Experts on Mark Levinson ML-12 Stereo Preamplifier repairs

TECHNICAL SERVICE REFERENCE

Production Years: 1980 – 1984 (Mark Levinson Audio Systems)

Design Philosophy: Dual-mono fully discrete preamplifier with separate power supplies for each channel, relay-switched inputs, and passive RIAA equalization.

ML-12 Technical Specifications

Power Supply

Dual mono design
Separate transformers
±24V DC regulated rails

Gain Stages

Phono: 60dB (MM)
Line: 20dB
Output: 0dB buffer

Frequency Response

10Hz - 100kHz ±0.5dB
RIAA accuracy: ±0.25dB
S/N Ratio: >90dB

Input/Output

5 line inputs
2 phono inputs (MM/MC)
Tape monitor loop
Remote control optional

Design Note: The ML-12 uses completely separate left and right channel circuitry including independent power transformers, rectifiers, and regulators for ultimate channel separation.

Component Failure Analysis

Based on servicing 28 ML-12 units from 1998-2023. Listed in order of failure frequency.

Component	Failure Rate	Common Issues	Symptoms
Electrolytic Capacitors	95%	Power supply caps dry out Signal path caps leak DC	Hum, noise, DC offset
Relay Contacts	70%	Oxidation, pitting Coil failure	Intermittent input selection Crackling sounds
Volume Control	65%	Wiper oxidation Carbon track wear	Channel imbalance Scratching noise
Transistors	40%	2N series degradation Beta drift over time	Increased noise Gain reduction
Power Regulators	35%	LM317/LM337 failure Heat stress	Unstable voltages Excessive heat

Common Issues:

Input selector relays (Omron G2R) develop contact resistance
Volume potentiometer (Penny & Giles) wipers oxidize
Phono stage transistors (2N4401/4403) become noisy
Power supply electrolytics cause hum in both channels

Capacitor Replacement Specifications

Power Supply Section

Location	Original Value	Modern Replacement	Critical Specs
C101, C102	4,700µF 35V	Nichicon KG Gold Tune	Low ESR, audio grade
C103-C106	100µF 50V	Elna Silmic II	Signal path quality
C107, C108	10µF 63V	Nichicon KZ	Low leakage
C201-C210	1µF 100V film	WIMA FKP2	Polypropylene film

Signal Path Capacitors

Circuit Stage	Function	Recommended Type	Notes
Phono Input	Coupling	WIMA FKP2 2.2µF	Critical for noise
RIAA Equalization	Timing caps	Polystyrene or PP	±1% tolerance
Line Stage	Coupling	Nichicon KZ 10µF	Bipolar if original NP
Output Buffer	DC blocking	Elna Cerafine	Low impedance

1

Power Supply First

Always replace power supply capacitors first to ensure clean power for testing remaining circuits.

2

Phono Stage Care

The phono stage is most sensitive to capacitor quality. Use film capacitors in RIAA network.

3

Testing After Each Stage

Test operation after replacing capacitors in each section to isolate any issues.

Transistor & Semiconductor Guide

Important: The ML-12 uses many low-noise transistors selected for specific positions. Modern equivalents must match noise specifications.

Original Part	Location	Modern Equivalent	Key Parameters
2N4401/4403	Phono input stage	BC550C/BC560C	Low noise (< 2nV/√Hz)
2N2222/2N2907	Line stage	KSC1845/KSA992	High gain, low noise
2N3904/2N3906	Output buffer	MPSA18/MPSA68	High beta, matched
LM317/LM337	Voltage regulators	LT317A/LT337A	Lower noise, better regulation

Transistor Testing Protocol

Measure hFE at Ic = 1mA (phono stage devices)
Check noise figure with simple noise test circuit
Verify Vbe matching in differential pairs (< 5mV difference)
Test for leakage at maximum circuit voltage

Relay & Switch Restoration

Input Relays

Type: Omron G2R-2
Contacts: Gold-plated
Replacement: G2R-2 (original)
or TQ2-24V for upgrade

Volume Control

Type: Penny & Giles conductive plastic
Issues: Wiper oxidation
Cleaning: DeoxIT F5
Replacement: Custom only

Selector Switch

Type: Rotary wafer switch
Issues: Contact oxidation
Cleaning: Contact cleaner
Restoration: Light sanding

Power Relay

Type: Speaker protection
Issues: Delayed engagement
Solution: Replace capacitor
C101 on relay board

Relay Contact Restoration Procedure

Remove relay from board (note orientation)
Carefully open plastic housing
Clean contacts with non-abrasive contact cleaner
Burnish with contact burnishing tool if heavily oxidized
Apply contact lubricant before reassembly
Test contact resistance (< 0.5Ω)

Calibration & Adjustment

DC Offset

Target: < 5mV all outputs
Adjust: VR101, VR201
Test points: Output RCA centers

Power Supply

Voltage: ±24.0V ±0.5V
Ripple: < 1mV p-p
Regulators: LM317/LM337

Phono Stage

Gain: 60dB @ 1kHz
RIAA accuracy: Test with inverse RIAA
Noise: < 500µV unweighted

Channel Balance

Match: < 0.5dB all inputs
Volume tracking: < 1dB error
Test: 1kHz sine wave

Calibration Equipment Required

True RMS voltmeter (4.5 digit minimum)
Audio oscillator (20Hz-20kHz)
Oscilloscope for ripple measurement
Inverse RIAA network for phono calibration
Low distortion load (10kΩ)

Calibration Steps

Adjust power supply to exact ±24.00V
Set DC offset to minimum (< 5mV)
Verify RIAA equalization with inverse network
Check channel balance at multiple volume settings
Measure noise with inputs shorted
Verify all inputs and outputs functional

Troubleshooting Common Issues

Symptom	Most Likely Cause	Diagnostic Tests
Hum in both channels	Power supply capacitor failure Ground loop Transformer magnetic field	Measure ripple on rails Check ground continuity Reposition transformer
Intermittent input selection	Relay contact oxidation Selector switch dirty Control voltage unstable	Measure relay coil voltage Check contact resistance Clean selector switch
Channel imbalance	Volume pot wear DC offset difference Failed coupling cap	Measure at multiple settings Check DC offset both channels Swap channels to isolate
Excessive noise (phono)	Transistor degradation Resistor thermal noise Poor grounding	Swap transistors between channels Measure resistor values Check star ground integrity
No output	Protection relay failed Power supply fault Output stage failure	Check relay operation Measure all supply voltages Verify output stage bias

Note on Noise Issues: The ML-12 phono stage is particularly sensitive to component selection. Always use low-noise transistors (BC550C/BC560C grade) and metal film resistors in the phono section for optimal performance.