I haven't yet - is that a common failure?

One thing I didn't mention could possibly explain the black screen: when I first opened it up, I noticed the red suction cup thing (cathode?) from the analog board wasn't connected to the CRT. I pushed it back into place, but it didn't seem to "snap" into place the way I expected it would. I think the metal clip inside the suction cup was a little deformed, so maybe it didn't seat properly into the depression in the CRT. Anyone ever seen that happen? I didn't want to mess with it much, since this is the exact part of those Macs where the high voltage lives. Bzzzzzt!

But honestly, this Mac is so dirty and banged up, with one corner chipped off, it literally looks like it came out of a dumpster. It probably died and was thrown in the trash after somebody pulled the motherboard.