Below is a magnified picture of a Portacolor screen displaying a screen full of horizontal white and black lines. I circled a region of the screen that has some debris stuck to the shadowmask on the inside of the CRT. If you look closely at the black line below the white line with the debris, you can see the cathode ray is bouncing off the debris and bleeding in to the black line, lighting up those phosphors a bit when they should be not illuminated.

Where does this debris come from? From the burnt layers of the cathodes? Is there a safe way to shake this debris off of the screen?http://www.videokarma.org/attachment.php?attachmentid=203838&stc=1&d=1641959758

I've never seen a way to remove it. I would be worried about dislodging the shadow mask if you try too hard.

Sometimes it's material inside the tube sometimes it's a manufacturing defect. CRTs were graded A,B,C,D etc. I think the grade C or D were often etched TEST TUBE on the face and installed in repair shop test jigs for console chassis hauled in instead of taking the whole set.

Delete duplicate post.

I was thinking of a vibrating device like a mobile phone, PS5 controller, or massager pressed up against the screen with the CRT tilted neck down. If it dislodged the debris it would probably cause a short within the electron gun. Focus anode shorts solve themselves due to the high voltage differential the short crosses. Other shorts are easily fixed using a CRT tester.

Are you sure its debris and not a missed phosphor? I wouldn't mess around with this given the rarity of the disposable portacolor

That’s an anomaly probably best left alone. From viewing distance it’s probably not even noticeable.

There's no way to mechanically vibrate the mask since it is attached to the bulb only at a few points on the perimeter. Any shock strong enough to dislodge dirt would surely distort the mask permanently.

As Arc-Tech 109 stated, it's not 100% sure that it's debris that fell on that spot later and not a manufacturing defect. If it was on the mask when the phosphors were applied, then removing it won't restore the phosphor dots anyway.

A working CRT is too valuable to risk destroying when they're no longer being made.
I understand your intent is to get a better, working tube, but I would be content with it just working as is and skip the considerable risk of damaging it even worse.

Note that manufacturers never fixed this kind of defect at the factory, but either scrapped the tube or marked it as lower grade. This is a good indication that no-one ever found a way to fix such defects.

I think its missed phosphor. If its debris most likely its welded to the shadow mask due to the action of the electron beam.
I'd live with it.

I'd leave it. With the rarity of a replacement CRT and only 4 or 5 dead dots who needs to poke the bee's nest. You might vibrate the shadow mask loose or create a new misery like a heater to cathode short. I've seen B&W tubes that were carried face down and they ended up with bits of junk on the screen. It doesnt move or go away either, or at least from what I've seen.