Red Screen Monitor 335

Colour temperature on a monitor can drift out of specs, if the CRT is going weak, or if there is a drive or bias problem in the circuits that feed the CRT. The most common problem is that the CRT goes weak.

For a well detailed writeup about CRT's used in TV and monitor equipment,

CRT Degradation CRT Aging - Effects on Electrical Characteristics and Performance Specifications for Philips CRTs can be found in the regular series of data books from Philips Components. Companies and universities usually have them. Usually the data sheets show typical Ik-Vk characteristics. They also list the spread on cutoff voltage and cathode gain, and this spread is quite large even on new CRTs. They also list phosphor sensitivity (Lum-Ik), this too has a large spread. But they almost never list anything about the aging process.

Here are some of the effects:

a.. Phosphor ages due to burn-in, particularly on static pictures, this is immediately obvious on visual inspection. If the aging is even (no pattern) then at least the efficiency is reduced. b.. Cathodes age due to loss of emission material, particularly for oxide cathodes. The central part of the cathode surface has carried the most current density and will wear out first. The surrounding area takes over, this will lead to an unsharp picture. Adjusting the focus voltage will not really improve it. The tube is worn out. c.. Also contagioning of cathode surface may occur. This can be cured temporarily by short-time overheating ("re-conditioning"). d.. The cathode that wears out first (often the red one) also loses gain, so the white point of the image will shift (to cyan). The white point can be re-adjusted with the gain potentiometers and the contrast, but peak brightness will not be as high as new. e.. The cutoff voltages of all cathodes will drift. Common drift is adjusted by the user by controlling the brightness. Different drift leads to a coloration of the black background level. In extreme cases vertical flyback lines will appear. Cutoff voltage can be adjusted with potentiometers, or there is automatic stabilisation. Still, the VG2 (screen) may need periodic adjustment too. f.. Leakage currents may disturb VG2 and focus voltage, re-adjustment has only a temporary effect. g.. VG2 and focus potentiometers may wear out due to electromigration etc. A hole may form under the wiper, re-adjustment is then impossible. h.. Some types of cathode wear (according to a friend in Philips Semiconductors) can cause the Ik-Vk transfer characteristic to divert so much from an ideal gamma function that no adjustment can compensate for it. Then the tube is really worn out. I hope that this helps you to distinguish between a really worn out tube and one that still has some life in it after re-adjustment.

It is best to engage your brain before operating your mouth!!!

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JANA

JANA "color temerature drift out of specs"????? Horsemanure!