We recently acquired this McIntosh MC2105 amplifier for restoration and repair. The unit was very good shape, and came installed in a walnut cabinet which also looked great. There were a few minor issues to address. The fuse holder was broken off the rear of the unit. The front panel glass had a few areas where the paint had peeled away. This was isolated to areas around the panlocs and headphone jack. The paint had also bubbled along the top and bottom edges of the front panel glass. These are areas where foam is attached to the inner surface of the glass. The headphone jack was loose. Lastly, the panloc rods were missing the cups that would normally hold them in the chassis.
Restoration
The minor issues aside, we wanted to restore this MC2105 from top to bottom to give it a fresh start and another 50 years of live. We started with the power supply, replacing the broken fuse holder and pulling the main filter capacitors to replace them with fresh Cornell Dubilier parts. While the heatsinks and main filter capacitors were out we gave the chassis a good polishing. We also replaced the power supply multi-cap with a model sourced from Hayseed Hamfest, built to match the original specifications. With parts replaced, we powered just the power supply with a variac and ensured all the supply voltages built to the correct values and there were no supply issues or shorts.
We removed and replaced the original electrolytic capacitors on the input board, meter board, and the driver boards. We also replaced the input board’s worn and cracked “tropical fish” film capacitors. Finally, we upgraded the driver boards per the McIntosh service bulletin, which mitigates the risk of thermal runaway in these units.
Reworked Power Supply 
Updated Input Board 
Updated Drivers with Original Heatsinks 
Updated Meter Board 
Driver Before Thermal Runaway Upgrade 
Updated with Thermal Runaway Changes
Functional Check
We verified diodes, transistors, and other key parts on input, meter, drivers, and heatsinks before powering the unit again. With power applied we verified bias to the heatsinks looked good, and the unit passed a good signal in both channels. Cleaning the meter switch resolved an intermittent issue with the left meter, and we set the unit aside to listen.
We addressed the few cosmetic issues by touching up the front panel paint. The new paint looks great and the front panel looks pristine. We installed new light barrier foam to keep the white front panel lighting from leaking into the blue light of the meters. We installed a new set of feet on the chassis. A pair of speed nuts were installed to retain the panloc rods in the chassis. The panlocs now look and feel like the originals from the front of the unit. We removed the worn felt on the feet of the walnut case and replaced it with fresh felt. The case now looks like new.
Performance
The performance is exceptional. We measured total Harmonic Distortionat under 0.1% across the full frequency range, 0.027% at its best. The unit runs to 120W output into 8 Ohms at 120VAC line voltage. We’ve been listening to the MC2105 pared with a McIntosh C32, shown here. Bob the Tech Audio also restored the C32 shown, read more at this link.
If you have a vintage piece like this McIntosh MC2105 amplifier that needs restoration, contact Bob the Tech Audio for your restoration and repair needs.
