Bose N123 Service Manual

The Bose N123 is a professional audio amplifier module designed for integration into high-performance sound systems. This service manual provides detailed technical information for qualified service personnel, including specifications, block diagrams, circuit descriptions, adjustment procedures, parts lists, and troubleshooting guidelines. Use this manual only if you are trained in electronic repair and safety procedures.

Safety Precautions

Before servicing, read all safety instructions. This unit contains hazardous voltages that can cause severe injury or death.

Hazard	Precaution
Electric Shock	Disconnect AC power before servicing. Discharge all primary power supply capacitors.
Heat & Fire	Allow unit to cool. Check for overheated components. Use correct fuse rating.
ESD Damage	Use grounded wrist strap and anti-static mat when handling PCBs.
Mechanical	Handle heatsinks and heavy components with care. Secure unit during service.

WARNING! Only qualified technicians with knowledge of high-voltage circuits should perform service. Incorrect repair can create additional hazards.

Product Overview & Specifications

The N123 is a Class-D amplifier module with integrated DSP and protection circuitry.

• Model: Bose N123 Amplifier Module.
• Amplifier Type: High-efficiency switching (Class D).
• Rated Power Output: 125W RMS per channel @ 8Ω, 20Hz-20kHz, <0.1% THD.
• Input Sensitivity: 1.0V RMS for rated output.
• Frequency Response: 10Hz - 50kHz (+0/-1 dB).
• Signal-to-Noise Ratio: >105 dB (A-weighted).
• Input Connectors: Balanced XLR & 1/4" TRS.
• Output Connectors: Heavy-duty binding posts.
• Protection: DC offset, short circuit, thermal, over-current.
• Power Supply: Internal switching PSU, 100-240V AC, 50/60Hz.
• Operating Temperature: 0°C to 40°C (32°F to 104°F).
• Dimensions (H x W x D): 2U Rack Height, 19" Rack Width, 12" Depth.

Internal Block Diagram

Main functional blocks: Input Buffer & Balancer -> DSP Processor -> PWM Modulator -> H-Bridge Output Stage -> Output Filter -> Load. Separate blocks for Power Supply, Microcontroller, and Protection Circuitry.

1. Input Stage: Receives balanced/unbalanced audio, provides gain staging and impedance matching.
2. DSP Core: Handles crossover, EQ, dynamics processing, and diagnostic functions.
3. Power Stage: MOSFET H-bridge converts PWM signal to high-power output.
4. Power Supply: Generates +/- 75V DC for output stage and low-voltage rails for control circuits.
5. Protection & Control: Monitors temperature, current, DC offset; triggers mute/shutdown.

Disassembly Instructions

Follow steps in order. Document cable locations and screw types.

1. Power Down & Disconnect: Turn off unit, unplug AC cord, disconnect all input/output cables.
2. Remove Top Cover: Remove (8) Phillips screws from rear and sides. Slide cover back and lift off.
3. Discharge Capacitors: Using a 10Ω, 25W resistor, discharge the main filter capacitors (C101, C102).
4. Access Main Board: Remove (4) screws securing the main amplifier PCB to the chassis heatsink.
5. Disconnect Harnesses: Carefully unplug the front panel ribbon cable, AC inlet connector, and speaker output leads.
6. Remove Board: Lift the main PCB assembly straight up and out of the chassis.

CAUTION! The heatsink may be hot. Handle the PCB by its edges to avoid ESD damage.

Circuit Description & Theory of Operation

Detailed analysis of key circuits.

Input Circuit (U101): IC NE5532 provides differential amplification and common-mode rejection. Gain set by R105/R106.

DSP (U201): Analog Devices SHARC processor. Handles all digital signal processing. Firmware stored in Flash memory (U202).

PWM Modulator (U301): Generates switching signals for the output stage based on DSP output.

Output Stage (Q401-Q404): Four N-channel MOSFETs in H-bridge configuration driven by gate driver IC U401.

Power Supply: PFC front-end (U501) followed by LLC resonant converter (U502) for main rails. Auxiliary flyback converter (U503) for +5V, +12V.

Protection: Current sensing via R451, R452. DC detection via C201, U203. Thermal sensing via TH101 on heatsink.

Test & Adjustment Procedures

Required equipment: Audio analyzer, oscilloscope, DMM, 8Ω dummy load.

1. Power-Up Test: With no load, apply AC power. Check all secondary voltages (+/-75V, +/-15V, +5V, +12V).
2. Bias Adjustment: Connect scope to test point TP301. Adjust RV101 for 50kHz carrier with minimal ringing.
3. Gain Calibration: Apply 1kHz, 1V RMS sine wave to input. Adjust RV102 for 28.3V RMS across 8Ω load (125W).
4. Protection Verification: Short output terminals; unit should mute, then recover after short is removed.
5. Full Performance Test: Measure THD+N, frequency response, and SNR per specification limits.

Tip: Always use an isolation transformer when probing the primary side of the power supply.

Parts List & Exploded View

Key replacement parts. For the complete list, refer to schematic package N123-100.

Ref Des	Part Number	Description	Qty
Q401-Q404	BOSE-123-MOS	Output MOSFET, N-Channel, 200V, 30A	4
U201	BOSE-123-DSP	DSP Processor, SHARC Core	1
U202	BOSE-123-FLASH	Flash Memory, 16Mb	1
U501	BOSE-123-PFC	PFC Controller IC	1
F101	BOSE-123-FUSE	Fuse, T5A, 250V, Slow-Blow	1
C101, C102	BOSE-123-CAP	Electrolytic Capacitor, 10000µF, 100V	2

Exploded View: Diagram shows assembly order of chassis, heatsink, PCB, front panel, and connectors.

Schematic Diagrams

Refer to separate PDF files: N123_Main_Amp.pdf, N123_Power_Supply.pdf, N123_Control.pdf.

1. Main Amplifier Schematic: Shows input stage, DSP, PWM modulator, and output MOSFETs.
2. Power Supply Schematic: Details PFC, LLC converter, and auxiliary supplies.
3. Control & Front Panel Schematic: Includes microcontroller, LEDs, and communication interfaces.

Note: Schematics are proprietary to Bose. Do not distribute.

Troubleshooting Flowcharts

Symptom	Possible Cause	Corrective Action
No Power (No LED)	Blown fuse, faulty AC inlet, dead PSU	Check F101. Check D501-D504 bridge rectifier. Verify U501 Vcc.
Power LED Blinks	Protection fault (thermal, short, DC)	Check output for short. Measure DC offset at output. Check thermistor TH101.
Distorted Audio	Failed output MOSFET, bad op-amp, low rail voltage	Check Q401-Q404 for short. Check U101 pins for correct voltage. Measure +/-75V rails.
One Channel Dead	Open input connector, failed DSP path, dead gate driver	Swap input cables to isolate. Probe signal at U201 input/output. Check U401 for proper drive signals.
Excessive Noise/Hum	Ground loop, failing filter capacitor, poor solder joint	Check C101, C102 for reduced capacitance. Reflow ground connections on input stage. Verify chassis grounding.

Reassembly & Verification

Reverse the disassembly procedure.

1. Install main PCB, ensuring it seats fully on the heatsink with thermal paste.
2. Reconnect all harnesses (front panel, AC, outputs).
3. Secure PCB with (4) screws.
4. Replace top cover and secure with (8) screws.
5. Final Verification: Perform a quick safety check (insulation, grounding). Power on without load, then with load, and verify normal operation.

WARNING! Ensure all screws are tightened and no wires are pinched before applying power.

Regulatory & Service Information

This product complies with FCC Part 15, Class B. Any changes not approved by Bose may void the user's authority to operate the equipment.

• Service Contact: Authorized Bose Service Centers only.
• Technical Support: For service providers, contact Bose Professional Technical Support.
• Manual Version: 2.1
• Copyright: © 2025 Bose Corporation. All rights reserved. Confidential and proprietary.

Disclaimer: Bose is not responsible for any damage or injury resulting from improper service procedures performed by unqualified personnel.