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Update: I located RCA's service bulletins for this chassis and there were several relating to burn-in, and I've performed the recommended preventative maintenance. There was also a bulletin about confirming proper operation of the spot-killer circuit, and I performed that testing and it works as intended.
I had come to the conclusion that the burn-in was likely from power cycling the set too often and too rapidly during testing of my modifications, since if you power cycle the set very quickly there is a bright vertical flash when it turns back on. So I've been trying to avoid that.
However, I've now got a SECOND burnt-in spot except this time its probably 2mm x 2mm in an almost perfect circle and the phosphors are straight up GONE. Entirely. Like not a trace of them left in that spot.
Other than that extremely brief flash if you restart it too fast, there have been absolutely no visible spots on the screen, and unlike the first burn-in which was vertical and almost certainly caused by the power-cycle flash described above, this new one is round. I'm trying to get this chassis fixed up so I have a known good chassis if I come across another tube, but I am just utterly stumped at this point for what could be causing this. All the service bulletins relating to burn-in have been performed, and I have followed RCA's guidance on confirming proper operation of the spot-killer circuit and it works fine.
If anyone has any additional thoughts it would be appreciated. Thanks.
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