Bob the Tech recently had this McIntosh MI254 in for repair. The MI254, the power amplifier at the bottom of the preamp pile in the photo above, is a class D design. Class D is a very power-efficient approach whereby the power devices act as switches. The input signal generates a series of square pulses of varying widths and intervals. That pulse train turns the power transistors on or off. The power transistors effectlvely switch the output between the two B+ and – voltage rails. Switching on and off, rather than operating in their linear regions, reduces the power dissipation. The output is filtered to eliminate the high frequency switching content, producing the output to the speakers. The high efficiency allows this unit to produce 4-channel output at 250 Watts per channel in such a small form factor. The result, when you open the unit, is that it looks like one big power supply inside.
This unit was not powering fully. The power switch resulted in the first few relay clicks of the startup sequence. The front panel lamps and logo lit. But that final “click” never came, and there was no audio output. The thermistor arced and damaged. Bob the Tech replaced that first, photo upper left. But if the thermistor failed, that typically means there is something else downstream that was the cause. We evaluated the components and isolated the issue to a failed power MOSFET in the power supply which had shorted the -85V B- supply to ground. The new FET, shown installed, top center in the photo.
Once replaced, and the power supply reassembled (top right), the unit performed perfectly. In the photo below it’s easy to see how much of the unit is power supply. We used this MI254 as the power amplifier in our McIntosh Preamplifier Shootout, and it sounded great. Click and read more about those preamplifiers and repairs.
If you have issues with your McIntosh MI254 and need repair help, contact Bob the Tech Audio!
