OK, that rogue cap is gone.
In preparation for powering up, I checked out the five-element ballast (Emerson # 397023). Most of the elements test bad. There's a lot of corrosion where the elements join the pin leads. Even if I could coax it back into life, this doesn't seem like a good bet for long-term reliability:
I found an eBay supplier with a Clarostat ballast, so let's hope that one works. Can't wait!
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html