I've been giving my soldering iron something of a flogging of late. It's a Metcal MX500P, one of the very finest irons ever to be made. It uses 13.56MHz RF to heat a teensy inductor right in the tip, with an incredibly ingenious temperature regulation system whereby a magnet in the tip, once it gets to temperature, hits it's Curie point and loses magnetism, changing the frequency the coil resonates at and thus making it get more or less hot with the applied 13.56 MHz. In any case, yesterday it simply stopped getting hot. I found an extremely good on-line analysis of the soldering iron here., and mention of dead driver transistors. in forums.
In any case, I opened it up and had a look, and there was a pretty obvious popped capacitor:
It's C4, a 330n 63V film cap that just does smoothing on a DC rail. Here it is after removal:
I replaced it with a 330n 100V one, and voila.