> Your description made me think BU208 which is a TO-3 TV line
> output transistor, it was sourced from SGS amongst many others
> and I've seen it used in ultrasonic cleaners

That's encouraging. I don't have any BU208 but somewhere around
here are some BU508, unfortunately in the TO3P

> Worth checking resistors as well, high value resistors tend to get 
> higher in value if used near their rating.

Couple of power resistors have gone quite flakey. Looks more like
an effect of sustained high voltage than wattage (ie no heat charring)
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