Hi Jeff, I am glad that I was able to contribute this this forum.
I would have to say, in my opinion, since you started to clean that section, you should keep cleaning it until it is spotless. I have read from another post that you don't clean this section well, that you would actually create leakage and that would throw your HV into chaos. Once it's all clean and dry, you should be able to power it up without any problems. My 545A was filthy, more so than yours most likely was. Mine was covered in all that soot, but was covered in cigarette tar so I had to clean it. I mean, it was pure yellow and sticky and I am not that knowledgeable so I learned the hard way that all this smoke and tar gave me a LOT of HV issues. I am sure that others on this group will chime in as I am still learning about these old scopes with my 535A being my favorite. I would also advise to check and reseat the 12AU7A and the 6AU5GT tubes that are on the underside of the HV oscillator, at least that is what I would do myself.
Brenda
