Marantz Model 18 Service Manual

The Marantz Model 18 is a classic stereo receiver from the golden age of hi-fi, featuring vacuum tube amplification, FM/AM tuner capabilities, and the renowned Marantz sound quality. This service manual covers technical specifications, circuit diagrams, alignment procedures, troubleshooting, and maintenance guidelines for qualified service technicians.

1. Technical Specifications 2. Circuit Description 3. Alignment Procedures 4. Tube Complement 5. Voltage Measurements 6. Schematic Diagrams 7. Parts List 8. Troubleshooting Guide 9. Safety Precautions 10. Service Notes

Technical Specifications

Complete electrical and performance specifications for the Marantz Model 18 stereo receiver.

Parameter	Specification
Power Output	35 watts per channel RMS
Frequency Response	20Hz - 20kHz ±1dB
Total Harmonic Distortion	<0.5% at rated output
Input Sensitivity	Phono: 2.5mV, Aux: 250mV
Signal-to-Noise Ratio	Phono: 65dB, Aux: 75dB
Tuning Range	FM: 88-108MHz, AM: 535-1605kHz
Tube Complement	12 tubes including output tubes
Power Requirements	117V AC, 60Hz, 200 watts
Dimensions	16" W × 5" H × 14" D
Weight	35 lbs

Circuit Description

Detailed explanation of the Model 18's circuit topology and signal path.

Preamplifier Stage: Dual triode tubes for phono and line amplification
Tone Control: Bass and treble controls with passive networks
Driver Stage: Phase inverter and driver tubes for push-pull operation
Power Amplifier: Push-pull output stage with beam power tetrodes
Power Supply: Heavy-duty transformer with tube rectification
Tuner Section: Separate FM and AM tuner circuits

WARNING! High voltages present - lethal shock hazard. Only qualified technicians should perform service.

Alignment Procedures

Step-by-step alignment instructions for FM and AM tuner sections.

FM IF Alignment: Use sweep generator at 10.7MHz
FM Front End: Align RF and oscillator circuits
AM Alignment: IF at 455kHz, antenna and oscillator tuning
Stereo Decoder: MPX alignment for proper stereo separation
Output Balance: Adjust for equal channel output

CAUTION! Use proper alignment tools and follow procedures exactly.

Tube Complement

Output Tubes: 4 × 7591 - Beam Power Tetrode
Driver Tubes: 2 × 12AX7 - Dual Triode
Phase Inverter: 1 × 12AU7 - Dual Triode
FM Tuner: 1 × 6BH6, 1 × 6AU6, 1 × 6BA6
AM Tuner: 1 × 6BE6 - Converter
Rectifier: 1 × 5U4GB - Full Wave Rectifier

Voltage Measurements

Reference voltage measurements at key test points with line voltage at 117V AC.

Power Supply: B+ voltage: 425V DC, Heater voltage: 6.3V AC

Output Tubes: Plate voltage: 410V, Screen voltage: 385V, Bias voltage: -18V

Preamplifier: Plate voltages: 180-220V, Cathode voltages: 1.2-2.5V

Schematic Diagrams

Complete schematic diagrams included for:

Main Amplifier Circuit
Preamplifier and Tone Control
FM Tuner Section
AM Tuner Section
Power Supply
Control Switches and Inputs

Note: Schematics show component values and test points.

Parts List

Complete listing of all components with manufacturer part numbers.

Component	Value	Part Number
Resistors	Various values 1/2W	Marantz R-series
Capacitors	Electrolytic and film	Marantz C-series
Tubes	See tube complement	Original Marantz
Transformers	Power and output	Marantz T-series
Switches	Rotary and pushbutton	Marantz S-series

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
No sound	Blown fuse, dead tubes	Check fuse and tubes, measure B+ voltage
Distortion	Bias incorrect, weak tubes	Check bias voltage, test output tubes
Hum	Filter capacitors, ground issues	Replace filter caps, check grounds
No FM reception	RF stage, oscillator failure	Check FM tubes, alignment
Intermittent operation	Loose connections, failing components	Resolder connections, check for heat damage

Service Tip: Always start with visual inspection and voltage measurements.

Safety Precautions

Essential safety procedures for working on tube equipment.

Always disconnect power before working on unit
Discharge filter capacitors before servicing
Use isolation transformer when testing
Work with one hand behind back when powered
Replace line cord if damaged

DANGER! Lethal voltages present - extreme caution required.

Service Notes

Additional technical notes and common modifications.

Bias Adjustment: Use specified test points and procedure
Capacitor Replacement: Modern replacements may improve performance
Tube Testing: Use quality tube tester for accurate results
Alignment Tools: Required for proper tuner service
Documentation: Record all measurements and replacements

For technical support contact Marantz service department.