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Interference to TV every P.M. revisited

Posted by catherinet (My Page) on
Sun, May 24, 09 at 15:47

Hi all,
For several years now, I get so much interference on my TV sets (all of them) from about 2p.m.- 6p.m. in the hot summers that I can't watch them, unless I turn off a circuit breaker that includes stuff in the attic. I need to have someone come figure it out, in case it might be a short that will burn the house down...??
but I just haven't gotten around to it.
Anyhow.....now some of our TVs have digital converter boxes. Today I realized that it doesn't seem to bother the TVs reception that are using the converter boxes. That really surprised me.
Does this say anything about the problem? Since we have to switch every TV to a digital box by June 12, is this even a problem I need to have checked out any more?
Thanks.


Follow-Up Postings:

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RE: Interference to TV every P.M. revisited

What's on that Circuit Breaker ?

Leave it OFF and find out what doesn't work ...


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RE: Interference to TV every P.M. revisited

There are a bunch of things on that circuit, but I'm wondering why all the interference doesn't show up when going through the digital converter.


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RE: Interference to TV every P.M. revisited

One of the reasons that the conversion to digital TV was that it's supposed to be free from interference compared to the old analog signal.


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RE: Interference to TV every P.M. revisited

I concur that it would be nice to have whatever is on that circuit checked out. Is it an attic fan?

The technical reason is that the older analog video signal is "amplitude modulated" (AM), just like the AM signal on your car radio. And just like your car radio, it is susceptible to all manner of clicks and pops. (The analog TV audio portion is "frequency modulated", FM, so does not have this problem. And yes, they made a mistake back in 1950 when they should have used FM for the video signal as well...)

With digital TV, the signal is sent in terms of digital pulses via a standard called ATSC (Advanced Television Systems Committee.) There are a bunch of schemes within ATSC for encoding portions of the signal - such as MPEG and MP3. While the signals are still sensitive to some forms of interference, there is enough error detection/correction built in that you won't normally notice it. When the signal does get weak enough, the reception simply "falls off a cliff" - and you will get a pixelated picture, or even no picture at all, until the converter gets back in synch with the signal. It is not like the analog days when things would just get fuzzy or ghosted.

But again, I would recommend figuring out the cause of the electrical noise in your attic. Noise like that is often caused by something arcing - which means heat can be building up and could eventually cause a fire. A cheap AM radio can help pinpoint it.


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RE: Interference to TV every P.M. revisited

Thanks wws944!
About the AM radio......would we just carry the radio around to the different things that are on that circuit, and the interference on the radio would get loud by the defective area?


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RE: Interference to TV every P.M. revisited

> About the AM radio......would we just carry the radio around to the different things that are on that circuit, and the interference on the radio would get loud by the defective area?

Basically, yes.

First, tune it between stations, and as high in the AM band as possible (e.g., 1600 khz or higher) while still receiving the attic noise.

Second, inside the box, AM radios typically have a 'loop' antenna - which looks like a small bar with wire wrapped around it. The loop is directional - as you turn the radio, you will hear more or less noise. The ends of the bar will point towards, and away from, the noise source. So you can use this as a tool to help pinpoint the problem.

A few years ago, I had a horrible problem at my house which I finally tracked down to my across the street neighbor. Turns out they had a bad dimmer switch in their kitchen that was causing electrical noise throughout our part of the neighborhood whenever they turned that light on. They knew the switch was broken, but hadn't realized the rest of the problem.

Another problem I had recently was the dumb proximity sensors for the outdoor lights that we were required to install as part of Title 24 here in Kalifornia. They generated huge amounts of electrical noise. They, ahem, don't anymore...


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RE: Interference to TV every P.M. revisited

Thanks wws944,
When this first started up about 6 years ago, I really thought it was interference from somewhere in the neighborhood. But nothing is closer than 1/4 mile. Its every day in the summer between 2p.m. and about 6pm. If the sun isn't shining, it usually doesn't happen, unless it is very hot out.
I finally figured out if I shut a certain circuit breaker off, it would go away. On this circuit are things like an attic fan (not used), closet lights, outdoor lights.
Should I call an electrician or a TV person? Our rotor on the antenna is dead. TV people seem to be hard to find around here, but I don't want to pay for an electrician if he can't know if the problem is with the antenna.
Our attic can get over 120 in the summers (maybe much higher, but that's all the higher the thermometer goes.)
Back to the AM radio........do we have to get real close to the problem for it interfere or does it just point you to a certain direction?
Would it most likely be in a junction box, or an actual thing like an outdoor light?
Thanks for your help!


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RE: Interference to TV every P.M. revisited

> ... it usually doesn't happen, unless it is very hot out. I finally figured out if I shut a certain circuit breaker off, it would go away. On this circuit are things like an attic fan (not used), closet lights, outdoor lights.

That was a nice bit of detective work - good job!

> Should I call an electrician or a TV person? Our rotor on the antenna is dead. TV people seem to be hard to find around here, but I don't want to pay for an electrician if he can't know if the problem is with the antenna.

Here is an idea: How about calling your power company? They might send someone out to do a 'safety inspection' for free.

> Our attic can get over 120 in the summers (maybe much higher, but that's all the higher the thermometer goes.)

Ouch! I suggest getting that attic fan repaired ASAP!

Might want to find all your attic vents, measure how big they are, and make sure there are enough of them to properly vent the attic. (The attic in our house, when we bought it, was seriously under vented. Like by a factor of 10 or more. You can find calculators around the internet to help you determine this.) Installing a radiant barrier under the roof rafters can also help a lot. Radiant barrier looks a lot like a big roll of aluminum foil. They unroll it and tack it onto the underside of the roof. It is not very expensive.

These could save you a lot of dinero on A/C costs. Or if you don't have A/C, could make the house much more comfortable in the summer.

> Back to the AM radio........do we have to get real close to the problem for it interfere or does it just point you to a certain direction?

Every situation is different. Can you hear interference on your AM radio when you are not in the attic?

Sometimes funny things can happen too. Your wiring is like a big antenna that radiates the noise. There may be places along a wire run where you get a lot of noise or no noise, depending on how the wires are criscrossing around up there.

> Would it most likely be in a junction box, or an actual thing like an outdoor light?

Cheap dimmers are notorious for EMI/RFI problems, and they live in junction boxes. A loose wire nut, which allows arcing, could also be living in a junction box.

What about your broken attic fan? Is it on a thermostat? When the temperature rises, it could be attempting to turn on the broken fan...


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RE: Interference to TV every P.M. revisited

Hi again,
The attic fan is in the ceiling of our hallway in the house, not the attic. It isn't broken, just not used, since we finally got central air. It doesn't have a thermostat....just an on/off switch.
I've been thinking about a solar roof fan. We have a ridge vent across the top of the roof, but I think those newer foam ones fill up with dust and dirt and end up not ventilating very well.
I talked to our REMC people last year, and they didn't know what it might be, but I think I just need to have an electrician out. The funny thing is, when its happening, the attic is too hot for human beings, so I put it off having anyone out. Then it goes away in the off-summer times, and I forget about it! Do you think an electrician could figure out the problem, when its not happening?


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RE: Interference to TV every P.M. revisited

> I've been thinking about a solar roof fan. We have a ridge vent across the top of the roof, but I think those newer foam ones fill up with dust and dirt and end up not ventilating very well.

Besides the ridge vent, check the ones under the eaves. Cool air has to enter under the eaves so that hot air can exit from the ridge vent. And again, make sure there are enough vents for your roof. A roofer or general contractor could help.

> Do you think an electrician could figure out the problem, when its not happening?

My impression is that not many electricians know or care much about EMI/RFI problems. The power companies are supposed to, but their records are often spotty too. It is the sort of thing where sometimes you have to show them where the problem is, then they will fix it for you. :(

Can you hear the noise on an AM radio? Or is it just on the TV picture? If you are hearing it on the radio, maybe you can localize the problem by walking around the house - without going into the attic.

The link below gives way more information than you probably want to know about EMI/RFI. It is intended for amateur (ham) radio operators, but has a lot of general information as well.

Here is a link that might be useful: American Radio Relay League - RFI info links


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