Luxman L-550AX II Integrated Amplifier Repair Guide

Model Overview

The Luxman L-550AX II is a premium pure Class A integrated amplifier representing the pinnacle of Luxman's audio engineering. This second-generation model features enhanced power supply regulation and improved thermal management over the original L-550AX. With 20 watts per channel of pure Class A power into 8 ohms, this amplifier delivers exceptional sonic performance with Luxman's renowned musicality and detail retrieval.

Common Issues and Solutions

Power Supply Problems

The L-550AX II power supply can develop specific issues due to Class A operation:

• Capacitor failure: High-temperature operation accelerates capacitor aging
• Rectifier issues: High-current rectifiers can fail under continuous load
• Transformer overheating: Power transformer may overheat in Class A operation
• Voltage regulator failure: Regulator circuits stressed by high current demands
• Thermal stress: Continuous high temperatures affect component lifespan

Class A Output Stage Problems

Specific to Class A amplifier issues:

• Bias current drift: Critical bias settings can drift over time
• Thermal runaway: Output transistors may overheat without proper bias control
• Output transistor failure: Power devices stressed by continuous operation
• Heat sink issues: Thermal interface degradation over time
• DC offset problems: Critical for speaker protection

Preamplifier Section Problems

The preamplifier section can develop issues:

• Potentiometer noise: Dirty volume/control pots cause crackling
• Selector switch issues: Input selection problems
• Phono stage problems: Issues with the MM/MC phono input
• Tone control issues: Problems with bass/treble controls (if equipped)
• Balance control problems: Channel balance issues

Thermal Management Issues

Critical for Class A operation:

• Fan failure: Cooling fan issues (if equipped)
• Thermal compound degradation: Dried thermal interface material
• Heat sink corrosion: Aluminum heat sink oxidation
• Temperature sensor failure: Protection circuit malfunctions

Capacitor Replacement Guide

The following capacitors should be replaced in any comprehensive restoration:

Critical Components to Check/Replace

Component	Reference	Location	Notes
Output Transistors	Q101-Q104	Amplifier Board	Check for failure - critical for Class A operation
Driver Transistors	Q201-Q204	Amplifier Board	Check for proper operation and thermal stress
Bias Transistors	Q301, Q302	Amplifier Board	Critical for Class A bias stability
Voltage Regulators	IC101, IC102	Power Supply	Check for proper output voltages
Protection Relay	RLY101	Protection Board	Check for contact oxidation
Power Resistors	R101-R104	Power Supply	Check for value drift and overheating
Thermal Sensors	TH101, TH102	Heat Sinks	Check for proper temperature sensing

Restoration Procedure

1. Disconnect power and allow unit to cool completely (minimum 2 hours)
2. Discharge all capacitors completely
3. Document all wiring with photos before disassembly
4. Replace all electrolytic capacitors with high-temperature rated equivalents
5. Check and replace out-of-spec resistors, especially in bias circuits
6. Clean all controls and switches with contact cleaner
7. Check output transistors and replace if necessary with matched pairs
8. Check driver transistors and replace if necessary
9. Clean protection relay contacts
10. Reapply high-quality thermal compound to output transistors
11. Clean heat sinks and check for proper thermal transfer
12. Reassemble and power up slowly using a variac
13. Adjust bias and DC offset according to service manual specifications
14. Monitor temperatures during extended operation

Calibration Procedure

After restoration, these calibrations are essential for Class A operation:

• Set DC offset to zero (adjust VR101, VR102)
• Adjust Class A bias current precisely (adjust VR103, VR104)
• Allow 30-minute warm-up before final bias adjustment
• Verify proper operation of thermal protection circuit
• Check phono stage gain and RIAA equalization
• Verify proper operation of all inputs and controls
• Monitor heat sink temperatures during extended operation
• Verify proper operation of protection circuits

Additional Tips

• Use only high-temperature rated capacitors (105°C minimum)
• Document all adjustments before making changes
• Allow ample warm-up time before making bias adjustments
• Use thermal imaging camera if available to check heat distribution
• Clean all controls thoroughly as they often cause noise
• Consider upgrading thermal compound for better heat transfer
• Check ventilation and ensure adequate airflow around the unit
• Monitor bias current stability over extended periods
• Use matched transistor pairs when replacing output devices
• Consider adding external cooling if ambient temperatures are high