Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site mgweed.UUCP Path: utzoo!watmath!clyde!cbosgd!ihnp4!mgnetp!mgweed!rjr From: rjr@mgweed.UUCP (Bob Roehrig) Newsgroups: net.audio Subject: ringing in my ears Message-ID: <20479@mgweed.UUCP> Date: Wed, 18-Sep-85 13:51:54 EDT Article-I.D.: mgweed.20479 Posted: Wed Sep 18 13:51:54 1985 Date-Received: Thu, 19-Sep-85 06:03:25 EDT Organization: AT&T Information Systems - Montgomery Illinois Lines: 37 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. Brought to you by Super Global Mega Corp .com