McIntosh MAC 1500 Receiver Repair Guide

Model Overview

The McIntosh MAC 1500 is a unique hybrid receiver introduced in 1965, featuring a tube-based FM tuner section and a solid-state amplifier. It was McIntosh's first solid-state receiver and is highly regarded for its build quality and sound. The unit delivers 30 watts per channel and features McIntosh's patented Unity Coupled output circuit.

Common Issues and Solutions

Tuner Section Problems (Tube)

The tube-based tuner section can develop specific issues:

• Tube failure: Tubes can become weak or fail over time
• Capacitor failure: Paper and electrolytic capacitors in the tuner section often need replacement
• Alignment issues: FM/AM alignment can drift over time
• Noisy operation: Dirty tube sockets or failing components cause noise

Amplifier Section Problems (Solid-State)

Common solid-state amplifier issues include:

• Output transistor failure: The germanium output transistors can fail
• Capacitor failure: Electrolytic capacitors dry out, causing various issues
• DC offset: Improper DC offset can damage speakers
• Potentiometer noise: Dirty controls cause crackling in audio

Power Supply Issues

The MAC 1500 has a complex power supply serving both tube and solid-state sections:

• Rectifier issues: Silicon rectifiers can fail over time
• Resistor drift: Carbon composition resistors can drift out of specification
• Filter capacitor failure: Main filter capacitors can dry out

Capacitor Replacement Guide

The following capacitors should be replaced in any comprehensive restoration:

Critical Components to Check/Replace

Component	Reference	Location	Notes
Output Transistors	Q9, Q10	Amplifier Board	Germanium transistors - check for leakage and failure
Driver Transistors	Q7, Q8	Amplifier Board	Check for proper operation
Tubes	V1-V6	Tuner Section	Test and replace weak tubes
Rectifiers	SR1, SR2	Power Supply	Silicon rectifiers - check for failure
Filter Resistors	R1, R2	Power Supply	Check for value drift

Tube Complement

The MAC 1500 uses the following tubes in the tuner section:

• V1: 6BH6 - FM RF Amplifier
• V2: 6BJ6 - FM Oscillator
• V3: 6BK7 - FM IF Amplifier
• V4: 6BK7 - FM IF Amplifier
• V5: 6BL8 - FM Detector/AM Converter
• V6: 6AQ8 - MPX Decoder

Restoration Procedure

1. Disconnect power and remove covers
2. Discharge all capacitors completely
3. Document all wiring with photos before disassembly
4. Replace all electrolytic and paper capacitors
5. Check and replace out-of-spec resistors
6. Clean all tube sockets and pins
7. Test all tubes and replace as necessary
8. Clean all controls and switches with contact cleaner
9. Check output transistors and replace if necessary
10. Reassemble and power up slowly using a variac
11. Adjust bias and DC offset according to service manual
12. Perform alignment of tuner section if necessary

Calibration Procedure

After restoration, these calibrations are essential:

• Set DC offset to zero (adjust R31, R32)
• Adjust bias current (adjust R33, R34)
• If necessary, perform FM alignment using proper equipment
• Check and adjust AM alignment if needed
• Verify proper operation of all inputs and controls

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 the silicon rectifiers with modern equivalents
• Pay special attention to the thermal compound on output transistors
• Consider adding a soft-start circuit to protect the power switch