TV buzzes with overly white video

Perform an ESR test on the caps in the power supply area, and especially in the area where the IF and tuner sections are.

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JANA _____

In the past few months I've been getting a loud buzz at certain instances when there's some overly white video. Picture flops around a bit too.

It's directly on analog cable and I don't doubt that Comcast may occasionally have signal levels that are out of spec but it doesn't seem to bother other sets in the house including one that is a similar Sony, maybe a few years older even. So I'm wondering if something is out of whack on the TV. Is there a solution (besides the eventual upgrade to digital HD flat screen)?

whites are saturating, this causes the buzz - adjust the AGC

Called a "sync buzz". Caused by white video being out of specification...or by the receiver's audio section being out of tune. Your Sony may have old-fashioned IF cans/coils, for video and sound IF and AGC. If so, then just tune it up. Naturally, you'll need service data or lots of experience. Or both. If your Sony is all digital, it will take some specific devices and jigs to tweak the alignment.

Find the sound QUAD coil or discriminator coil and give it a little tweak until the sound clears up.

If all your TVs buzz on a certain channel when there is heavy white in the screen, then call Comcast and thell them they are overmodulating the video.

Mark

On Jan 17, 1:29=A0pm, Mark wrote: > On Jan 15, 1:10=A0am, Steve Kraus > wrote: >

Don't dick with the RF alignments. Someone already pointed out the AGC is a strong culprit. If that doesn't do the trick, I'd want the whole IF swept to find out what is _really_ going on.

GG

to

No one has established from the OP whether noise from the speaker or direct from the back. My TV has always buzzed on peak whites, a sort of metallic buzz, presumably not from the LOPT as no other problems. I've never had a more pressing reason to get in there and explore. Almost as though the scan coils are moving but that would not respond to beam current variation

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Buzz is in the audio from the speaker.

I'd like to explore the AGC issue before messing with anything else. Is this something that has a physical adjustment? That would seem to be the way to go with least downside if it's wrong.

BTW, how do you get into the setup/service menu? I've tried pushing the button via the small hole in back but it doesn't seem to do anything. Again, this is a Sony KV-27EXR25. If the service manual is available free online anywhere please point me.

Also, if it is an AGC issue would putting a pad on the RF (cable) input solve it in simple fashion? Right now I have the cable looped through a VCR (pass through) hoping it might help and maybe it has but not enough.

ee

A pad shouldn't help as the 'automatic gain control' will just push up the gain to make up for the pad. If it's in an extremely high RF condition, the pad might help but it's rare to get that much signal.

GG

Have you ever tried a pad? The AGC system in a TV is a very basic system that is designed to keep the level within a range, and driving it to the upper limit can cause sync buzz. I have seen complex AGC systems with 14 op amps, and the DC offset was limited to 1.5 mV over the entire range. It could maintain the output level to less than a .1 dB change over the entire useful RF input range. You WILL NOT see a AGC system like this in a consumer level receiver.

Yeah I've tried pads and they do make a difference if the signal is so strong as to overload the front end but as I said earlier, that condition is not very common. More common for the AGC to be pushing the visual carrier to cutoff causing the buzz.

GG

How would I adjust it or is that even possible? I'm hoping for a little trimpot in there but I don't have a schematic.