Check with Frank Gow at Audioclassics.com - he may be able to help or steer you in the right direction - they sell and refurbish Mcintosh equipment and have done so for years.
McIntosh hum problem
Any of you MAC people own a MCD 751 CD Transport and MDA 700 D/A Converter combo? I do and both hum, wondered if any others with same problem? The converter hums louder and MAC gave me a new power transformer for it, but did not help. The hum is from the units themselves vs. speaker hum. Any way to reduce the hum. Any advice appreciated.