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-   -   CRT decay when old ? (http://eab.abime.net/showthread.php?t=106072)

LeCaravage 03 March 2021 16:51

CRT decay when old ?

I noticed a pb while coding on my Amiga wired to a 1083S. I tried to fix a copper pb during several days and when suddenly had a doubt. May be it's my 1083's fault ? I mean the display is wrong. So I made a test on WinUAE and I was right, my 1083 is probably too old and the image begings to be bad.

Anyone where it come from ? Is it fixable ?

Here is screenshots, the 1083 is blurring but you can notice the red line seems higher and the blue one seems too low (almost 1 vertical line pixel error).



Rotareneg 04 March 2021 22:33

Colors not being aligned on an CRT is due to poor convergence, which means the electron beams for red, green, and blue are not aimed exactly where they're supposed to. This is generally adjusted by rotating the magnets that surround the neck of the CRT, but it's a notoriously tricky process that can take a while to do even if you know what you're doing, and that's assuming the 1083 even has adjustable convergence (not all CRTs do.)

As far as CRTs "decaying", yes, they do. The values of various electronic components can change with age, the electron guns wear out, and the phosphors degrade from use (that's what causes burn-in.)

Bruce Abbott 05 March 2021 03:02

1 Attachment(s)
A bit of convergence error at the edges of the screen is normal. Each electron gun aims at the screen from slightly different position, and the screen surface is not perfectly hemispherical, so the geometry is distorted in different directions for each color. This is corrected with a combination of adjustable permanent magnets around the tube, and dynamic correction by modulating the deflection waveforms. It is very difficult to get all 3 beams perfectly aligned, particularly in the corners of the screen.

As electronic components on the PCB and parts of the tube age the convergence often drifts further out of alignment. However if your 'almost 1 line' is only in the corners that may be how it has always been.

To check convergence over the whole screen and gauge whether adjusting the alignment might be necessary, display an image with a grid pattern of white lines on a black background. Where convergence is not perfect you will see the white separating into colored lines, like this:-

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