Buzz/overload in high contrast scenes/text - Videokarma.org TV - Video

Carmine · #1 04-13-2020, 09:22 AM

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.

dieseljeep · #2 04-13-2020, 11:39 AM

Quote:

Originally Posted by 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.

That's one thing I don't miss on the newer sets, the audio buzz at certain scenes. It always seemed to be associated with AGC. Some older sets, the gain is controlled by the contrast setting.
Since the manufacturers went to keyed AGC, it helped a whole lot.

old_tv_nut · #3 04-13-2020, 01:23 PM

What is your signal source? Cabled into the RF input, I presume?

The only thing I can think of that would reduce the problem with a professional modulator is to deliberately reduce the modulation percentage - nothing to do with cabled RF connection vs. antenna.

Glad you found a fix.

old_coot88 · #4 04-13-2020, 03:12 PM

Wonder if this uses a ss version of Zenith's (in)famous quadrature sound detector which was notorious for buzz.

zeno · #5 04-13-2020, 03:44 PM

Quote:

Originally Posted by old_coot88

Wonder if this uses a ss version of Zenith's (in)famous quadrature sound detector which was notorious for buzz.

25EC58 is the last year flat SS chassis. The best ever built. They were replaced by the 25EC45 upright CC2 sets mid model year. Never known for buzz like
the all tube sets. Audio basically same as hybrid. Normal no equipment
alignment is.
1) set RF & IF AGC controls. ( some chassis only have one )
2) Adjust detector coil for minimum / slowest buzz.
3) Adjust SIF coil for minimum hiss.

73 Zeno

LFOD !

Electronic M · #6 04-13-2020, 03:35 PM

You don't need 25 modulators unless you already have 25 DTV boxes and want to use 25 DTV boxes at once... Before I got my Blonder Tongue modulators I had 3 cheap consumer modulators feeding 5 sets with splitters and RF switches. If you don't mind cabling wired RF is one method (it will work better with a Blonder Tongue AM series since the output is higher and it can take the attenuation of being split to 20+ TVs).

My AM series Blonder Tongues have the power to transmit over my entire property (and then some if I crank the output pot) and 1-2 are all I need (though I have more than that). How many different TV channels would your family ever watch simultaneously?...That is the number of modulators you need if you do wireless RF. If you have TVs in every room you could get RF IR remote repeaters in every room, universal remotes, and keep the modulators and boxes in a central location.

In the case of that CCII it looks like the buzz was in the TV, but sometimes it is baked into the modulator...cheap consumer modulators send the same carrier through both AM and FM modulation stages...if the AM video overmodulates it inverts the carrier phase and creates phase modulation with a 60Hz repetition. Any FM modulation creates phase modulation and any phase modulation creates frequency modulation and any FM demodulator is equally sensitive to both phase and frequency modulation so if the modulator creates phase modulation from video over modulation then it is hard to eliminate on the reciever end.