television malfunction mystery

Hello-
I have an 8 or 9 year old Magnavox television that has a very conditional problem. During the daytime, the picture is great, but when it is dark outside, not just dark inside, the picture gets interferance in the top 1/3 of the screen, in the shape of a thick black line that is shaped like an arrowhead, that jumps from the left side of the screen to the right. Anyone have a theory?
Thanx, Todd
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Just a couple of wild-ass guesses, but here goes...
1. A change in line voltage after sunset. Does the same thing happen if you plug the set into a receptacle that's on a different circuit?
2. If you're on cable, could it be a problem with your cable service provider?
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Only theory I have is that a new set is probably about the cost of repairing what you have. There is a newsgroup that is for TV and radio, You may get better help there.
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I had a similar problem on my SONY that I couldn't recreate. The only clue was it sometimes crapped when the forced air furnace came on thus indicating it had something to do with external temperature.
I had taken some night courses on TV repair and tried many times over 3 to 4 years to locate the problem without success. There wasn't much to look at anyway because everything was on one PCB and the parts were unidentifiably teeny. The set was very compactly assembled as to be barely reachable with any tool. If I couldn't reproduce the problem and couldn't locate the circuitry area I wasn't about to bring that hernia box to the repair shop because they would likely encounter the same difficulties.
On the final try I got lucky. My hand brushed the cable harness to the CRT display tube and that reproduced the problem. A close look at the harness connector pins on the PCB revealed hairline cracks on the solder pads. I resoldered the pin pads and my TV set image is now clearer and sharper than it had ever been.
For your repair attempt open up the cover. Use a wooden dowel to jiggle all the wire harness connections to see if you can reproduce the problem. It may be the same problem and have the same solution as mine. Beyond that don't spend any more time and money on repairing an old TV set. Its cheaper to buy a new one. Or you can live with your present one.
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I saw a problem like this once at a vacation cottage on Cape Cod. In the evenings a powerful radar system was turned on probably by the coast guard or military which made a mess of the TV picture. Steve

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Depending on where he lives there are alot of Military Ultra high powered long range comuniction equipment around. He should talk to neighbors , see if any have problems or run apliances at night. I dought it could be a refrigerator defrost cycle , its a coil. Ive had CB radios come in my tv from 1 mile away.
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jon wrote:

My guess is some lighting, interior or exterior, maybe even street lighting near your home or more likely near the cable line, is causing interference. Can you reproduce it with a second TV set? Newer sets may be more resistant to the interference. You might also ask your neighbors if they are having a like problem.
Call your cable company (I suspect it is cable) and have them check their cable lines run on the same poles as street lights and I'll bet if they look they will find one of their amplifiers is on the same pole as a light with a defective ballast.
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Its something that turns on at night, and inducing RF or line interference. As Joe sais maybe street lights. Time the lights, have tv on , wait for the lights. Or something else that turns on at night. A neighbors air filter , AC, fan , or something. Try a different circuit . Maybe a Ham radio, or CB radio, operator in the area, look for a big antenna , See what time it goes on and off. A radio operator will have a sleep time he leaves. street lights have an exact time. call your cable co they will have better ideas, but talk to a Tek, not sales. Unlikely but a frige has a defrost cycle. I say Radio operator.
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m Ransley wrote:

Not around here. Most are on light sensors.

Also a good chance. Radio operators are on the air at night because long distance communications are generally better.
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Mine are photocell to , dusk or dark is more correct.
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Maybe you hit on something IIRC, some TV's used to have a sensor to adjust the contrast or brightness level depending on the room lighting levels. Could this set have such a thing?
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Questions to ask:
Is this pattern visible on one station or all stations?
Does the pattern crawl up or down the screen?
Is anyone else in the area having similar problems?
Can you change the pattern by bumping the set? On a healthy set you should be able to wack it with the flat of your hand without any visual or audible problems. (As long as you don't cause any physical damage to the cabinet, controls, or your hand, hit it as hard as you can. Sets are not particularly fragile.) Be sure to hit in all directions.
Is the set sensitive to jiggling any of the connecting wires?
If you have other TV sets in the area, do they have any similar problems?
Are you keeping a written log of problems? If so, can you correlate TV problems with anything? You mentioned time of day. What about weather, day of week, other appliances in the area, computer use, video games, power tools. Look outside; what is happening out there?
Did anything in the area change at about the same time as the problem started? (construction, new neighbor, utility crew working in the area)
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jon wrote:

Do you have a VCR or DVD? try to play a disk or a tape without the antenna connected and see if it still does it.
Is the picture full and squished and the stuff on the top of the screen just garbage? If so, you have a vertical problem. More than likely bad capacitors. You really need to know what your doing to work on a TV. THe CRT is really just a big capacitor that can store 25,000 volts or so. SO careful is the word.
Bob
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jon wrote:

Worse on one channel than on others, right?
You're picking up another station on the same channel from a distant station, at the same time you're getting the local channel. The "thick black line" is the horizontal blanking bar from the distant station. Rather than "jumping" from one side to the other, I bet it floats gradually from one side to the other, then appears at the one side again to start over.
You *may* be able to turn your antenna a little to alleviate this, but that might cause other problems. You'll probably want to get an attenuator, but what type depends on whether you're using coax or flat twinlead, and how much attenuation has to be determined by some experimentation. Electronics parts jobbers who sell to TV shops, or stores that are really good with antennas, should have them.
Just on the off chance that you're using an antenna amplifier with a strength control, try turning that down. (Few have them.)
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