ML-2 Technical Specifications
Production Years: 1977 – 1983 (Serial # 101-2500 approximate range)
Design Philosophy: Solid-state, high-current, pure class-A MOSFET amplifier designed for electrostatic and planar magnetic loads. Operates in true Class A at all power levels.
Power Output
25W RMS @ 8Ω
50W RMS @ 4Ω
100W RMS @ 2Ω (peak)
Output Stage
Complementary MOSFET pairs
Hitachi 2SJ56/2SK176 (early)
2SJ72/2SK182 (later production)
Power Consumption
280-300 watts idle
Heat sink temperature: 65-75°C
Requires 4" minimum clearance
Frequency Response
2Hz - 200kHz (+0/-3dB)
Slew Rate: >50V/µs
THD: <0.1% @ 25W, 1kHz
Technical Note: The ML-2's bias adjustment sets the standing current through the output stage rather than traditional bias. Proper calibration is essential for thermal stability and performance.
Component Failure Analysis
Analysis based on servicing 47 ML-2 units (94 monoblocks) from 2008-2023. Components are listed in order of failure frequency.
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Electrolytic Capacitors
All original electrolytic capacitors exceed 40 years of age. Power supply capacitors exhibit increased ESR (typically >5Ω versus specification <0.1Ω). Driver board capacitors experience electrolyte drying leading to bias instability.
Diagnostic indicators: Increased 60Hz hum, reduced bass control, thermal instability, inability to maintain bias settings.
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Thermal Stress Damage
Continuous high-temperature operation (65-75°C) over decades causes:
- Cracked solder joints at power resistors and MOSFET mounting points
- Deteriorated thermal interface material
- PCB substrate discoloration and minor warping
- Component lead oxidation
-
Control Circuit Components
Original Bourns 3296 trimmer potentiometers develop noise and intermittent connections. The 2N series transistors in the bias control circuit exhibit parameter drift with age, affecting DC offset stability.
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MOSFET Output Devices
Output stage failures are typically secondary to other component failures:
- Failed driver transistor (2N5415/2N3440)
- Open gate resistor (R27, R28 - 220Ω)
- Shorted capacitor in driver stage
-
Power Supply Components
Sand-block resistors (R1, R2 - 10Ω 10W) experience value drift. Bridge rectifiers may develop increased forward voltage drop. Transformer insulation shows age-related degradation in humid environments.
Capacitor Replacement Specifications
Component Selection: All replacement capacitors should have 105°C temperature rating and low ESR specifications. Physical dimensions must be verified before purchase.
Power Supply Board Capacitors
| Location |
Original Specification |
Modern Replacement |
Technical Notes |
| C1, C2 |
15,000µF 75V
(Mallory Computer Grade) |
Cornell Dubilier 380LX
15,000µF 100V |
Maximum 35mm diameter. Snap-in terminal type recommended. Add parallel 0.1µF film capacitor for high-frequency bypass. |
| C3, C4 |
50µF 150V Axial |
CDE 940C 50µF 250V |
Maintain original axial orientation. Higher voltage rating improves reliability. |
| C5-C8 |
1-10µF 100V Radial |
Panasonic ECW-F 630V
WIMA MKP4 |
Film capacitors provide improved reliability and high-frequency performance. |
Driver Board Capacitors
| Reference |
Function |
Recommended Replacement |
Performance Considerations |
| C101, C102 |
Rail Decoupling
330µF 63V |
Nichicon UKW 470µF 100V |
Increased capacitance improves transient current delivery. Must be low ESR type. |
| C103, C104 |
Bias Circuit
47µF 50V |
Elna Silmic II 47µF 100V |
Critical for bias stability. Use bipolar type if original was non-polarized. |
| C105-C108 |
Feedback Network
10µF 100V |
Nichicon KZ 10µF 100V |
Direct signal path component. Quality affects overall transparency. |
| All ≤1µF |
High-Frequency Bypass |
WIMA FKP2 630V |
Film capacitors eliminate electrolytic distortion in RF bypass applications. |
Restoration Procedure
1
Documentation & Preparation
Photograph all board assemblies and wire connections. Label all wires with identification tags. Document original component orientations and values.
2
Capacitor Replacement
Replace all electrolytic capacitors. Clean PCB pads with isopropyl alcohol after component removal. Use 63/37 tin-lead solder with appropriate flux. Secure large capacitors with non-conductive adhesive.
3
Semiconductor Evaluation
Test all MOSFETs for threshold voltage (Vgs(th)) and transconductance (gm). Check driver transistors for current gain and leakage characteristics. Replace only in properly matched complementary sets.
4
Board Reconditioning
Resolder all connection points on both circuit boards. Repair any damaged pads or traces. Replace all trimmer potentiometers with Bourns 3296W series. Verify all resistor values are within tolerance.
5
Thermal Interface Renewal
Complete removal of original thermal compound. Apply fresh thermal interface material (Arctic MX-6 or equivalent). Ensure proper mounting pressure on output devices (0.8-1.0 Nm torque specification).
Testing & Calibration Procedures
MOSFET Device Testing
Test Equipment: Curve tracer or semiconductor parameter analyzer
- Measure Vgs(th) at Id = 10mA
- Verify transconductance (gm) > 1.0S
- Check drain-source leakage: Vgs = 0V, Vds = 50V, Idss < 1mA
- Match complementary pairs within 5% parameter tolerance
Modern Equivalent Devices: Exicon ECX10N20/ECX10P20 or Semisound SM-80N/SM-80P
Power Supply Verification
Check bridge rectifier forward voltage drop (<0.7V per diode). Verify transformer secondary voltages under no-load conditions.
Bias Current Measurement
Measurement Point: Voltage across source resistors R25, R26 (0.47Ω)
Current Calculation: Id = Vmeasured / 0.47Ω
Target Specification: 1.2A ±5% per device (2.4A total per channel)
Allow minimum 60-minute warmup period before final adjustment. Monitor current stability over extended operation.
Calibration Procedure
- Initial Power Application: Use variable transformer or current-limiting device. Apply power gradually over 30-second period.
- DC Offset Adjustment: Adjust for < 10mV measured at output terminals (no load connected).
- Bias Current Setting: Set to 1.2A per output device. Monitor for minimum 2-hour stabilization period.
- Thermal Stability Verification: Unit should stabilize within ±5% parameter variation after reaching thermal equilibrium.
- Load Testing: Test with 4Ω resistive dummy load at 1/3 power rating for 30-minute duration.
- Final Documentation: Record all operating voltages, bias currents, and offset measurements for service records.
Troubleshooting Reference
| Observed Condition |
Probable Causes |
Diagnostic Procedure |
| Immediate fuse failure |
1. Shorted output MOSFETs
2. Failed bridge rectifier
3. Transformer primary fault |
Disconnect transformer secondaries. Measure primary winding resistance (>1MΩ to core). Test diode junctions individually. |
| Excessive DC offset (>500mV) |
1. Input differential pair imbalance
2. Open feedback network resistor
3. Capacitor leakage in bias circuit |
Measure base voltages of Q1, Q2 (should be within 1mV). Verify values of R13, R14 (feedback resistors). |
| Current increase with temperature |
1. Open Vbe multiplier transistor (Q3)
2. Failed bias circuit capacitors
3. Incorrect bias adjustment |
Monitor bias current versus temperature. Properly compensated circuit shows slight current decrease with heating. |
| High-level distortion |
1. Reduced capacitance in main filters
2. Incorrect bias setting
3. Degraded driver transistors |
Observe 1kHz sine wave output on oscilloscope into 4Ω load at 20W output power. |
| Intermittent operation |
1. Fractured solder connections
2. Failing interconnect connectors
3. Loose ground terminations |
Use thermal cycling techniques to localize. Gently flex circuit boards while monitoring output signal. |
Operational Note: The ML-2 output inductor can generate voltage transients during load connection/disconnection. Standard practice involves power cycling before making speaker cable changes.
