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I took a poke at the schematic, and I am also scratching my head. The fact that the voltage droops to that extent usually means something upstream is severely limiting the current, but the only thing I can see "upstream" aside from the choke would be a weak 5U4G rectifier, also possibly one side of the line transformer may be disconnected or open, since if you were to try to pull full current over a half-functioning rectifier in that configuration you'd get somewhere around 90v and a whole boatload of ripple. Can you hook a scope up to the B+ line and examine it unloaded and loaded (without and with the caps). If it gets all wiggly when you load it then I could be on the right track.
Easiest thing though is pull the rectifier and confirm you have both sides of the B+ winding present and functioning, then scrutinize the rectifier itself.
Hope this helps, or at least gives you a few new trees to bark up.
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