This vintage amp dates from around 1985, and hails from the Japanese manufacturer Marantz. It was placed at the top of their product range at the time and is very similar to the slightly lower powered PM-84. Its unique in running in both class-A and class A/B, that is for the first quarter of its output it runs exclusively in class-a mode, essentially most of the time. This gives it a trade mark warm and low distortion sound characteristic of class-a, but obviously it runs hot, hence the huge internal heatsinks and top grill.
Strangely this amp didn’t sell well locally in Japan, but did very well in export markets or US, UK and Germany. It was universally rated as a great sound sounding amp. In 1998 this amp retailed for $2900 in the US, hence it is now quite a rare and exclusive item. Also look out for a limited edition dating from about 1989, which featured higher spec and entirely copper chassis and fascia.