The Bose 1800 Power Amplifier is a professional-grade audio amplifier designed for high-fidelity sound reinforcement. This service manual provides detailed technical information for qualified service personnel, including safety precautions, specifications, block diagrams, circuit descriptions, adjustment procedures, parts lists, and troubleshooting guides. The amplifier features robust power output, comprehensive protection circuits, and modular design for serviceability.
This equipment contains hazardous voltages and components that can retain a charge even after disconnection from mains power. Service should only be performed by qualified personnel familiar with audio amplifier circuitry and safety procedures.
| Warning | Description |
|---|---|
| Lethal Voltage | High voltages are present on the power supply and output stages. Use an insulated tool set. |
| Capacitor Discharge | Large filter capacitors can hold a charge. Always discharge them using a suitable resistor before servicing. |
| Heat Sinks | Heat sinks and power transistors may become extremely hot during operation. Allow to cool before handling. |
| Fuse Replacement | Replace fuses only with the specified type and rating. Never bypass fuses. |
| Grounding | Ensure the chassis is properly grounded via the 3-prong AC power cord. Do not defeat the ground. |
The Bose 1800 is a high-power, low-distortion stereo amplifier built for professional touring and installed sound applications.
WARNING! This manual is for service purposes only. Incorrect adjustments can damage the amplifier and connected speakers.
The amplifier consists of several key modules: Input Buffer/Attenuator, Voltage Gain Stage, Driver Stage, Complementary Symmetry Output Stage, Switch-Mode Power Supply, and Protection/Monitoring Logic.
Signal Path: Balanced Input -> Input Attenuator -> Differential Input Stage -> Voltage Amplifier -> Driver Transistors -> Output Transistors (Multiple pairs) -> Output Inductor -> Speaker Terminals.
Protection Path: Output DC Sensor -> Current Limit Sensors -> Thermal Sensor -> Logic Controller -> Mute/Disable Relays.
Input Stage: Utilizes a low-noise op-amp (e.g., NE5532) configured as a balanced differential amplifier with gain set by feedback resistors. Includes RF filtering.
Voltage Gain Stage: Discrete Class-A amplifier providing the majority of voltage gain. Biased for linear operation.
Driver & Output Stage: Complementary Darlington configuration using multiple MJ15024/MJ15025 (or equivalent) transistor pairs in parallel for high current delivery. Emitter resistors ensure current sharing.
Power Supply: High-frequency switching supply for efficiency. Generates +/- 85V DC rails for the output stage and lower voltage rails (+/- 15V, +5V) for the control circuitry.
Protection Circuits: DC offset protection uses comparator to monitor output for DC > +/- 2V. Current limiting uses sensing resistors in the emitter paths of output transistors. Thermal protection uses a sensor on the main heat sink.
Required Tools: Phillips screwdrivers, hex keys, nut drivers, anti-static wrist strap.
Tip: Take photos during disassembly to aid reassembly. Keep screws organized.
Required Test Equipment: Audio oscillator, DMM, oscilloscope, 8Ω dummy load (rated for full power), true RMS AC voltmeter.
WARNING! Use insulated adjustment tools. Re-check adjustments after amplifier has reached normal operating temperature.
Key replacement parts and their manufacturer part numbers. Refer to separate exploded view diagrams for mechanical part locations and numbers.
| Ref Designator | Description | Mfr. Part Number | Qty |
|---|---|---|---|
| Q101, Q102 | Output Transistor NPN | MJ15024G (or BOSE XXX-XXXX) | 8 |
| Q103, Q104 | Output Transistor PNP | MJ15025G (or BOSE XXX-XXXX) | 8 |
| U101 | Op-Amp, Dual | NE5532AP | 2 |
| F101, F102 | Fuse, Slow-Blow, 10A | Littelfuse 10AT | 2 |
| C101, C102 | Capacitor, Electrolytic, 10,000µF/100V | BOSE XXX-XXXX | 4 |
| VR101 | Potentiometer, 500Ω, DC Offset | Bourns 3296W-1-501LF | 2 |
| VR102 | Potentiometer, 200Ω, Bias | Bourns 3296W-1-201LF | 2 |
| PSU Module | Switching Power Supply Assembly | BOSE 1800-PSU-01 | 1 |
Complete schematics are provided in separate documents included with this manual. Key diagrams:
Note: Component values and reference designators on the schematic take precedence over any listed in the general text.
| Symptom | Possible Cause | Corrective Action |
|---|---|---|
| No Power (No LEDs) | AC cord, fuse, power switch, primary side of PSU. | Check AC inlet fuse F101/F102. Check for ~320V DC across main filter caps in PSU. If absent, check rectifier bridge, inrush thermistor, and switching transistors. |
| Protection Light On | DC offset, short circuit, thermal fault. | Measure DC at outputs. If >2V, check input stage op-amps and differential pair transistors. Check emitter resistors for open circuit. Verify heat sink temperature sensor. |
| Distorted Output | Incorrect bias, faulty output transistor, failing capacitor. | Adjust bias per procedure. Check for leaking or bulging capacitors in signal path. Test output transistors in-circuit with DMM diode test function. |
| Low Output / One Channel Dead | Failed driver transistor, open feedback resistor, broken trace. | Check driver stage transistors (usually smaller TO-126 packages). Check the 22kΩ feedback resistor from output to input stage. Visually inspect PCB for cracked solder joints. |
| Excessive Hum/Buzz | Ground loop, failing filter capacitor, poor solder joint on ground plane. | Check main filter capacitors for reduced capacitance or high ESR. Reflow ground connections on the PCB, especially where the star ground connects. |
Reset: Cycling power resets protection latch. If fault is persistent, do not repeatedly attempt to power on.
This Bose 1800 amplifier is designed for professional service. Always use genuine Bose replacement parts for critical components like output transistors and power supply modules to ensure performance and safety specifications are met.
Service Support: For technical assistance, schematic requests, or to order genuine replacement parts, contact:
Bose Professional Systems Division
Service Department
Phone: 1-800-999-2673
Email: [email protected]
Website: www.boseprofessional.com
Copyright & Disclaimer: © [Year] Bose Corporation. All rights reserved. This service manual is proprietary and intended for use by authorized service centers. Bose reserves the right to make changes to product design and documentation without notice.