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TV pictures get worse as a
set’s cathode ray tube ages. As electrons boil off the cathode, a “blooming”
effect on the surface gradually reduces the distance between the cathode and a
beam-steering electrode. This changes the field strength between them and
degrades focusing, making pictures less crisp and bright. Now Thomson-Brandt of
Germany (EP 969 663) says that the voltage difference between the cathode and
steering electrode should be continually measured and compared with the reading
when the set was new. The difference can be used to control the cathode current,
so pictures stay crisp and bright.