Hi All. I have some updates.
First, I checked the wiring yet again, and didn't find anything.
I re-checked my voltages, and the grid at pin 7 of the 12AU7 vertical output tube ends up with around +20V on it. It should be zero. The plate voltage started out high at around 420V, but as the grid voltage increased and the tube current went up, it seemed to draw down the power supply to the low 300V range. This is when the picture deteriorated and waveforms became irregular. If I wait a minute more, R77 starts smoking.
1. The grid bias is set by a single 2.2 meg resistor. I replace the resistor and got the same results.
2. I disconnected the input cap (I've never seen a new cap go bad, but thought I should rule this out). Same results.
3. I decided to make sure the yoke wasn't supplying too small of a load, so I replace the yoke in the circuit with an appropriately sized resistor. Same results. (the DC resistance on the vertical yoke is within spec)
4. I decided to make sure their wasn't something going on in the vertical output transformer (DC resistances were within spec), so I replaced it in the circuit with an appropriately sized resistor. Same results.
5. Just for kicks, I disconnected the input coupling cap and tied the control grid directly to ground. Nothing smoked, and the other stages in the vertical chain remained stable for a good while.
6. I had already tried 2 different 12AU7 tube, so I decided to sub in a third. This time it worked for a while. The plate voltage stayed high at around 410V, but the grid voltage stayed near zero. After a while it reverted back to the previous behavior. The 12AU7 also got REALLY hot.
I'm starting to wonder if this particular might be really sensitive to B+. The 20/40/80 uF electrolytics were replaced with 22/47/82 uF modern caps, and this often raises the B+ somewhat. If so, I may have to take some steps to drop the B+ back down to the usual range. That said, I'm still getting my head wrapped around boosted B+ power supplies, and I may be missing something in the theory.
Comments are greatly appreciated.
I put a copy of the schematic online at:
http://metro706.hostmetro.com/~archr.../schematic.pdf
(I maintain the website for our local antique radio club, and it dawned on me that I could simply put the file on our server)