Bob the Tech Audio had this McIntosh MC2205 in for repair and restoration. When it came in the complaint was no output to the speakers. A front panel speaker switch enables the outputs. When the speaker switch is on, a driver transistor energizes the speaker relay coil. This close the relay and connects the speakers. A third switch in the same relay bypasses the thermistor inrush current limiter once the unit is up and running. This allows the power supply a bit more capacity to drive the speakers. This relay was apparently not working.
Power Supply
Bob the Tech pulled the covers on the unit and evaluated the power supply. The thermistor, (actually a pair wired in parallel), was damaged. Some prior rework done on the unit resulted in some mis-wiring in this area. At some point the unit had taken a strike on the rear, in the area of the fuse, and bent the chassis a bit. As a result the unswitched outlet had broken loose.
The customer requested a replacement of the electrolytic capacitors. We started by reworking the power supply capacitors and resolving the wiring issues. Bob the Tech replaced the original thermistor with a factory recommended replacement. We replaced the power cord and grommet with a modern polarized version. The broken accessory outlet was repaired and reinstalled. The photo top center illustrates these repairs. We installed new line RFI and power supply filter capacitors and resolved the supply wiring issues. the photograph top right shows the original tropical fish capacitor removed, and the replacement polyester film line capacitor installed on the rectifier. We bench tested the relay, verified it was operational, and reinstalled. The power supply area rework is shown in the large photo below.
Re-capping
Bob the Tech evaluated the power supply board and driver boards for bad components. We replaced electrolytic capacitors in power and signal path. The boards with their new capacitors are shown in the before and after photos below. We removed the heatinks, verified good, and reinstalled. We polished the chassis while the heatsinks and mains supply capacitors were out of the way.
With rework complete we powered the unit and verified the main power supply. We reconnected the heatsinks to the supply and verified the unit successfully passes a signal. With the unit fully operational, we cleaned and lubricated all of the controls. We adjusted meter 0, and calibrated the meters to properly track output power in both channels. The limit lamps were verified to be operational. Frequency response was flat across the full range. The unit provided about 230 Watts per Channel before significant clipping occurred. Distortion measurements at rated output power showed performance well within specification, shown below.
Performance Evaluation
We had the pleasure of listening to this unit for about a week before returning it to a happy customer. If you have a McIntosh MC2205 that requires repair or restoration, contact Bob the Tech Audio – we are happy to help.
