What could cause a handful of dead phosphors seemingly overnight? RCA MM36110
 

Hello, I've got an RCA MM36110 TV and seemingly overnight it developed a small vertical row of dead phosphors (maybe 1/4" to 3/8"). I've not been displaying anything that would have caused burn-in in that specific spot, at least not that I'm aware of.

Sometimes on startup there is a brief vertical flash in the center of the screen, but it's an inch or two ABOVE these burnt-out phosphors so I doubt that caused it.

I'm assuming at this point that its not fixable, I'm just very curious what might have caused such a thing to happen.

Image here: https://imgur.com/a/VKnfOSA

It's very small, but dead in the middle of the screen and extremely noticeable.

I'm guessing that one time your vertical flash wasn't very tall and fell on this spot, burning it, but I have to say I've never seen a color CRT burn like this.

Has anyone else seen a color set get a burn from deflection failure? Does this set maybe have a spot killer circuit that's not working?

That was my thought. I remember watching a YT video about center burn-in being caused by a bad cap that eases the shut off process when the beam collapses, or something like that anyway.

In any case, the burned spot in the center of the screen basically looked like that.

Yeah, I've posted this question in a few places and screwed up spot killer circuit seems to be the leading theory. I'll have to dive into the service manual tonight and read up on that.

At this point I've basically written off the tube as a loss. Since the black spot is dead center, it's going to be very distracting any time there's a solid color in the middle. And its extremely distracting on 31.5khz inputs where there's not big thick scanlines to help hide part of it. But I'd still like to get the chassis working properly to prevent further damage, and also because I've started searching for anyone who might have a dead MM36100/36110 with a good tube that I could use for a replacement.

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.

Just a dumb question - do you always turn it on and off with the power switch, or do you sometimes pull the power cord with the switch on?