Marantz 3250B Service Manual

The Marantz 3250B is a professional stereo preamplifier featuring high-quality audio performance, multiple input/output options, tone controls, and advanced switching capabilities. This service manual provides comprehensive technical information for service technicians, including schematics, alignment procedures, parts lists, and troubleshooting guidelines for proper maintenance and repair.

1. Technical Specifications 2. Block Diagram & System Overview 3. Circuit Description 4. Alignment & Adjustment Procedures 5. Parts List & Replacement Guide 6. Schematic Diagrams 7. PCB Layouts 8. Test Points & Voltage Charts 9. Troubleshooting Guide 10. Service Notes & Modifications

Technical Specifications

Complete electrical and mechanical specifications for the Marantz 3250B preamplifier.

Parameter	Specification
Frequency Response	10Hz - 100kHz ±1dB
Total Harmonic Distortion	<0.05% at 1kHz
Signal-to-Noise Ratio	>90dB (IHF-A weighted)
Input Sensitivity	2.5mV (MM), 150mV (line)
Input Impedance	47kΩ (MM), 100kΩ (line)
Output Voltage	1V RMS (max 8V)
Tone Control Range	±10dB at 100Hz and 10kHz
Power Requirements	120V AC, 60Hz, 15W
Dimensions	420mm W × 110mm H × 300mm D
Weight	5.2kg

Block Diagram & System Overview

Functional block diagram showing signal flow and major circuit sections.

Input Selector: Phono, Tuner, Aux, Tape 1, Tape 2
Phono Equalization: RIAA correction network
Tone Control: Bass, Mid, Treble adjustment circuits
Volume/Balance: Dual concentric controls
Output Stage: Buffer amplifier and main output
Power Supply: Regulated ±15V DC rails
Tape Monitor: Switching and recording circuits

WARNING! Disconnect from AC power before performing any internal inspections or measurements.

Circuit Description

Detailed explanation of each circuit section and component function.

Phono Stage: Low-noise transistor array with RIAA equalization
Line Stage: High-impedance JFET input buffer
Tone Control: Active Baxandall-type circuit with selectable turnover frequencies
Power Supply: Transformer, bridge rectifier, and regulator ICs
Selector Switches: Gold-contact rotary switches for reliable operation
Output Circuit: DC-coupled emitter follower with protection diodes

CAUTION! High voltages present in power supply section - exercise extreme caution.

Alignment & Adjustment Procedures

DC Offset Adjustment: Adjust VR101 for 0V ±10mV at output
Phono Gain: Verify 40dB gain with 1kHz test signal
RIAA Equalization: Check response at 50Hz, 1kHz, 20kHz
Tone Control Center: Adjust for flat response at mid-position
Power Supply Voltages: Verify ±15V DC at regulator outputs
Balance Control: Center for equal channel output

Parts List & Replacement Guide

Complete component listing with manufacturer part numbers and recommended substitutes.

Reference	Description	Original Part	Replacement
Q101-Q104	Transistor 2SC2240	2SC2240BL	KSC1845
Q201-Q204	Transistor 2SA970	2SA970BL	KSA992
IC101, IC201	Op-amp NJM4558	NJM4558D	NJM4558A
D101-D104	Diode 1S2473	1S2473	1N4148
C101, C201	Capacitor 100μF/16V	ECE-A1CGS101	United Chemi-Con
R101, R102	Resistor 47kΩ 1%	RN55D	MF1/4W47K0F

Schematic Diagrams

Complete schematic diagrams for all circuit boards including main amplifier, power supply, and control sections.

Main Schematic: Overall system interconnection
Phono Equalizer: Detailed RIAA network
Tone Control: Active filter circuits
Power Supply: Regulator and protection circuits
Selector Switches: Input/output routing
Control Panel: Front panel switches and indicators

Tip: Use high-resolution prints for accurate component identification.

PCB Layouts

Printed circuit board layouts with component placement and trace routing.

Main Board: Component side and solder side views
Power Supply Board: Transformer mounting and regulator section
Control Board: Switch and potentiometer connections
Phono Board: Low-level signal routing with ground plane
Interconnection: Cable routing and connector positions

WARNING! Observe proper grounding techniques when working on PCB assemblies.

Test Points & Voltage Charts

Designated test points and expected voltage measurements under normal operating conditions.

Test Point	Expected Voltage	Tolerance
TP1 (+15V)	+15.0V DC	±0.5V
TP2 (-15V)	-15.0V DC	±0.5V
TP3 (Phono Out)	0V DC	±50mV
TP4 (Line Out)	0V DC	±50mV
TP5 (Ref Ground)	0V	-
TP6 (Regulator In)	±18V DC	±1V

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
No output	Power supply failure	Check fuse F101; measure ±15V rails
Distorted sound	DC offset incorrect	Adjust VR101/VR201 for 0V output
Hum in phono	Grounding issue	Check phono ground wire; verify shield connections
Intermittent operation	Oxidized contacts	Clean selector switches with contact cleaner
One channel dead	Transistor failure	Check Q101-Q104 or Q201-Q204 for shorts/open
Noise in controls	Dirty potentiometers	Clean volume/tone controls with DeoxIT

Reset: None required - no microprocessor controls.

Service: Authorized Marantz service centers only for warranty work.

Service Notes & Modifications

Factory service bulletins and recommended performance enhancements.

Service Bulletin 3250B-1: Updated capacitor C207 to 220μF for improved bass response

Modification M1: Op-amp upgrade to NJM2068 for lower noise

Modification M2:Added power supply bypass capacitors for high-frequency stability