What are the most common problems with the McIntosh MR 78 tuner?

Common issues include dried-out electrolytic capacitors causing hum and power problems, noisy or failed transistors in the RF/IF stages, drift in alignment, dirty switches and controls, and lamp failures. The power supply capacitors are particularly prone to failure after decades.

Which capacitors should be replaced in an MR 78 restoration?

All electrolytic capacitors should be replaced, including: main filter capacitors, secondary filter capacitors, regulation capacitors, audio output section capacitors, RF/IF section capacitors, and control circuit capacitors. Use high-quality audio-grade capacitors for optimal performance and longevity.

What safety precautions are necessary when repairing a McIntosh MR 78?

Always disconnect from power before opening. The MR 78 contains lethal high voltages. Discharge all large capacitors completely before handling any components. Exercise extreme caution around the power supply section. Use a variac for initial power-up after repairs. If you're not experienced with high-voltage electronics, consult a qualified professional technician.

How do I calibrate a McIntosh MR 78 after restoration?

Essential calibrations include: aligning the RF/IF and multiplex sections using proper test equipment, adjusting the stereo decoder for proper channel separation, checking meter calibration, and verifying sensitivity and selectivity. Allow the unit to warm up for 30 minutes before making final adjustments.

What are the specifications of the McIntosh MR 78?

The MR 78 features legendary FM selectivity and sensitivity, solid-state RF/IF and audio stages, 0.2% total harmonic distortion, and a classic McIntosh glass front panel with blue meters. Introduced in 1972, it is considered one of the finest FM tuners ever made.

When was the McIntosh MR 78 manufactured?

The McIntosh MR 78 was introduced in 1972 and is renowned for its selectivity, sensitivity, and sound quality.

McIntosh MR 78 Tuner Repair Advice

IMPORTANT DISCLAIMER: This article is provided for informational purposes only. I am not responsible for the accuracy of this information. The content may or may not be accurate, and any repairs attempted based on this information are done at your own risk. Always consult a qualified technician for electronic repairs, especially with vintage equipment like the McIntosh MR 78.

SAFETY WARNING: Always disconnect the unit from power before working internally. This unit contains high voltages that can be lethal. Discharge large capacitors before handling. If unsure, consult a professional technician.

CRITICAL NOTE: The McIntosh MR 78 is a sophisticated solid-state FM tuner. Exercise extreme caution around the power supply and RF/IF sections. Always use a variac for initial power-up after repairs.

Model Overview

The McIntosh MR 78 is a legendary solid-state FM tuner introduced in 1972. Renowned for its selectivity, sensitivity, and sound quality, the MR 78 is considered one of the finest FM tuners ever made. It features advanced RF/IF and multiplex circuitry, a classic McIntosh glass front panel, blue meters, and robust build quality.

Selectivity

Exceptional

Tuner Section

Solid-state FM with multiplex

Year Introduced

1972

Total Harmonic Distortion

0.2%

Common Issues and Solutions

Power Supply Problems

The MR 78 power supply can develop specific issues:

Capacitor failure: Large electrolytic capacitors dry out, causing hum and power issues
Rectifier issues: Rectifiers can fail under load
Resistor drift: Power resistors can drift out of specification or overheat
Voltage regulator failure: Regulator transistors can fail

RF/IF Section Problems

Common RF/IF issues include:

Transistor failure: Noisy or failed RF/IF transistors
Alignment drift: IF alignment can drift, reducing selectivity
Capacitor drift: Small-value capacitors in tuned circuits can drift
Switch contact issues: Dirty switches cause signal loss

Audio Output Section Problems

Common audio output issues include:

Capacitor failure: Output coupling capacitors can degrade
Transistor noise: Noisy output transistors
Potentiometer noise: Dirty controls cause crackling

Display & Control Problems

Meter lamp failure: Burned out dial or meter lamps
Switch contact issues: Dirty or worn switches

Note: WE DO NOT SELL COMPONENTS: Every repair is unique; what resolves an issue for one unit may not apply to others, especially if there are discrepancies in component value or underlying quality concerns. The following is a recommendation, not a guaranteed solution.

Capacitor Replacement Guide

The following capacitors should be replaced in any comprehensive restoration:

Power Supply Section

C1, C2: 2200μF 50V - Main filter capacitors

C3, C4: 1000μF 35V - Secondary filter capacitors

C5, C6: 470μF 25V - Regulation capacitors

C7, C8: 220μF 16V - Low voltage supply capacitors

RF/IF Section

C101-C110: Various values - Tuning and alignment capacitors

C111-C115: 4.7μF 50V - Coupling capacitors

C116-C120: 22μF 25V - Bypass capacitors

C121-C125: 0.47μF 100V - Decoupling capacitors

Audio Output Section

C201, C202: 10μF 25V - Output coupling capacitors

C203, C204: 47μF 16V - Output stage capacitors

C205, C206: 1.0μF 50V - Compensation capacitors

Control & Display Section

C301, C302: 4.7μF 25V - Meter circuit capacitors

C303, C304: 22μF 16V - Control circuit capacitors

Critical Components to Check/Replace

Component Inspection and Replacement Guide
Component	Reference	Location	Notes
RF/IF Transistors	Q101-Q110	RF/IF Boards	Check for noise and proper operation
Multiplex Decoder	IC201 or discrete	Multiplex Board	Check for proper stereo separation
Voltage Regulators	Q201, Q202	Power Supply	Check for proper regulation and heat sinking
Power Resistors	R1, R2, R3	Power Supply	Check for value drift and overheating
Meter Lamps	L1, L2	Display Board	Replace if burned out
Switches	S1-S4	Front Panel	Clean or replace if noisy

Note: The MR 78 uses high-quality components throughout. When replacing parts, use equivalent or better quality components. Pay special attention to the RF/IF alignment procedures which require specialized equipment.

Restoration Procedure

Disconnect power and remove covers
Discharge all capacitors completely
Document all wiring with photos before disassembly
Replace all electrolytic capacitors
Check and replace out-of-spec resistors, especially power resistors
Clean all controls and switches with contact cleaner
Check RF/IF and audio transistors and replace if necessary
Reapply thermal compound to output transistors and heat sinks
Check and clean all heat sinks
Reassemble and power up slowly using a variac
Align RF/IF and multiplex sections according to service manual
Verify proper operation of all functions

Calibration Procedure

After restoration, these calibrations are essential:

Align RF/IF and multiplex sections using proper equipment
Adjust stereo decoder for proper separation
Check meter calibration for signal strength and tuning
Verify proper operation of all outputs and controls
Test sensitivity and selectivity

Additional Tips

Use high-quality, audio-grade capacitors for best performance
Document all adjustments before making changes
Check all resistors near replaced components; they may have drifted in value
Clean all controls thoroughly as they often cause noise
Allow the unit to warm up for 30 minutes before making final adjustments
Consider replacing rectifiers with modern equivalents
Pay special attention to the alignment of RF/IF and multiplex circuits
Inspect all PC boards for cracked traces or solder joints
Clean the meter movements carefully if they're sticking
Check all lamp holders and replace burnt-out lamps with appropriate replacements

DISCLAIMER REPEATED FOR EMPHASIS: This information is provided as-is without warranty of any kind. I am not responsible for the accuracy of this information or any damage that may occur from its use. Always consult a qualified technician for repairs on vintage audio equipment.