This McIntosh C47 was in for repair for severe output distortion. The issue was intermittent. Sometimes the signal was fine. At other times the distortion was so bad that it pushed the power amplifier that followed into Power Guard. The intermittent nature reminded me of similar issues where faulty board grounding was to blame. The grounding proved to be the case here, and it’s a simple fix.
Take a look at the grounding screw and star washer on the rear of the unit in the pictures below. These screws are designed to provide a single point where the internal board signal ground point is tied to chassis ground. In some cases, the star washers are unable to bite through the black chassis paint. I measured 2-3 Ohms from audio board signal ground point to chassis on this unit. I’ve seen other cases where it measured 15 Ohms or more relative to chassis. Isolating the signal ground in this way provides an unreliable voltage reference for the audio board circuitry. When this happens the outputs can float or oscillate to inappropriate levels, which caused high output distortion in this McIntosh C47.
This is the lone grounding screw for the C47 audio board. If it’s not tied well to the chassis, performance will suffer 
Internally the screws ties to a ground point on the board
I’ve found the fix is simple. Scraping a bit of paint off the chassis under the screw head allows the star washers to get a good bite on the chassis when installed. This enables a reliable contact between the audio board signal ground point and the chassis, and a distortion-free output signal. If you have issues like this and need some help resolving it, contact service here.
