Repairing old electronics and test equipment down to the component level with a friend of mine who's been doing it since the 1950s.

You'd be surprised how many times it turns out to be connections that look ok, actually aren't. I've seen bolted ground lugs that were open circuits. I've learned to despise silver mica capacitors.

Oh, and with vacuum tube based gear, letting some smoke out doesn't necessarily kill things dead. The smoke does help localize the problem though.

I like that also. The smell of some of the old gear is unmistakable a lot of the time too.