Bose 901 Equalizer Service Manual

The Bose 901 Active Equalizer is a critical component designed specifically for use with the Bose 901 Series VI Direct/Reflecting® speaker system. This manual provides detailed service information, including technical specifications, circuit descriptions, alignment procedures, troubleshooting guides, and parts lists for qualified service technicians. Proper use of this equalizer is essential to achieve the intended acoustic performance of the 901 speaker system.

1. Introduction & Safety 2. Technical Specifications 3. Front & Rear Panel Overview 4. Internal Circuit Description 5. Test & Alignment Procedures 6. Block Diagram & Schematics 7. Troubleshooting Guide 8. Parts List & Exploded View 9. Replacement Procedures 10. Performance Verification

Introduction & Safety

The Bose 901 Active Equalizer corrects the acoustic response of the 901 speakers, providing the necessary equalization for flat overall system response. It includes level controls, contour switches, and is integral to the system's patented design.

Warning	Description
High Voltage	Internal components may contain hazardous voltages even when unplugged. Discharge power supply capacitors before servicing.
Qualified Personnel	This manual is for use by qualified service technicians only.
ESD Precautions	Use anti-static precautions when handling circuit boards and semiconductors.
Original Parts	Use only Bose-specified replacement parts to maintain performance and safety.

Technical Specifications

Model: Bose 901 Series VI Active Equalizer.
Input Impedance: 47 kΩ (unbalanced).
Output Impedance: 1 kΩ (maximum).
Frequency Response: 20 Hz - 20 kHz (±0.5 dB with equalization engaged).
Total Harmonic Distortion: < 0.1% (20 Hz - 20 kHz at 1V output).
Signal-to-Noise Ratio: > 90 dB (A-weighted).
Power Requirements: 120V AC, 60Hz, 10W.
Controls: Level (L/R), Contour (On/Off), Bypass Switch.
Connectors: RCA input/output jacks, AC power inlet.

Front & Rear Panel Overview

Front Panel: LEVEL control (dual concentric for left/right channels), CONTOUR switch (Engage/Defeat), BYPASS switch (Equalizer In/Out), POWER indicator (LED).

Rear Panel: INPUT L/R (RCA jacks), OUTPUT L/R (RCA jacks), AC POWER socket, FUSE holder (2A, 250V).

CAUTION! Always connect the equalizer between the preamplifier tape monitor output and power amplifier input. Incorrect connection can cause oscillation or damage.

Internal Circuit Description

The equalizer uses active filter networks based on operational amplifiers (Op-Amps). The circuit provides substantial boost at low and high frequencies to compensate for the 901 speaker's natural response.

Input Buffer: High-impedance non-inverting amplifier stage.
Bass Equalization: Shelving filter providing up to +12 dB boost below 100 Hz.
Treble Equalization: Shelving filter providing up to +9 dB boost above 2 kHz.
Output Stage: Low-impedance driver amplifier.
Power Supply: Linear regulated ±15V DC supply for Op-Amps.

Test & Alignment Procedures

Required Equipment: Audio oscillator (20 Hz-20 kHz), True RMS voltmeter, Oscilloscope, Distortion analyzer, 600-ohm load resistors.

Power Supply Check: Measure ±15V DC at test points TP1 (+) and TP2 (-). Tolerance ±0.5V.
Gain Calibration: Apply 1 kHz, 0.5V signal. Adjust R101 (Left) and R201 (Right) for 0 dB gain (0.5V output) in BYPASS mode.
Frequency Response Verification: With CONTOUR ON, verify boost per specification sheet at 50 Hz and 10 kHz.
Distortion Check: At 1 kHz, 1V output, verify THD < 0.1%.

Block Diagram & Schematics

Block Diagram: Shows signal flow from Input > Buffer > Bass EQ > Treble EQ > Output Stage.

Main Schematic: Includes all active components (Op-Amps IC101-IC104), resistors, capacitors, and switches. Reference designators: R for resistors, C for capacitors, IC for integrated circuits, SW for switches.

Power Supply Schematic: Transformer T1, Bridge Rectifier BR1, Voltage Regulators U1 (+15V) and U2 (-15V), filter capacitors.

Note: Refer to separate fold-out schematic diagram included with this manual.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
No output, power LED off	Blown fuse, faulty power cord, transformer failure	Check/Replace F1 (2A). Verify AC input to transformer primary. Check DC outputs.
Distorted sound in one channel	Faulty Op-Amp, leaky capacitor in signal path	Check IC101 (Left) or IC102 (Right) for proper DC voltages at pins. Replace suspect IC.
Insufficient bass or treble boost	Contour switch failure, open capacitor in EQ network	Verify switch continuity. Check capacitors C105, C106 (Bass) and C115, C116 (Treble).
Hum or buzz	Ground loop, failed filter capacitor in power supply	Check C501, C502 (2200µF). Ensure chassis ground connection is secure.
Bypass function not working	Faulty bypass switch or relay	Check SW3 (Bypass switch) for continuity. Check relay K1 operation.

Parts List & Exploded View

Key Components:

IC101, IC102: TL072CP Dual Op-Amp (Signal Path).
IC103, IC104: TL072CP Dual Op-Amp (EQ Networks).
U1: 7815 +15V Voltage Regulator.
U2: 7915 -15V Voltage Regulator.
T1: Power Transformer, Primary 120V, Secondary 24V CT.
C501, C502: 2200µF, 25V (Power Supply Filter).
F1: Fuse, 2A, 250V, Slow-Blow.

Exploded View: Shows assembly order of chassis, front/rear panels, circuit board, and transformer.

Replacement Procedures

Circuit Board Removal: Disconnect AC power. Remove top cover. Disconnect all wiring harnesses noting positions. Remove four mounting screws to free main PCB.
Op-Amp Replacement: Desolder old IC using appropriate tools. Insert new IC observing correct orientation (notch/pin 1). Solder and trim leads.
Power Supply Capacitors: Discharge capacitors before handling. Note polarity when installing new electrolytic capacitors.
Control Potentiometers: Unsolder from rear of PCB. Replace with identical part number (Bose 123456).

WARNING! Always re-verify calibration after component replacement.

Performance Verification

After service, perform full functional test:

Connect equalizer into a test system with 901 speakers or equivalent resistive load.
Cycle all switches and controls, checking for noise or intermittent operation.
Verify correct operation of Bypass switch (flat response) and Contour switch (boosted response).
Listen for any distortion or channel imbalance across the frequency range.
Complete and attach service tag with date, technician initials, and tests performed.

Service Support: For technical assistance or genuine Bose parts, contact Bose Corporation Professional Systems Division.