Buzz In Chromacolor II Sound- 25GC45

My sincere thanks to those who helped me solve my horizontal output problem on this set. For the sake of perfection, I'd like to solve a less serious problem that has been plaguing me for awhile.
I have a standard definition satellite feed (directv) going into channel 3 via RF. When the picture has bright white areas (particularly annoying on the news networks when there are constant banners at the bottom of the screen) there is an annoying background buzz/hum, as though there is video leaking into the audio or the signal overloads the set. (I tend to discount the latter because some channels, particularly the local ones, seem better than others.) Fine tuning doesn't help because when the picture is optimal, the sound is at max. volume but so is the buzz. There is no problem at all when the screen material has moderate contrast throughout.
Could someone please offer advice here? I should point out that I replaced the sound board on this set recently, and the buzz seemed worse afterwards.
There may always have been a little but not nearly so noticeable.
The chassis on this set is a 25GC45/45Z.
As usual, help much appreciated!
Frank

[MRX37]

If you replaced the sound board, you might want to check your soldering/wire connections.

I've found that in every TV I've ever listened to, there is usually a faint buzz in the sound, especally when the picture is bright.

These are plug-in boards and I have checked the contacts. As for the faint buzz I know what you mean but this is far from faint!

[kx250rider]

I've seen that on many older sets, not just Zeniths. They weren't designed to have the captions and boxes displayed as we have today, and they don't filter that "noise" out of the sound so well. You can try swapping out the IF module, or even VERY SLIGHTLY turning the sound discriminator transformer slug (located on the IF board, if that set has one that's adjustable). Be sure to count and mark exactly where the slug was set, and frankly, I'd want a spare IF module lying around in case you get the alignment all screwed up accidentally.

All of my 70s Zeniths have buzzed somewhat when displaying boxed text and sharp white objects.

Charles

[wa2ise]

Originally Posted by frank1492

When the picture has bright white areas (particularly annoying on the news networks when there are constant banners at the bottom of the screen) there is an annoying background buzz/hum, as though there is video leaking into the audio or the signal overloads the set.
Frank

What usually happens to cause that is the picture carrier getting cut off in the IF amp during the bright white parts of the video. White in TV RF is at minimum video RF carrier, and if biasing in a stage in the IF amp is off, all signal disapears for the time it takes to raster scan thru the white areas. And the sound IF loses the sound signal, as the intercarrier system depends on the picture carrier being there all the time. And when that happens you get noise spikes in the sound. It sounds like a 60Hz buzz as the video raster scans in the vertical dimension repeadly at about 60Hz. So look in the video IF strip for a weak tube, or a bias problem. The AGC might be too agressive, if say on of 3 tubes in the IF is weak.

[Chad Hauris]

I know it is possible for the Chromacolor II to have noise free performance as I've got one from 1979 and even when white captions or text come on the screen there is no noise at all...I wonder if there may be bad electrolytics on the sound module or even in the power supply...it is probably getting to the age where bad caps can start to show up.