The box can be opened and the transistor inside replaced. It's easy to do, once you know the right kind of transistor. There was a page up at
www.mbz.org that had the info. It got lost in one of the system crashes a few years back.
I found a nice page that describes rebuilding a distributor cap. A handy thing for an M189...
http://articles.mbz.org/engine/gas/dist/
But that's a subject for a different thread. -CTH