Why do old TV’s always have that annoying high pitch noise when you turn them on?


Why do old TV’s always have that annoying high pitch noise when you turn them on?

In: 3

That is because of the trasformer which transforms the high voltage for the anode. This was mainly in old CRT TV s (not flat)

In order to generate a picture, the electron beam inside the CRT has to sweep back and forth across the entire screen very fast–on a PAL TV it has to scan 625 lines every 1/25th of a second, which is over 15kHz. The high voltage components that sweep the beam make noise while doing that, which you hear as the high-pitched noise.

Old TVs used a cathode ray tube. It’s a giant glass envelop, and formed the picture on one end of it. That’s why if you tapped on the TV screen, it seemed like a giant glass bottle. It’s also why these TVs were so deep and heavy. Inside there was a vacuum, so that electrons could be launched across the tube to form the picture on the front.

It was a pretty long way from the back of the tube to the front, so it took a lot of voltage to fire an electron that far without any distortion. this is also why a magnet distorted the TV screen. It’s about 30,000 volts in a family room size set. That’s a super dangerous voltage, so when it’s on wires they are held up by the Godzilla Stopper towers. That’s why the TV said “No user serviceable parts inside” Taking the covers off could expose one to this lethal voltage.

The voltage was made with a flyback transformer. The power from the wall was modulated to about 40 Khz before it was fed into this transformer, so that the transformer could be smaller. A line frequency transformer that cold step up to 30KV is the size of the thing you see at the local power distribution station. When the transformer and the circuit that powers it were powering up, this frequency was applied to many parts of the high voltage part of the TV. Any little gap would vibrate at that frequency and all it’s harmonics, and you’d hear the result. Once things warmed up and got to their normal operating power level, the sound typically died down.

Please don’t answer if you don’t know.

The picture is drawn on the phosphor screen of the tube by bending an electron beam with electromagnets. The beam is scanned across horizontally to make lines, and then much more slowly vertically, the scan position moves to the next line and the next line is scanned across the display, to make 60 fields per second.

The horizontal scan rate is 15.734kHz, a very high frequency or pitch within the hearing range of young listeners. The beam jumps from the right of the screen to the left 15000 times a second. The magnetic fields of electromagnets can cause internal vibrations that are radiated into the air: sound.