Bose 901 Repair Manual

The Bose 901 Direct/Reflecting® Speaker System is a high-fidelity loudspeaker known for its unique acoustic design utilizing nine full-range drivers and an active equalizer. This manual provides comprehensive repair, maintenance, and troubleshooting guidance for the Bose 901 Series I through VI speakers, covering driver replacement, crossover network service, equalizer diagnostics, cabinet repair, and system alignment. Below are key sections for specifications, disassembly, component testing, repair procedures, and calibration.

System Overview & Specifications

Key components: Nine 4.5-inch full-range drivers (eight facing rear, one forward), passive crossover network, proprietary active equalizer, tuned port cabinet.

Component	Specification
Driver Type	4.5" full-range, paper cone, rubber surround
Driver Configuration	9 drivers per cabinet (1 front, 8 rear)
Impedance	8 ohms nominal
Power Handling	Series I-III: 75W RMS; Series IV-VI: 100W RMS
Crossover Frequency	Passive network: 500 Hz (varies by series)
Equalizer	Active, required for frequency response correction
Cabinet Material	Veneered particle board
Dimensions	Approx. 25" H x 13.5" W x 12.5" D
Weight	Approx. 45 lbs per speaker
Frequency Response	With equalizer: 35 Hz - 20 kHz (±3 dB)

Safety Precautions

Disconnect all power sources before servicing. Handle drivers and components with care.

1. Disconnect Speakers: Unplug speaker wires from amplifier and equalizer.
2. Disconnect Equalizer: Unplug AC power and audio cables from active equalizer.
3. Static Discharge: Use anti-static wrist strap when handling electronic components.
4. Safe Workspace: Use clean, well-lit area with adequate ventilation.

WARNING! High voltages may be present in connected amplifiers. Ensure all equipment is powered off and unplugged. The active equalizer contains internal capacitors that can hold a charge.

Tools & Test Equipment Required

Gather necessary tools before beginning repair.

1. Phillips screwdrivers (#1, #2).
2. Flat-head screwdriver.
3. Needle-nose pliers.
4. Wire cutters/strippers.
5. Multimeter (for continuity and resistance checks).
6. Audio signal generator (optional, for driver testing).
7. AC voltmeter (for equalizer testing).
8. Soldering iron and solder (rosin core).
9. Speaker sealant (non-hardening type).
10. Cloth for handling drivers.

CAUTION! Use proper tools to avoid damage to drivers, terminals, and cabinet.

Cabinet Disassembly

• Step 1: Remove grille cloth. Most models have Velcro or plastic retainers.
• Step 2: Locate and remove rear panel screws (typically 8-10 Phillips screws).
• Step 3: Carefully separate rear panel from cabinet. Note any sealant or gasket material.
• Step 4: Identify driver wiring. Label or photograph connections before disconnecting.
• Step 5: Remove faulty driver(s) by unscrewing mounting screws (usually 4 per driver).
• Step 6: Disconnect speaker wires from driver terminals carefully.

Driver Identification & Testing

Each driver must be tested for continuity and voice coil rub.

Visual Inspection: Check for torn surrounds, damaged cones, or burnt voice coils.

Multimeter Test: Set to ohms (Ω) setting. Measure resistance across driver terminals. Nominal reading should be approximately 6-7 ohms for an 8-ohm driver. An open circuit (OL) indicates a broken voice coil. A very low reading indicates a short.

Driver Replacement: Use only genuine Bose 901 replacement drivers for your specific series. Mixing series may alter sound characteristics.

Crossover Network Service

The passive crossover is mounted inside the cabinet. Series I-III use a simpler network than IV-VI.

1. Locate crossover board (typically near input terminals).
2. Inspect for burnt components, cracked solder joints, or swollen capacitors.
3. Test inductors for continuity (should read near 0 ohms).
4. Test capacitors with a multimeter capable of measuring capacitance, or replace if suspected faulty.
5. Resolder any dry or cracked joints using rosin core solder.

Tip: Refer to the specific series schematic for component values. Do not substitute values arbitrarily.

Active Equalizer Diagnostics

The equalizer is critical for proper frequency response. Common issues: no output, distorted output, hum.

1. Power Check: Ensure equalizer is receiving AC power and power LED is illuminated.
2. Connection Check: Verify all audio cables are securely connected to Tape Monitor loop or Pre-Out/Main-In.
3. Internal Fuse: Some models have an internal fuse. Check and replace if blown with same rating.
4. Capacitor Replacement: Old electrolytic capacitors can dry out, causing hum or loss of bass. Consider recapping.
5. Potentiometer Cleaning: Scratchy controls can be cleaned with contact cleaner.

WARNING! Internal repair of the equalizer requires advanced electronics knowledge. If unsure, seek professional service.

Reassembly & Sealing

Proper sealing is essential for acoustic integrity.

1. Install new or repaired driver(s). Ensure gasket or sealant is in place.
2. Reconnect wiring according to your labels or photos.
3. Apply a fresh bead of non-hardening speaker sealant (like Mortite) around the rear panel mating surface.
4. Carefully align and press rear panel into place.
5. Reinstall and tighten all screws evenly in a cross pattern.
6. Reattach grille cloth.

CAUTION! Do not overtighten driver or panel screws, as this can warp the frame or cabinet.

System Calibration & Testing

After repair, the system must be calibrated with the equalizer.

Step	Procedure
1. Reconnect	Connect speakers to equalizer, equalizer to amplifier per manual.
2. Set Controls	Set equalizer slide controls to flat (center) position.
3. Play Test Tone	Use a pink noise or test tone source.
4. Adjust Equalizer	Following the original Bose calibration guide, adjust slides for flat response in your room (requires test microphone for accurate results).
5. Listen Test	Play familiar music to check for balance, distortion, or rattles.

Reset: If equalizer has been serviced, ensure all internal trim pots are set to factory default.

Common Faults & Solutions

Symptom	Possible Cause	Corrective Action
No sound from one cabinet	Blown driver, broken wire, faulty crossover	Test each driver for continuity; inspect internal wiring; check crossover components.
Distorted sound	Voice coil rub, damaged cone, overdriven amplifier	Replace damaged driver(s); ensure amplifier power is within speaker rating.
Weak or no bass	Failed equalizer, incorrect hookup, port blockage	Bypass equalizer to test; verify Tape Monitor loop is engaged; check cabinet port for obstructions.
Rattling noise	Loose driver, internal wire touching cone, broken brace	Secure all drivers and internal components; ensure no wires are loose.
Intermittent sound	Loose terminal connection, failing solder joint	Check and tighten all input terminals and internal solder points.

Service: For genuine Bose parts and advanced technical support, contact Bose Corporation or authorized service centers.

Parts List & Ordering

Genuine replacement parts are recommended for optimal performance.

• Driver, 4.5" Full-Range: Specify Series (I/II/III or IV/V/VI).
• Crossover Network Assembly: Complete board or individual components (capacitors, inductors).
• Equalizer Repair Kit: Capacitor kit, potentiometer, fuse.
• Grille Cloth: Original replacement fabric.
• Speaker Terminal Cup: Input terminal assembly.
• Sealant: Non-hardening acoustic sealant.
• Hardware: Mounting screws, gaskets.

Note: Part numbers vary by series. Have your speaker serial number ready when ordering.

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