Question about Standard (CRT) Televisions

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KV-28CL10U colour circles

My sony KV-28CL10U television has coloured concentric circles on the screen.

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  • PETE0629 Mar 04, 2009

    I have the same problem with 28in Beko widescreen TV. Is degaussing thermosistor located as in Sony?

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There are two things that commonly cause the fault that you are describing. Below is the main one.

It is very possible that the degaussing circuit or the degaussing coil is no longer working. A common fault is that the degaussing thermosistor has gone defective and the CRT has become magnetized. The earth's magnetic field along with other environmental things cause the CRT's internal mask to become magnetized over time.

The thermosistor in most Sony TV sets is located in the power supply area. It looks like a little black square box with 3 solder posts on it. In some models it has 2 solder posts. After a number of years this part may fail.

When the power is first turned on, the thermosistor is very conductive. After it heats up, it goes very high in resistance. It drives a high decaying AC mains current through the degausing coil (located around the shroud of the CRT assy) for about 50 to 60 ms. It then allows the current to decay to zero over the next 50 ms. When you turn on the TV set, you normaly hear a kind of a loud thump noise. This is the degaussing action.

Once the degausser is serviced and is working properly, it may take a number of power cycles to eventualy demagnetize the CRT. After shutting off the TV, it takes about 20 minutes before the degausser can work again. The thremosistor needs some time to cool down.

If you are not experience and trained for doing TV service, I would suggest you give the set out to an experienced TV service tech to change this part. There are serious safety issues when working inside of TV sets.

It is possible to purchase a manual hand held degaussing coil. There is a proper procedure for using this type of coil. If you were to manualy degausse your CRT, you would have to do it on an almost daily basis. The manual degaussing coil is fairly expensive to purchase. It would be better to service the set.

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The second thing that causes the CRT to show colour purity problems (color splotches, or circles, or patches), is that the mask inside on the face of the tube has become dislodged off its supports or became warped. This can happen if the set has been dropped, or hit hard enough to do some damage to the mask. I have seen this where TV sets were roughly handled during moving them.

The only fix for a damaged CRT mask is to replace the CRT. The cost would be as much or more than the set is worth. This is if you can even find a replacement CRT. Most all of the manufactures no longer manufacture replacement CRT's for consumer products.

Only specialized CRT's are still being produced on a limited bases for specific industrial and military equipment that must still be kept in service. These are for some types of radar, medical, aerospace, and other related industries.


Jerry G.




Posted on Apr 16, 2008

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