The heatsink is a large aluminum fin style with Q7, Q8, and Q9 attached. When I received the amplifier for repair it had what looked like white lithium grease between the transistors and the heatsink and no mica insulators. I cleaned and replaced the grease with Arctic MX-6 Thermal Compound when I replaced the transistors. Could that be the problem? Does Q9 need to be isolated?
Small update: The version of the Q9 D600K that I used has a metal backplate that is connected to pin-2 the Collector (the original did not have an exposed backplate). I'm going to make an insulating wafer and will update soon.
Small update: The version of the Q9 D600K that I used has a metal backplate that is connected to pin-2 the Collector (the original did not have an exposed backplate). I'm going to make an insulating wafer and will update soon.