jabusch@uiucdcsb.Uiuc.ARPA (09/16/85)
Yes, all TV's are this way... the high voltage required to drive
the electron guns has to be DC. The standard way to create this voltage
is to generate a low-frequency signal that is above hearing range and then
amplify it. The amplification is done through a transformer, usually re-
ferred to as the 'flyback' transformer. It's basically just an AC stepup
device. The next stage is a rectifier of some sort, which supplies the
final DC needed for the CRT.
The problem with these circuits usually starts when the original
oscillator goes off-frequency a little. Since this oscillator runs around
20KHz, any variation can throw it into audible range. The ringing sound
is usually caused by the windings of the transformer, the laminations of
the core, and occasionally nearby components vibrating slightly in the
strong magnetic field. Remember, the output of the flybak must be at least
1000VAC to operate most of the CRT's in use, and must be much higher, gen-
erally speaking, for color CRT's. Some stereo amps use the same style of
high-voltage generation, but this is rare today. Dimmers are not always
potentiometers, but are occasionally made of oddball parts. I have seen
the internals of a few, and would hesitate to guess what exactly is being
done internally.
John W. Jabusch
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ARPA: jabusch@uiuc.arparjr@mgweed.UUCP (Bob Roehrig) (09/18/85)
Subject: Hearing TV sets (etc): In a TV set, and most video monitors, The High frequency signal heard is normally 15734 Hz. (In the old Black & White transmission days it was 15750 Hz). This is the frequency used for the horizontal deflection circuits. This signal (basically a sawtooth waveform) is used to drive the horizontal coil in the deflection yoke. This is usually a high current circuit. The same signal is used to develop the high voltage used to power the 2nd anode in the CRT. The high voltage is developed either by a winding on the flyback transformer (then rectified) or by a transformer and a voltage multiplier circuit. The reason for using this high frequency signal for the high voltage source is that it needs little filtering after rectification. It is also safer and cheaper than using a separate high voltage power supply. (Many years ago the high voltage supply was separate and people were killed by its high current capability). In the case of color tv's, this rectified voltage often runs near 24 kilovolts. Any component, typically the transformers involved, can vibrate at this frequency and be heard for quite a distance. The problem is normally eliminated in manufacture by proprer potting of the devices that can vibrate. (By the way, The horizontal TV oscillator is locked on frequency to the received signal. The only time it will be off frequency is if the horizontal hold control is WAY off, or there is a malfunction in the set). Light dimmers: Light dimmers use a TRIAC type device to control the lamp intensity. They are controlled by a pot. Unlike a variable autotransformer or a pot, the peak voltage out of the dimmer is always the same. What is actually being varied is the duty cycle of the waveform (or the pulse width). When the triac fires, The edge of the waveform is sharp, not sine shapped. This sudden "kick" is what can cause the lamp filament to vibrate.
bmw@aesat.UUCP (Bruce Walker) (09/19/85)
[Articles are packed by weight; contents may settle during shipping] In article <5700094@uiucdcsb> jabusch@uiucdcsb.Uiuc.ARPA writes: > > Yes, all TV's are this way... the high voltage required to drive > the electron guns has to be DC. The standard way to create this voltage > is to generate a low-frequency signal [...stuff removed...] [...] > The problem with these circuits usually starts when the original > oscillator goes off-frequency a little. Since this oscillator runs around > 20KHz, any variation can throw it into audible range. [...] Not quite. The flyback transformer is part of a sawtooth waveform generator that is phase-locked to the horizontal sweep frequency which, in the North American TV system, is supposed to be 15.75 KHz. This is *already* audible to most young people and a lot of older folk as well. I agree with your other comments about vibrating parts, ageing components can cause the most horrendous squeals from TV's. > Dimmers are not always > potentiometers, but are occasionally made of oddball parts. I have seen > the internals of a few, and would hesitate to guess what exactly is being > done internally. Dimmers are *never* pots (at least not since the thyristor (1960's?)). The average dimmer uses a triac (sort of a bi-directional silicon switch) to control how many degrees of each half-cycle of the 60Hz line will be applied to the light bulb. If the bulb "sees" 180 degrees, it's full on; if it sees 90 degrees, it's about half bright, etc. The pot controls the phase angle (number of degrees) until "firing" the triac. Buzzing sounds coming from a dimmer are probably caused by some component breaking down (ageing) a bit inside. My guess is that one of the larger capacitors' plates are vibrating. Most cheap light bulbs will whine or buzz when dimmed sufficiently, this is just an indication of the poor design of the filament. If one springs for a $6 Splendor spot light (for track lighting), you will never get any annoying sounds from your lights. Bruce Walker {allegra,ihnp4,linus,decvax}!utzoo!aesat!bmw Join the march to save individuality!
hrs@homxb.UUCP (H.SILBIGER) (09/25/85)
Ringing in the ears is a hearing disorder with the name tinnitus. Hearing the 15750 Hz tone emiited by most TV receivers and many vdt's is just good hearing ability. If this bothers you, wait 10 years and it will go away. There is no evidence that it will harm your hearing, unless it is very loud. By loud I mean subjectively equual of the loudness of an automibile horn one foot away from your ear. I doubt that is the case. Since the subject of hearing and hearing protection from loud sounds often comes up, here are a few guidelines if you want to preserve your hearing as long as possible, although some high-frequency losses are inevitable with aging. 1: Use hearing protectors (earmuffs) around any noisy equipment such as lawnmowers, electric saws (especially circular), pneumatic drills, chain saws, and any environment where you have to shout to be heard. 2: If, after listening to earphones, you experience some feeling of dullness in your ears, or ringing, or if you find yourself turning up the volume after a while, it's too loud! 3: Do not stick anything in your ear smaller than your elbow. Herman Silbiger CGE ihnp4!homxb!hrs
hrs@homxb.UUCP (H.SILBIGER) (09/25/85)
Re: dimmer noise. Dimmers use scr's or triacs which convert the ac to pulses. While not changing frequency, the pulses are clipped and thus geth high frequency components. These drive the lightbulb filaments to generate the sound. This is lower in frequency than the TV flyback, and also will not damage your hearing. Herman Silbiger, CGE ihnp4!homxb!hrs