This evening, I changed the two BU406 power transistors in the ultrasonic cleaner. They're TO220 parts on quite large heatsinks, and both of them were shorted collector-emitter. I just put in new ones, and replaced the fuse, hoping that they wouldn't simply blow again. Well, they didn't, and the circuit worked, and the fuse was OK. So now I have an ultrasonic cleaner that not only gives the cleaning bath a good shake, but also illuminates it with blue LED light while doing so. Makes it work better, of course.
So, now I'm looking out for small dirty things in my house.