Every receiver or integrated amplifier generate heat of varying levels. The output processors on the amplifier board are just one of the more heat-sensitive items. This heat increases as you crank up the volume, usually. The output transistors are mounted to a heatsink and some of those have aggressive cooling fins. These are almost always passive which means no fan actively moving air. It is very important that the output transistors have the ability to move heat as efficiently as possible. Over time the factory thermal grease dries out and loses the ability to transfer heat efficiently so the outputs can retain the heat they should be shedding.
The output transistors need a way to help move the heat from themselves to the heatsink and that is by way of a mica insulator and thermal grease. The mica insulator (electrically insulated) is used as a barrier so the conductive properties of the transistor can't touch the metal heatsink. It also helps move the heat along with the thermal grease, which is also non-electrically conductive, into the heatsink.
Showing one of four output transistors (Pioneer SX-750) removed from heatsink and showing the old, dried-up factory thermal grease and mica insulator. You can see that the factory did not apply enough thermal grease at time of the build. There should be enough to thinly coat the entire surface of the transistor and heatsink surface, but not too much as to act as an insulator itself!
Heatsink pad cleaned and ready.
Output transistor cleaned and a very thin layer of silicone-based thermal grease applied. New mica insulator ready to be installed. A very thin layer of thermal grease will be applied to that, too, after it is installed to the transistor.
The transistor is now ready to be re-mounted. 3 more to go!