Marantz 115B Service Manual

The Marantz 115B is a high-performance stereo integrated amplifier featuring 70 watts per channel RMS, advanced phono preamplifier, tone controls, loudness compensation, and comprehensive input/output connectivity. This service manual provides detailed technical information, schematics, alignment procedures, and troubleshooting guidance for qualified service technicians. Below are key sections covering specifications, circuit descriptions, alignment procedures, and service information.

1. Technical Specifications 2. Circuit Description 3. Block Diagrams 4. Alignment Procedures 5. Parts List 6. Schematic Diagrams 7. PCB Layouts 8. Mechanical Assembly 9. Troubleshooting Guide 10. Service Notes

Technical Specifications

Complete electrical and mechanical specifications for the Marantz 115B integrated amplifier.

ParameterSpecification
Power Output70W RMS per channel (8Ω, 20Hz-20kHz)
Total Harmonic Distortion≤0.05% at rated output
Frequency Response10Hz-50kHz (+0/-1dB)
Input SensitivityPhono: 2.5mV; Aux: 150mV
Signal-to-Noise RatioPhono: 75dB; Aux: 90dB
Tone ControlsBass: ±10dB at 100Hz; Treble: ±10dB at 10kHz
Power Requirements120V AC, 60Hz, 450W maximum
Dimensions17" W × 5" H × 14" D
Weight28 lbs

Circuit Description

Detailed analysis of the Marantz 115B circuit topology and design principles.

  1. Preamplifier Section: Dual-stage phono equalization with RIAA accuracy within ±0.5dB
  2. Power Amplifier: Complementary symmetry output stage with thermal tracking
  3. Power Supply: Heavy-duty toroidal transformer with dual secondary windings
  4. Protection Circuits: DC offset protection and thermal shutdown
  5. Tone Control: Baxandall-type bass and treble controls with turnover frequencies

WARNING! High voltages present - qualified technicians only. Always discharge power supply capacitors before servicing.

Block Diagrams

System-level block diagrams showing signal flow and power distribution.

Alignment Procedures

Step-by-step calibration and alignment instructions for optimal performance.

  1. DC Offset Adjustment: Adjust VR401/VR402 for 0V ±10mV at speaker terminals
  2. Bias Current Adjustment: Set idle current to 20mA using test points TP1/TP2
  3. Phono Equalization: Verify RIAA response using 1kHz reference signal
  4. Input Level Matching: Balance sensitivity across all input sources
  5. Protection Circuit Test: Verify proper operation of DC offset and thermal protection

CAUTION! Use calibrated test equipment. Follow procedures exactly as specified.

Parts List

Complete bill of materials with manufacturer part numbers and substitutions.

ReferenceDescriptionPart NumberSubstitution
Q101-Q104Input Differential Transistors2SA872KSA992
Q105-Q108Voltage Amplification2SC1775KSC1845
Q109-Q112Driver Transistors2SA968/2SC2238MJE15032/33
Q113-Q116Output Transistors2SA1106/2SC2581MJ21193/94
C401-C404Power Supply Capacitors10,000μF/63VSame value
IC101Phono EqualizationNJM2068NJM2068D

Schematic Diagrams

Complete schematic diagrams for all circuit sections.

  1. Main Amplifier Schematic: Complete power amplifier circuit
  2. Preamplifier Schematic: Phono and line-level input circuits
  3. Power Supply Schematic: Rectification and regulation circuits
  4. Control Section Schematic: Input selection and tone control circuits
  5. Protection Circuit Schematic: Speaker protection and relay control

Note: Schematics show component values, test points, and voltage references.

PCB Layouts

Printed circuit board layouts with component placement and trace routing.

Tip: Use layouts for component identification and troubleshooting.

Mechanical Assembly

Exploded views and assembly instructions for chassis and mechanical components.

  1. Chassis Assembly: Main frame and heat sink mounting
  2. Front Panel Assembly: Control knobs, switches, and indicators
  3. Rear Panel Assembly: Input/output connectors and binding posts
  4. Internal Assembly: PCB mounting and wiring harness routing
  5. Heat Sink Installation: Thermal compound application and transistor mounting

WARNING! Ensure proper thermal contact between output transistors and heat sink.

Troubleshooting Guide

Systematic approach to diagnosing and repairing common issues.

SymptomPossible CauseCorrective Action
No outputProtection relay, blown fusesCheck speaker relay, measure output DC offset, replace fuses
Distorted soundBias current, damaged transistorsAdjust bias current, check output transistors, verify supply voltages
Hum/buzzGround loops, filter capacitorsCheck ground connections, measure power supply ripple, replace filter caps
Intermittent operationCold solder joints, failing componentsInspect PCB for cracked solder, test transistors under load
One channel deadChannel-specific componentsCompare voltages between channels, check input switching

Service Notes

Important service information and technical updates.

Factory Updates: Revised bias adjustment procedure for improved thermal stability. Updated output transistor mounting torque specification to 6-8 inch-pounds.

Common Failures: Output transistors (2SA1106/2SC2581) prone to failure under short-circuit conditions. Power supply capacitors may require replacement after 15+ years of service.

Test Equipment Requirements: Audio oscillator, distortion analyzer, dual-trace oscilloscope, digital multimeter, variac.

Service Contacts: Marantz Technical Support: 1-800-MARANTZ. Replacement parts: authorized Marantz service centers only.

Marantz 115B Service Manual © - All Rights Reserved 2025