection
Power Supply: Regulated ±42V rails
Tuner Section: FM/AM demodulation and IF processing

NOTE: Refer to schematic diagrams for detailed circuit analysis.

Circuit Description

Detailed analysis of major circuit sections and their operation.

Power Supply: Transformer T001, bridge rectifier D001-D004, filtering capacitors C001-C004
Amplifier Section: Differential input stage (Q101-Q102), voltage amplifier (Q103), driver stage (Q104-Q105), output transistors (Q106-Q109)
Protection Circuit: DC detection (IC201), relay driver (Q201-Q202), thermal sensor (TH201)
Tuner Circuit: FM front end (IC301), IF amplification (IC302), multiplex decoder (IC303)
Control Section: Input selector (SW101), tone controls (IC401), volume/balance (VR101-VR102)

CAUTION! High voltage present in power supply section - discharge capacitors before servicing.

Alignment Procedures

Step-by-step alignment and adjustment procedures for optimal performance.

FM Tuner Alignment: Adjust L301 for 10.7MHz IF; set L302 for proper stereo separation
AM Tuner Alignment: Adjust L401 for 455kHz IF; set antenna trimmer for maximum sensitivity
Power Amplifier Bias: Adjust VR101 for 20mA idle current across R115
DC Offset: Adjust VR102 for 0V DC at speaker outputs
Phono Equalization: Verify RIAA curve accuracy using test equipment

Disassembly Instructions

Proper disassembly sequence to access internal components.

Remove top cover: 6 Phillips screws on rear panel
Remove front panel: 4 screws along top edge, disconnect control knobs
Remove main chassis: 8 screws securing chassis to bottom plate
Access power supply: Remove 4 screws holding transformer bracket
Remove amplifier board: Desolder 12 connections and remove 6 board mounting screws

WARNING! Always disconnect power and discharge capacitors before disassembly.

Replacement Parts List

Original manufacturer part numbers and recommended substitutes.

Part Number	Description	Substitute
2SC2240	Small signal transistor	KSC1845
2SA970	PNP small signal transistor	KSA992
2SC2581	Output transistor	MJ21194
2SA1106	Output transistor	MJ21193
IC301	FM Tuner IC	LA1260
IC401	Tone Control IC	NJM2068
C001-C004	6800μF/63V capacitor	United Chemi-Con
F001	3A slow-blow fuse	Littelfuse 3AG

Schematic Diagrams

Complete schematic diagrams for all circuit sections.

Main Amplifier Circuit: Pages 8-12
Power Supply: Page 13
Preamplifier Section: Pages 14-16
FM Tuner: Pages 17-20
AM Tuner: Pages 21-22
Control Circuits: Pages 23-25
PCB Layout: Pages 26-28

NOTE: All schematics use standard electronic symbols per IEC standards.

Voltage Measurements

Reference voltage measurements at key test points.

Test Point	Voltage (DC)	Conditions
TP1 (+B)	+42V ±1V	No signal, volume minimum
TP2 (-B)	-42V ±1V	No signal, volume minimum
TP3 (Vcc)	+15V ±0.5V	All modes
TP4 (Speaker Out)	0V ±50mV	No signal
TP5 (Phono Out)	0V ±10mV	No signal
TP6 (Tuner B+)	+12V ±0.3V	Tuner selected

Troubleshooting Guide

Systematic approach to diagnosing and repairing common issues.

Symptom	Possible Cause	Corrective Action
No power	Blown fuse, power switch, transformer	Check F001, test power switch, measure transformer primary
Distorted audio	Bias current, output transistors, power supply	Adjust VR101, test Q106-Q109, check ±42V rails
No FM reception	Antenna, tuner IC, power to tuner	Check antenna connection, test IC301 pin voltages
Hum in audio	Ground loops, filter capacitors, shielding	Check ground connections, test C001-C004, verify cable routing
Protection relay trips	DC offset, short circuit, thermal overload	Measure DC at outputs, check speaker wiring, test TH201

Safety Precautions

Essential safety procedures for service technicians.

Always disconnect AC power before servicing
Use isolation transformer when testing live circuits
Discharge large capacitors before handling
Wear safety glasses when soldering or cutting
Ensure proper grounding of test equipment
Follow electrostatic discharge (ESD) precautions for semiconductors

WARNING! Lethal voltages present - experienced technicians only.

Service Notes

Important notes and observations from field service experience.

Common failure: Output transistors 2SC2581/2SA1106 due to aging
Electrolytic capacitors C001-C004 should be replaced after 15 years
Volume control VR101 often develops noise - clean with contact cleaner
Thermal compound between output transistors and heatsink degrades over time
Original fuses must be replaced with identical specifications
Factory service bulletins: Check for updates at authorized service centers

Service Support: Authorized Marantz service centers only. Unauthorized modifications void warranty.