Rog, thanks for your write ups & pictures. When a component “let’s go” on circuit boards it would seem that the spike or over voltage causes surprising failures elsewhere on the board.
During my education at A level. Me & a mate did a project testing transistors to destruction, testing their heat tolerance. Using different manufacturers & working out what heat syncs would be required.
Simple circuit used on a universal circuit board. Only the transistor was being heat treated.
70% of the time another component failed when the transistor gave up. Often included the circuit board, sometimes difficult to see.
IIRC some transistors failed at temps around 70c. Was in 1970 so the components were quite large compared to todays.
