Q: If my old TV set is switched on for any length of time it begins to emit a high-pitched noise. If I hit the TV it solves the problem, but a few minutes later the noise returns. What is happening?
A: The problem lies with a high frequency vibration in the field coils which are used to deflect the electron beam in the picture tube. Resonances within the supporting structures (possibly because of a slight loosening of fittings) cause this to become audible. The amount by which the fittings are loosened does not have to be great or even significant within the TV circuitry, so hitting the TV set disturbs whatever arrangement of mechanical forces happen to predispose the TV to these resonances and so, for a while, you can鈥檛 hear anything.
A: The high-pitched noise is the television whimpering because, being old and abused, it knows it鈥檚 going to be struck before the evening is out.
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A: This is a common phenomenon in my area. It even has a name, according to a local expert: 鈥溾�榮 on the blink, mate鈥�. What鈥檚 happening, it seems, is that 鈥渢he wossname鈥檚 arcing against the thing at the back what regulates the current鈥�. It can be stopped, but not, apparently, if you expect to see much change out of 拢40. You鈥檇 be better advised either to put up with the noise without asking too many questions, or to take your TV to the local dump.