Now if somehow the sparking was outside the tube, it would mean that there was some conductive path for the HV to follow, which I think is unlikely.

Sparking inside the tube, looks like where the HV comes in, would make me think that the tube is no longer a vacuum. Which means, "it's dead Jim"

That does look like vacuum is lost, in which case there's nothing to do...

The blue stuff on the pots on the flyback is glue so the pots would stay where they are after adjustment at the factory.

BactaTankAddict wrote on 2025-01-19, 07:09:

A third theory would be that the anode cap needs regreasing, since there's a dark shaded circle around the tube's anode. I can wipe this circle off with my finger, not sure if I should be doing that, so I left it alone.

HV attracts dust and soot. You should clean it with paper towels or some soft cloth (which will end up in the bin afterwards). Soft painting brush might help getting to hard to reach places like yoke/neck area. Be careful not to disturb anything on the yoke (although all the magnets and knobs here should be glued in place with paint). Clean the HV cup and wire too, but preferably discharge the CRT first or you might find any weak spots on the insulation with your hands.

Note there's a special conductive coating on the outside of the CRT - aquadag. Needs to be grounded by wires/springs touching it. As the name suggest it might be water soluble, usually is but some are more resistant than others. Which means you should be very careful about using any water on it, preferably just don't. What is dirt and what is aquadag? You can spot that most easily around the cup, the aquadag doesn't reach there on purpose to prevent HV arcs. Clean a bit of that area and you'll know what is what. HV wire and cup can be cleaned with water and soap, it actually helps a lot, just make sure it's bone dry before reconnecting. Not all cups are installed using silicone grease, dry install also works on clean glass. But adding some silicone grease might help.

Color CRTs have another HV lead, it's the focusing electrode in the neck. Runs at about 6kV or so. Sometimes dust can accumulate in the CRT socket and cause arcing between guns and electrodes, that's also bad. Gently pull the socket (might be hot-glued to the neck!) and clean it, you'll know which is the HV pin because it's better insulated from others. Make sure it's not burned, that happens too but usually you'd see the picture loose focus if that happens.

All in all though it does look like a gassy tube. I'd still clean it just in case, and that at least you can do.

With the cover off, discharge the tube clean everything up.

Fire up the screen once it’s clean and dry with the cover off.

Most of the time the flyback or the big windings short out.

If there is a short in the windings it’s only repairable by rewinding it which is tremendously time consuming and difficult by hand

If it truly is inside the tube it’s gassy and nobody fixes that anymore, would be a tube swap scenario now days

That looks pretty similar to a problem I once had:

CRT randomly makes popping noise

Long story short: I looked around for a TV repair shop that still could deal with CRTs, handed it in, and after a few days the technician said that it was the tube itself (he diagnosed it with a tube tester that he still had). The monitor was disposed of and I got another used CRT monitor as a replacement.
That's all I know.

Concur with the others, it looks like the tube has lost vacuum an is arcing through... unless somehow the HV anode has come disconnected or there's a break in its insulation.

To confirm, unfortunately, you would have to have to completely risk your life here...
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Nah, that's a joke 🤣

There won't be that much of a risk if you follow some proper procedures here.
Start by connecting the monitor to power and push the On/Off button to turn it on. Once it does its sparking thing/dance, DO NOT press the On/Off button again. Instead, directly unplug the monitor from the wall.
Next, wait a day or so, then proceed to open the monitor. This typically takes 4 screws holding the back cover on. I like to open mine with the tube screen facing down to the floor, so that way if the monitor doesn't have a bottom support below the back cover, the tube won't fall back and break.
Next, connect a power cable to the monitor, but DO NOT plug it into the wall. Connect the power cable to a an extension cord/tap that has an On/Off switch (make sure the switch is in the OFF position)... or if the extension cord does not have this switch, then only connect the power cable to the extension cord/tap once instructed to do so.
Then turn off the lights in the room. Preferable you do this in the late afternoon or early evening with less light, but not when its completely dark in the room.
Now set up a camera to film what happens when you plug the monitor into the extension cord/tap and/or turn the switch on the extension cord to the ON position. Make sure the camera is a good 1 meter away or more from the CRT monitor, and you yourself just as far.
Finally, plug/switch on power to the monitor to film what happens. Also probably worth observing yourself too. May want to repeat that several times so we can see it better. Upload the video when done.

With a little luck, maybe it's only the anode wire that has a break in the insulation and is arcing to the CRT's case/shield somewhere. But if you can't see any bright arcing above the tube or the arc appears to come from within the tube, then that's a sign of a CRT that's lost its vacuum. A lot of times, a CRT that has lost vacuum will also appear to show arcing inside close to the socket on the neck board.

For those of you who have a little more technical knowledge: another test for a (badly) leaked tube is to remove the neck board and apply about 5V to the tube heater pins. If there's no orange glow after 5-15 seconds, that could be an indication the tube has gone to air. Unfortunately, this test is only useful if the tube has lost its vacuum very badly. If it's only leaked a little, this test might not show it.