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After examination, I saw Q10 had a crack in it.? I imagine this was from excessive heat, plus the bolt being tightened down too much, putting excess physical stress on the plastic case of the transistor.?

How much do transistors expand when heated?? Perhaps what seemed like moderate pressure when I was tightening the bolt became extreme pressure when the transistor case expanded?

I'll be replacing all of the BS170s to?be safe.??

I've been reading comments about thermal management using heat sinks, thermal tape, etc.? What about changing the shape of the transistor?? My thought is to sand down the rounded side of the transistor to make a flat surface that will be easier to connect to a heat sink, and will less susceptible to odd stresses from a compression force being applied to the rounded surface.

So far, I have sanded down over 1mm from that cracked BS170, with no sign of reaching any internal components.? It is now a nice rectangular shape.? If I can sand all 4 new transistors to the same thickness, adding a heat sink over the top of them will be easy.? Perhaps some soft copper tape as well, which will conform to the shapes and fill in any inconsistencies in my sanding.

Would this be worth experimenting with, or am I being silly?