With this kind of stuff, there's some electronics, and then a big rugged high-voltage output transistor. 99% of the time, the output transistor fails, sometimes from age, sometimes from it's control circuitry leaving it on all the time and burning it up (a lot like the horizontal output transistor on a CRT-based TV set).

I'll bet it is a Bosch P/N and not available as a separate item. But there are functional replacements out there.

If the unit is disassembleable, it would be interesting to do a post-mortem on it...

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