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I am looking at the sams photofact for that model...it is based on a slightly different model (8-PT-7030) but that is where the sams index directed me. The vertical output stage gets its power from boost voltage...when you say disable the hv do you mean remove the damper or HO tube...if so then the vertical stage is running on B+ alone as boost is not being developed.
Sounds like the stage is not oscillating and is drawing too much current and dragging down the supply voltage as no bias is being developed...you might try feeding 60 hz ac filament voltage into the grids of the tube through a .047 600 volt capacitor and see if you get any vertical deflection or negative bias voltage on the stages...may need to check the feedback loop between the discharge and output sides of the oscillator.
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