I just went and looked at the schematic. Here it is BTW, thanks to a thread Phil Nelson posted a while back:
http://antiqueradio.org/art/temp/RCA630TSSchematic2.gif
http://antiqueradio.org/art/temp/RCA630TSSchematic1.gif
I don't know exactly why that capacitor is there, but my best guess is that it's bypassing various trash from the Horizontal and Vertical scanning, and keeping it out of the rest of the set. Nothing looks like a voltage doubler to me.
It looks like in normal use, that cap would have 5vdc across it. However if the Horiz or Vertical centering pots opened up, most of the *unloaded* voltage of the power supply, maybe 350vdc would appear across that poor little 50vdc cap. That might have got it pissed.
If it was the vertical centering pot that opened up, c225b might not be too happy either.
All the best,
John