TV Repair

Monday, January 13, 2014

Q. I am studying tuners in the TV lessons. In some of the schematic diagrams, there are heavy lines boxing in the components. What are these?

A. The heavy lines represent shielding. Higher VHF and UHF circuits are so sensitive to stray coupling that they are enclosed in compartments to isolate them. Small holes in the compartments allow signals to be coupled in and out of them. Otherwise, merely bringing your hand close to the controls on the TV would detune the circuits. Also, the circuits emit radio-frequency signals that can interfere with other services such as aircraft radio, police, and fire. The shielding suppresses this kind of radiation.

Also, instead of using coils and capacitors for tuned circuits, they use strips of metal. The combined self-inductance and capacitance of these strips is all that is needed to make tank coils. A high-frequency transformer is made by placing two strips close together. This is indicated on some of the schematics by the letter "M" and some arrows.

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