Buzz/overload in high contrast scenes/text

[Carmine]

I've read for many years that this problem could not be fixed, other than using an OTA signal generator. In fact, I even came to this sub-forum seeking circuit modifications. Transmitting my own signal really isn't practical. My home is all-CRT, no subscription TV, and all Zenith. I'm not going to run 25 modulators for all of the "useful" OTA channels.

My primary set for the past few months has been the Zenith that Doug brought out from Oregon 9 years ago... The Chassis is a 25EC58. And like every one of these sets I've owned, lots of buzz when text is on the screen, especially black on a white background.

Yesterday I got fed up enough that I committed a sin. I diddled with my stick without a scope.

Specifically the two coils on the sound board. Since I already put the set back together, I can't tell you the board number, but maybe someone knows from the chassis number I posted. Before starting, I marked the position an vowed to only move the coil perhaps 360 degrees in each direction, returning to the original spot if there was no improvement.

I put a very buzzy image on the screen from an old Direct TV box that a rental tenant left behind. Basically their logo on a white screen. Within 5 degrees, the buzz "mostly" went away. Hmm... Did I now just kill the audio entirely?

Nope. Returning to the convertor signal resulted in no change to the sound.

Going back to the logo I decided that I'd try to get the remaining 30% of buzz. The original coil I turned (at the top of the board in my set) had a very tiny sweet spot with the lowest buzz. Turning past it in either direction increased the buzz. But it could not be eliminated.

So I went back to the same process with the second (lower) coil and viola! The buzz was gone. Again, no effect on normal audio. Color me overjoyed.

I can now look forward to commercial breaks and their associated catheter, senior life insurance and Roundup-lawsuit ads. My thinking is these coils were probably adjusted for their peak at the factory, but placing them just off the peak works much better in the post-analog OTA era.

I hope this helps others and will attempt more of the same on my other sets with the same issue. Look forward to any explanation by the more technical members here.