Marantz 125 Tuner Service Manual

The Marantz 125 is a high-performance stereo FM tuner featuring advanced circuitry, precise tuning mechanisms, and exceptional audio quality. 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, block diagrams, alignment procedures, troubleshooting, and parts list.

1. Technical Specifications 2. Block Diagrams 3. Circuit Description 4. Alignment Procedures 5. Schematic Diagrams 6. Parts List 7. Troubleshooting Guide 8. Mechanical Assembly 9. Test Points & Voltages 10. Service Notes 11. Warranty Information

Technical Specifications

Key performance characteristics and electrical parameters of the Marantz 125 tuner.

Parameter	Specification
Tuning Range	87.5 - 108 MHz (FM)
Frequency Response	20 Hz - 15 kHz (±1 dB)
Sensitivity	1.8 μV (IHF)
Signal-to-Noise Ratio	70 dB (stereo)
Capture Ratio	1.5 dB
Selectivity	75 dB (alternate channel)
Image Rejection	80 dB
IF Rejection	90 dB
AM Suppression	55 dB
Output Level	750 mV (at 100% modulation)

Block Diagrams

Functional block diagrams showing signal flow and major circuit sections.

RF Amplifier Stage: Front-end tuning and amplification
Mixer/Oscillator: Frequency conversion to 10.7 MHz IF
IF Amplifier: Multi-stage ceramic filter amplification
FM Detector: Ratio detector circuit
Stereo Decoder: Multiplex separation and decoding
Audio Output: Buffer and output stage

WARNING! High voltages present in power supply section. Exercise caution when servicing.

Circuit Description

Detailed explanation of each circuit section and component function.

RF Input Circuit: Variable capacitor tuning with FET amplifier
Local Oscillator: Phase-locked loop synthesizer
IF Strip: Three-stage amplifier with ceramic filters
Detector Circuit: Foster-Seeley discriminator
Stereo Decoder: IC-based matrix decoder
Power Supply: Regulated ±15V DC supply

CAUTION! Use proper grounding techniques when testing live circuits.

Alignment Procedures

FM Front End: Adjust RF coils for maximum sensitivity
IF Alignment: Peak IF transformers at 10.7 MHz
Detector Balance: Adjust for zero DC at center frequency
Stereo Decoder: Set 19 kHz pilot carrier and separation
Meter Calibration: Adjust tuning meter for center indication
Oscillator Tracking: Align across FM band
Required Equipment: FM signal generator, oscilloscope, VTVM
Test Points: TP1-TP8 as indicated on schematic

Schematic Diagrams

Complete schematic diagrams for all circuit boards and power supply.

Main Board: RF/IF and detector circuits

Decoder Board: Stereo multiplex section

Power Supply: Transformer, rectifier, and regulators

Control Board: Tuning mechanism and display circuits

Parts List

Comprehensive list of components with manufacturer part numbers.

Semiconductors: Transistors, diodes, ICs with specifications
Capacitors: Electrolytic, ceramic, film types and values
Resistors: Carbon film and metal oxide values
Inductors: RF coils and IF transformers
Mechanical Parts: Tuning capacitor, switches, knobs

Tip: Use only Marantz-approved replacement parts for authentic performance.

Troubleshooting Guide

Systematic approach to diagnosing and repairing common issues.

Symptom	Possible Cause	Corrective Action
No reception	RF stage failure	Check FET Q1, test RF amplifier voltages
Weak sensitivity	Alignment/component drift	Realign front end, replace weak transistors
No stereo separation	Decoder IC failure	Test IC301 voltages, replace if necessary
Hum in output	Power supply ripple	Check filter capacitors C501-C504
Meter not working	Meter movement	Test meter coil, check driver transistors

Mechanical Assembly

Disassembly and reassembly instructions with exploded views.

Test Points & Voltages

Reference voltages and waveforms at critical test points.

CAUTION! Measure voltages with proper grounding to prevent damage.

Service Notes

Factory service bulletins and common modification procedures.

Warranty Information

Marantz limited warranty terms and service center locations. This manual is intended for qualified service technicians only. Unauthorized modifications may void warranty. For authorized service, contact Marantz service centers. Copyright © Marantz. All specifications subject to change without notice.