[Maarten]

I'm not surprised the TDA8351 failed. They are run very near their maximum ratings (in some sets from Vestel and Grundig, they even failed routinely. The cure was to lower its supply a few volts by adding a small coil in series). Also it is normally fed from the line output transformer so if the tuning cap is suddenly low or open, the tension rises above the maximum value even before the protection kicks in.

As to the BU508D, it depends on the design of the set whether it will work reliably without modifications or at all (Sony sets are quite critical, I don't know about Formenti). If you get the TV going again, switch off after 10 minutes and touch the heat sink. If you can't comfortably hold it, chances are you need a S2055, BU2508D or even a BU2520D.