Red Screen Monitor 334

Jana is right!

The following is for everyone who is interested.

A CRT has 3 electron guns, each for each of its primary colour, red, green, and glue. In the modern CRT's the colour guns are in one buttembly, and referred to as a single electron gun. In this case, they each have their own control grid, and each have their own cathode. They share the same screen, accelerator grid, and focus grid.

As the CRT is used, the cathodes wear down, and thus the emission is lowered. They do not necessarily wear down at the same rate. Also there is some phosphor wear as well. Each electron beam falls in to its own phosphor dots or stripes, depending on the type of CRT.

As the CRT emissions, are decreasing with age, there are some circuits involved to compensate, and thus maintain the colour temperature. There are circuits involved that measure the cathode currents and compare them to a reference. As there is wear, the cathode current will decrease. Eventually the cathodes will wear down to the point where it is not possible to get more emission out of the electron gun buttembly cathodes. At this point, visible colour drift will start to be apparent. This colour drift is in effect the colour temperature that is viewed on the screen.

Inside of the monitor there are some bias and drive set-ups to adjust the electron gun drive circuits to match as exact as possible to the particular CRT when the monitor was manufactured. In the modern monitors, these adjustments are software setup, and there are no mechanical pots, as like in the older monitors. If these adjustments have to be tweaked, then there is a fault condition, since they should have been properly set up when the monitor was manufactured. The automatic bias circuits are supposed to compensate for the offsets of the CRT aging. If a monitor can be made to look good after doing some extra tweaking, the fix will only be temporary, since there has to be a fault condition.

Also, when the CRT wears down, the sharpness will also decrease. This is because the overall beam current is decreased, and thus the electron beam width gets wider. Sometimes adjusting the focus supply to the CRT will temporarily fix the problem. This is because by raising the focus voltage a little, the beam current will be increased.

As the tube ages, its overall brightness will also decrease. This where when a monitor gets older, the brightness controls has to be turned up a little. If the colour guns age evenly, then the colour temperature will not have so much drift, as there will be a decrease in the overall brightness displayed. Sometimes the service tech will turn up the screen bias to compensate for this. In effect, the screen grid voltage is being increased to force more emission out of the CRT. This actually accelerates the wearing of the electron gun buttembly, and thus hastens the CRT's failure. Overdriving the screen grid voltage, will also produce visible vertical blanking lines. This is because the reference bias for the blanking insertion is exceeded for the CRT's design.

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Back the early 70's I worked in R&D for television monitor and receiver design at RCA.

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Jerry G. ======

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