Marantz Model 27 Service Manual

The Marantz Model 27 is a classic stereo receiver featuring high-quality audio performance, robust construction, and advanced circuitry for its era. This service manual covers technical specifications, circuit diagrams, alignment procedures, troubleshooting guides, and maintenance instructions for qualified service technicians. Below are key sections for technical overview, schematics, alignment, troubleshooting, and parts list.

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

Technical Specifications

Key performance characteristics and electrical parameters of the Marantz Model 27 stereo receiver.

Parameter	Specification
Power Output	30 Watts per channel (RMS, 8Ω)
Frequency Response	20Hz - 20kHz ±1dB
Total Harmonic Distortion	<0.3% at rated output
Input Sensitivity	Phono: 2.5mV; Aux: 150mV
Signal-to-Noise Ratio	Phono: 70dB; Aux: 85dB
Damping Factor	>40 at 8Ω
Tuner Section	AM/FM with multiplex stereo
Power Requirements	120V AC, 60Hz, 180W
Dimensions	17" W × 5" H × 13" D
Weight	22 lbs

Circuit Description

Detailed explanation of the electronic circuits and functional blocks.

Preamplifier Stage: High-gain phono and line-level amplification circuits
Power Amplifier: Complementary symmetry output stage with protection circuitry
Tuner Section: FM front end, IF stages, and stereo decoder
Power Supply: Regulated DC supplies for various stages
Protection Circuits: Overload and thermal protection systems

WARNING! High voltages present - exercise extreme caution when servicing live circuits.

Alignment Procedures

Step-by-step instructions for proper alignment and calibration.

FM Tuner Alignment: Using standard test equipment and signal generator
AM Tuner Alignment: Antenna and oscillator coil adjustments
Stereo Decoder Alignment: 19kHz pilot carrier and separation adjustment
Preamplifier Calibration: Gain and RIAA equalization settings
Power Amplifier Bias: DC offset and idle current adjustment

CAUTION! Use proper test equipment and follow procedures exactly.

Block Diagrams

Overall System: Complete signal flow from input to output
Preamplifier Section: Phono and auxiliary input processing
Power Amplifier: Voltage amplification and output stages
FM Tuner: RF amplification, mixing, and detection
Power Supply: Transformer, rectification, and regulation
Protection Circuits: Overcurrent and thermal protection
Control Systems: Volume, balance, tone controls

Schematic Diagrams

Complete electronic schematics for all circuit boards and subsystems.

Main Amplifier: Power output stage with driver circuits and protection.

Preamplifier: Phono equalization, tone controls, and input switching.

FM Tuner: RF amplifier, local oscillator, IF stages, and detector.

Power Supply: Transformer, bridge rectifier, and filter capacitors.

Control Circuits: Selector switches, volume, balance, and tone controls.

Parts List

Complete component listing with manufacturer part numbers.

Semiconductors: Transistors, diodes, integrated circuits
Passive Components: Resistors, capacitors, inductors
Electromechanical: Switches, potentiometers, relays
Hardware: Screws, brackets, connectors
Special Parts: Tuning capacitors, transformers, meters

Tip: Use original Marantz parts for best performance and reliability.

Troubleshooting Guide

Systematic approach to diagnosing and repairing common issues.

No Power: Check AC cord, fuse, power switch, and transformer
No Audio: Verify speaker connections, protection relay, output transistors
Distorted Sound: Test output stages, check bias current, examine capacitors
Weak FM Reception: Inspect antenna connection, align RF stages
Hum/Buzz: Check ground connections, filter capacitors, shielding
Intermittent Operation: Examine solder joints, connectors, switches
Overheating: Verify bias settings, check for shorted outputs
Meter Issues: Calibrate meter movement, check driver circuits

WARNING! Always disconnect power before replacing components.

Maintenance Instructions

Routine maintenance procedures for optimal performance.

Cleaning: Use compressed air for dust removal; contact cleaner for controls.

Lubrication: Apply appropriate lubricants to tuning mechanism and switches.

Safety Precautions

Essential safety guidelines for service technicians.

Always use isolation transformer when servicing live circuits. Discharge large capacitors before handling. Use proper grounding techniques. Wear safety glasses when soldering or cutting.

Service Notes

Problem Area	Common Issues	Recommended Solutions
Power Supply	Failed rectifiers, bulging capacitors	Replace with specified voltage/current ratings
Output Stage	Shorted transistors, blown fuses	Check driver stages; replace complementary pairs
Preamplifier	Noisy transistors, leaky capacitors	Replace with low-noise equivalents
FM Tuner	Poor sensitivity, stereo issues	Realign IF transformers; check multiplex decoder
Controls	Scratchy pots, dirty switches	Clean with contact cleaner; replace if worn

Calibration: Use specified test points and procedures for accurate results.

Service Support: Refer to Marantz technical bulletins for updated information.