"I bought a 29" TV a couple of weeks ago and I have noticed that after being switched on for > about 15/20 minutes, whenever the picture changes from a "light" scene to a darker scene, the set makes a crackling noise. It sounds as though there has been a build-up of static and it is being discharged. I have never noticed this in a TV before and I was wondering if this is normal and acceptable behaviour for a large-screen TV?"
(From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)
It probably is normal. Whether it is acceptable is a personal matter. In some geographic areas no countermeasures are taken at all...
When the scene changes from bright to dark, the beam current is reduced to practically zero. As a result, the high voltage rises. (The high voltage supply has a relatively high internal impedance.) The high voltage is connected to the inside layer of the picture tube. A voltage change on the inside will also cause a voltage change on uncovered parts of the outside, especially on the part of the picture tube that is hidden under the deflection coils. This causes little sparks between the picture tube surface and the inside of the deflection coils and this is accompanied by a crackling sound.
On the better picture tubes, a dark "anti-crackle coating" is painted on the picture tube near the deflection coil. This is a very high impedance coating, dark black, much darker than the usual aquadag coating over the rest of the picture tube. You should be able to see the difference.
If, on the other hand, the outside of the picture tube near the deflection coil is not coated then you have a problem. Then you will hear strong crackling also at switch-on and switch-off. Normally you shouldn't see such a 'cheap' picture tube on the European market...
The area of the picture tube around the anode connector is also not coated, for obvious reasons. Normally that should not cause any significant sound. Same goes for the front of the screen and neither should the anode cable crackle.
In a dark room you should be able to see from the tiny blue flashes where the sound comes from. This is perhaps best observed at switch-on and switch-off (with a black picture on the screen). Try and keep the back cover mounted !