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blurry corners and old CRTs

Jamieson

Well-known member
In the thread "Fan replacement for a Mac SE/30" user techknight wrote:

Also with CRTs, the more you use the CRT and the brighter its used at, the more cathode material that gets burned away and oxidized. Eventually you will lose enough cathode material to cause a drop in emissions. Emissions drop ever so slightly as the tube wears. But there is a point at which it begins to affect the picture.

The first thing that happens is the corners get blurry/out of focus, and when adjusting focus, you can sharpen the corners but the middle goes blurry and vice versa. Then it get so weak that the image begins to "bleed" from oversaturation of the drive signal. The saturation point drops as electron emissions drop.

On color tubes, the first to go is usually red and green. But not always. You can tell when the emissions drop below 25% is when the color bleeds like fire off to the left side of images. Such as if you have a nice bright sun in a movie scene, the sun has a blurred fiery flare coming off the left side, and it looks "washed out". Thats the sign of a dead tube, again from over-saturation of the cathode drive from the loss of emissions.

When a color tube starts looking like that, that means your emissions are now lower than 25% of health.

black and white tubes (like our compacts) will "bloom" and look fuzzy around the edges, and it WONT sharpen up with the focus control. But the brightness level has a severe effect. Thats a weak black and white tube.

I'm seeing this behavior on my SE/30. Blurry corners, gets worse when turning the brightness up. Adjusting the focus pot on the analog board can make the center or the corners better, but not both.

Also seeing a dark streak/smudge to the right of bright screen elements that becomes more pronounced when the brightness is turned up.

I have yet to recap the analog board. Is it reasonable to expect any improvement after a recap, or is this just a symptom of an old/bad CRT?
 
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