I finally got tired of the noise and switched off the supply. The bit of
FR4 which had 23KV per inch on it shows no signs of a problem. This is a
lot longer than I need it to survive in the production case.
I had a couple of old analog Kikusui scopes that had an awful pcb
layout in the CRT supply, probably 40-50 KV/inch. They would
eventually accumulate dust, arc, and zap some exotic diodes [1]. The
fix was to clean carefully and glop with thick conformal coating.
John
[1] This was in the cheapie Z-axis coupling circuit. Classic scopes
had two transformer-isolated negative supplies, one for the CRT
cathode and one for the grid, with the cold ends driven by the
intensity control and the unblanking pulse. The cheapie circuit
combines AC coupling with a modulated-level ac carrier that's
rectified on the high side to achieve sorta-net DC coupling to the
grid.