This very well built, nice sounding 25 year old came in for a service this week. Not much wrong: checked and set the bias and DC offset was on spec and a visual inspection of all other components and circuit boards. the manufacturer was helpful in supplying the bias settings but claims they don’t have a circuit diagram for the amp any more! It’s an interesting design and its main distinguishing feature is it uses a single MOSFET as the output device giving 100 watts per channel. It also has an excellent on board protection system against unexpected transients, shorts etc.