Thanks for the suggestions everyone, I will start with cleaning the soot.
Is this the storage version?The D2 is the non-storage. The front-panel intensity pot is very linear, delivering -30..30V to
the HV board. But the intensity range trimmer under the shield is hard to turn and acts oddly,
I wonder if it is contaminated with soot just like everything else there.
I hadn't thought of soot as conductive, though I guess that's obvious in hindsight. The HV adjust
is also in the sooty area, and HV range is also not working.