Usually it is a relay on the board or its driver circuit. You could probably get any decent repair shop to jump it out(bypass the mute circuit.) You would at least have sound but no more mute. In the absence of a replacement board===HMMM Can't believe they didn't keep replacements. Seems kinda of questionable on their part.
marantz SR-18 receiver defect
My Marantz Sr-18 receiver went into the mute mood. I am told that the small board conrolling the mute and set-up memory is bad and must be replaced. However, despite being only 5 years old, Marantz said that they do not have this board any more! Any words of wisdom ou there?