The burn was worse than first thought, had to take the POT apart and scrape out the carbon burn.
https://i.imgur.com/SrGOwaf.jpg
Seems that they did not use a high enough voltage rated POT and it arced to the metal frame, also the voltage on the center pin seems to be a bit higher than shown in the SAMs.
https://i.imgur.com/YidsRwG.jpg
cut metal away from center pin area to keep it from happening again.
https://i.imgur.com/Uu0v6oY.jpg
repair done
this seems odd, cause the linearity pot which has MUCH less voltage on it, seems to be built with better isolation and higher voltage capacity.
Anyway, once it was dry, put it back in and ran it OK for 90 min.