Bob the Tech Audio recently addressed this McIntosh MC257 Power Supply Repair. The customer complained that on startup the unit buzzed and the meters pegged.
Bob the Tech takes power supply issues seriously. The last thing we want is added damage occuring by powering the unit. We isolate the mains transformer and verify there are no shorts between windings or to ground. Then we check the rectifier and main power supply to ensure it’s operational and no shorts. Last, we pull the heatsinks and evaluate for failed power transistors or diodes. These can short the high voltage supplies to ground. The photo shows the work in progress.
We didn’t find anything wrong with the supply or heatsinks, so we reassembled and powered to evaluate. A bubbling sound and a bit of a stink helped us isolate the issue to the -12V regulator. This is located on the backup power supply board. We replaced the rectifier diode, capacitor, and regulator. But smoke from the front panel surprised us when we powered it up. We isolated this, in turn, to a failed op amp on the front panel. The op-amp runs off of the -12V supply. The op amp could have been the cause of the -12V supply failure, or the other way around. In any event, replacing that op amp had the unit running fine. The photo at left is the backup power supply rectifier diode and other -12V supply parts. The photo at right shows the reworked op amp.
If you need an MC257 power supply repair, or repair of any other McIntosh unit, contact Bob the Tech Audio at our service link.
