Sorry to post in an old thread, but I finally got the thing working.
After fixing the two diodes that failed as I was evaluating the thing, and fixing the things that I broke and/or were broken when I got it, I was back to the original problem - no regulation.
I'd like to say I used proper troubleshooting technique, but I didn't. I was looking at the error amplifier section and noticed that a ballast tube was open. I checked the schematic, and found this was in series with the 12V to the filaments of the tubes in the error amp. So, I ordered a new one, installed it and it's working.
I don't know why the two diodes failed. I do know that it's tight working on this thing, and it's easy to do damage. (I broke off a wire from the circuit board I was working on, broke the ceramic terminal strip for the mains cord, broke/further broke a tube socket) Other than that, it was a simple failure of a ballast tube.
I don't know what the specs are at the moment (I'll look them up) but it drifts a couple of tenths of a volt at 500v. (I did adjust the trimpot for least error, but it moves around a bit and gains a tenth from 500 volts to 700. I didn't get out my high voltage probe so I didn't go higher.)