McIntosh MX115 Service Guide

Part 1: Common Symptoms and Their Meanings

In the MX115, "error codes" are audible or functional problems. Here are the most common ones:

Part 2: Repair Solutions

The solutions almost always follow a standard restoration procedure:

1. Initial Assessment: Do not power the unit on for long periods. Check for obvious signs of damage (burned resistors, bulging capacitors).
2. Replace the Power Supply Capacitors FIRST: This is non-negotiable for safety and basic operation. Replace the large multi-section can capacitor and the other main electrolytics.
3. Replace All Critical Coupling Capacitors: The plastic-film "Mustard" caps and other small coupling caps are known to become leaky and can damage other components.
4. Transistor Replacement (Crucial for FM): It is standard practice to proactively replace the transistors in the MPX decoder (H17-H20) and the tuning meter circuit (H13, H14) with modern, stable equivalents.
5. Clean Controls and Tube Sockets: Use a high-quality contact cleaner on all switches, potentiometers, and tube sockets.
6. Bias Adjustment (Headroom Control): After recapping, the "Headroom" control must be readjusted using a multimeter to the specified voltage.

Part 3: Capacitors That Need Replacing

This is the core of an MX115 restoration. Here are the critical capacitors, organized by section.

A. Power Supply (Top Priority)

• C1A, C1B: 500µF 50V (Dual Section Can Capacitor) - The main cause of loud hum.
• C2A, C2B: 500µF 50V (Dual Section Can Capacitor) - Often contained in the same can as C1.
• C3: 80µF 350V
• C4: 40µF 350V
• C5: 20µF 350V
• C6, C7: 100µF 25V

Note: The large multi-section can capacitor (C1/C2) is often replaced with individual modern capacitors mounted on a terminal strip for reliability.

B. Audio Path (Preamp & Control Amp)

These are critical for sound quality and preventing damage.

• C11, C12: 0.1µF 400V (Coupling caps from 12AX7 plates) - Must replace.
• C13, C14: 0.1µF 400V (Coupling caps to output) - Must replace.
• C9, C10: 25µF 25V (Cathode bypass capacitors for the 12AX7s) - Cause hum if bad.
• Various other small-value film caps (e.g., C15, C16, C17, C18, C21, C22).

C. FM Tuner & MPX Section

Replacing caps here restores FM performance.

• C101, C102, etc.: All electrolytic and film capacitors in the tuner boards should be replaced.
• MPX Board Caps: Pay special attention to capacitors around the failed transistors (H17-H20).

D. General Electrolytics

• Replace all other electrolytic capacitors throughout the unit, as they are well past their life expectancy.

Recommended Capacitor Types for Replacement:

• Power Supply: Use high-temperature (105°C) radial lead electrolytic capacitors from reputable brands (Nichicon, Panasonic, United Chemi-Con).
• Audio Coupling: Use modern polypropylene film capacitors (e.g., Illinois Capacitor / Illinois MPW, WIMA MKP10). These have excellent sonic characteristics.
• General Purpose: Use high-quality film capacitors (Mylar/Polyester) for small-value caps and low-voltage electrolytics for bypass duties.

Final Advice

A full restoration of an MX115 is a significant but rewarding project. The result is a preamp/tuner that rivals modern units costing thousands of dollars.

1. Get the Documentation: Download the Service Manual from a site like HiFiEngine. It's essential for the schematic, parts list, and adjustment procedures.
2. Work Methodically: Replace one section at a time and double-check your work. Take high-resolution photos before you desolder anything.
3. Beware of the PCBs: The circuit boards are fragile. Use a good temperature-controlled soldering iron and avoid excessive heat.
4. Consider a Professional: If you are not confident, investing in a qualified vintage audio technician to perform this restoration is money well spent.