Expert Mark Levinson No.27 / No.29 Repair

Model Overview

No.27 75W MONO

Production: 1995-1998

Design: 75W monoblock power amplifier with fully balanced differential design, high-bias Class AB operation, and dual power supplies.

Key Features:

75W RMS @ 8Ω, 150W @ 4Ω
Fully balanced differential design
Dual power supply (separate positive/negative)
High-bias Class AB operation
No overall negative feedback
Massive heatsinking required

No.29 100W MONO

Production: 1996-1999

Design: 100W monoblock power amplifier with enhanced power supply, higher bias current, and improved thermal management over No.27.

Key Features:

100W RMS @ 8Ω, 200W @ 4Ω
Enhanced power supply with more capacitance
Higher bias current for more Class A operation
Improved thermal management
Same basic circuit topology as No.27
More output devices for higher current

Shared Architecture: Both models share identical circuit topology with fully balanced differential design from input to output. The No.29 is essentially an upgraded No.27 with larger power supply, more output devices, higher bias, and improved cooling for increased power output and reliability.

Technical Specifications

Parameter	No.27 Specification	No.29 Specification	Notes
Power Output	75W @ 8Ω 150W @ 4Ω 300W @ 2Ω	100W @ 8Ω 200W @ 4Ω 400W @ 2Ω	Continuous RMS, 20Hz-20kHz
Class A Region	First 15W	First 25W	High-bias Class AB design
Frequency Response	10Hz-20kHz ±0.1dB 5Hz-100kHz ±3dB	10Hz-20kHz ±0.1dB 5Hz-100kHz ±3dB	No overall feedback
Total Harmonic Distortion	<0.05% @ full power	<0.05% @ full power	20Hz-20kHz, typical
Signal-to-Noise Ratio	>115dB (A-weighted)	>115dB (A-weighted)	Reference: 1W output
Input Sensitivity	1.5V for 75W out	1.5V for 100W out	Balanced inputs only
Damping Factor	>200 @ 20Hz	>200 @ 20Hz	8Ω load

Common Failure Modes

Output Transistors

Failure Rate: 80%

Thermal stress causes secondary breakdown. Beta degradation over time. Matching drifts causing imbalance.

Power Supply Capacitors

Failure Rate: 75%

Main filter caps develop high ESR. Bias supply caps leak. Decoupling caps dry out causing oscillation.

Driver Transistors

Failure Rate: 60%

Thermal cycling stress. Current gain reduction. Vbe drift affecting bias stability.

Protection Circuit

Failure Rate: 55%

DC offset protection triggers. Current sensing resistors drift. Thermal sensors degrade.

No.29 Specific Issues

1

Higher Thermal Stress

Additional output devices and higher bias generate more heat. Thermal compound dries faster. Heatsink joints crack.

2

Power Supply Strain

Larger transformer and capacitors experience more stress. Bridge rectifiers run hotter. Soft-start components fail.

3

Fan System Issues

Forced air cooling fans wear out. Fan controllers fail. Air filters clog reducing cooling efficiency.

Power Supply Restoration

Power Supply Components Comparison

Component	No.27 Specification	No.29 Specification	Modern Replacement
Main Transformer	800VA toroidal	1000VA toroidal	Custom wound or equivalent
Main Filter Caps	22,000µF 100V (x4)	33,000µF 100V (x4)	Cornell Dubilier 381LX
Secondary Filters	10,000µF 63V (x4)	15,000µF 63V (x4)	Nichicon LGU
Bias Supply Caps	2,200µF 50V (x4)	3,300µF 50V (x4)	Elna Silmic II
Bridge Rectifiers	35A 400V	50A 400V	KBPC5010 or similar

Soft Start & Protection Circuits

Soft Start Circuit

Thermistor: CL-140 or CL-150
Relay: Time-delay type
Resistors: Current limiting
Test: Inrush current limitation

DC Offset Protection

Detection: ±2.5V DC
Response: < 100ms
Action: Relay opens
Reset: Manual power cycle

Overcurrent Protection

Sensing: 0.1Ω resistors
Threshold: 15A (No.27)
Threshold: 20A (No.29)
Response: < 50ms

Thermal Protection

Sensors: NTC on heatsink
Warning: 75°C
Shutdown: 85°C
Control: Fan speed adjustment

Output Stage Components

Output Device Configuration

Component	No.27 Configuration	No.29 Configuration	Modern Equivalents
Output Transistors	8 pairs (16 devices) MJ15024/MJ15025 TO-3 package	10 pairs (20 devices) MJ21193/MJ21194 TO-247 package	MJ21193G/MJ21194G Higher SOA rating
Driver Transistors	2 pairs (4 devices) MJE15032/MJE15033	2 pairs (4 devices) MJE15032/MJE15033	Same (still available) Check for counterfeit parts
VAS Stage	2SC3423/2SA1360 High voltage types	2SC3423/2SA1360 Same as No.27	KSC3503/KSA1381 300V Vceo rating
Input Stage	2SK170/2SJ74 JFETs Matched pairs	2SK170/2SJ74 JFETs Matched pairs	LSK170/LSJ74 Linear Systems

Transistor Matching Requirements

1

Output Device Matching

Match Vbe within 20mV at 100mA. Match hFE within 15% for parallel devices. Complementary pairs should have similar curves.

2

JFET Input Matching

Match Idss within 0.5mA for differential pairs. Match Vgs at operating current within 10mV. Critical for DC offset.

3

Thermal Tracking

Ensure Vbe multiplier tracks output devices. Mount on same heatsink. Use fresh thermal compound.

Bias & Thermal Management

Bias Current

No.27: 1.5A total quiescent
No.29: 2.0A total quiescent
Measurement: Emitter resistors
Target: 100-150mA per device

Class A Operation

No.27: 15W pure Class A
No.29: 25W pure Class A
Crossover: Seamless transition
Bias adjustment: Critical

Thermal Design

No.27: Large external heatsinks
No.29: Larger heatsinks + fan
Temperature: 55-65°C normal
Monitoring: Thermal sensors

DC Offset Control

Target: < 20mV
Adjustment: Input balance
Protection: ±2.5V detection
Monitoring: Continuous

Bias Adjustment Procedure

Allow amplifier to warm up for 60 minutes at idle
Measure voltage across emitter resistors (0.22Ω or 0.47Ω)
Calculate bias current: I = V / R (emitter resistor value)
Adjust for target current: 100mA/device for No.27, 120mA/device for No.29
Monitor temperature rise and adjust Vbe multiplier for thermal tracking
Verify DC offset remains < 20mV during adjustment
Run for 4 hours and readjust if necessary

Calibration Procedures

Required Test Equipment

True RMS multimeter (0.5% DC accuracy minimum)
Dual trace oscilloscope (100MHz)
Audio analyzer or low distortion generator
8Ω/4Ω 300W non-inductive dummy loads
Infrared thermometer
Variac or current-limiting device
Isolation transformer (recommended)

Calibration Steps for Both Models

Step	Procedure	No.27 Target	No.29 Target
1. Protection Test	Verify DC offset, overcurrent, thermal protection	±2.5V trigger	±2.5V trigger
2. Bias Adjustment	Set quiescent current	1.5A total	2.0A total
3. DC Offset	Adjust input balance	< 20mV	< 20mV
4. Power Test	Full power into dummy load	75W @ 8Ω	100W @ 8Ω
5. Thermal Stability	Monitor over 4 hours	< 10% drift	< 10% drift

Troubleshooting Guide

Symptom	No.27 Likely Causes	No.29 Likely Causes	Diagnostic Steps
Blows main fuse immediately	Shorted output devices Failed bridge rectifier Transformer primary short	Same as No.27 More devices = more likely Larger transformer failure	Disconnect transformer secondaries Test all output devices Check bridge rectifier diodes
Protection relay won't engage	DC offset fault Overcurrent detection Thermal sensor fault	Same as No.27 Higher current threshold Fan system fault	Measure DC offset Check current sense resistors Test thermal sensors
Distortion at high power	Insufficient bias Failed output devices Power supply sag	Same as No.27 More complex failure patterns Larger supply but higher demand	Measure bias current Test output devices in-circuit Monitor rail voltages under load
Overheating	Bias too high Insufficient ventilation Failed thermal compound	Same as No.27 + Fan failure Clogged air filters Higher heat output	Measure bias current Check fan operation Monitor heatsink temperature Clean air passages
Intermittent shutdown	Thermal protection Intermittent component Protection circuit fault	Same as No.27 Fan controller issues Higher thermal stress	Monitor temperature Test protection circuit Check all connectors

Thermal Management System

No.27 Cooling System

Heatsinks

Type: External aluminum
Size: Large passive design
Thermal resistance: <0.5°C/W
Mounting: Vertical orientation

Thermal Interface

Compound: Silicone based
Application: Even layer
Replacement: Every 5-7 years
Cleaning: Isopropyl alcohol

Monitoring

Sensors: NTC thermistors
Location: Heatsink center
Calibration: Factory set
Protection: Graduated response

No.29 Enhanced Cooling

Forced Air Cooling

Fans: 1-2 axial fans
Speed: Temperature controlled
Control: Microprocessor
Filters: Removable foam

Enhanced Heatsinks

Size: Larger than No.27
Design: Finned for airflow
Orientation: Optimized for fans
Mounting: More secure

Fan Control System

Controller: PWM or voltage
Sensors: Multiple locations
Response: Progressive speed increase
Failure detection: Built-in

Cooling Maintenance: The No.29's fan system requires regular maintenance. Clean air filters every 6 months, lubricate fan bearings if not sealed, and test fan operation regularly. Fan failure can lead to thermal shutdown and potential damage.

Replacement Parts Guide

Critical Replacement Components

Component Type	No.27 Part Numbers	No.29 Part Numbers	Modern Equivalents
Output Transistors	MJ15024/MJ15025 (TO-3 package)	MJ21193/MJ21194 (TO-247 package)	MJ21193G/MJ21194G (Higher SOA rating)
Driver Transistors	MJE15032/MJE15033	Same as No.27	Same (genuine On Semi)
Input JFETs	2SK170/2SJ74	Same as No.27	LSK170/LSJ74 (Linear Systems)
Power Capacitors	Various values	Larger values than No.27	Nichicon, CDE, Panasonic
Thermal Sensors	NTC thermistors	Same type	10kΩ NTC, B=3950
Cooling Fans (No.29)	N/A	120mm axial fans	Noctua or similar quiet fans

Important Note: When replacing output transistors in the No.27/No.29, always replace the entire set in one channel. Mixing old and new devices or mixing different production batches can cause current imbalance and thermal runaway. Always use matched sets from the same production batch.

Experts on Mark Levinson No.27 & No.29 Power Amplifiers repairs