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TV Problem?

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  • 14-04-2011 5:16pm
    #1
    francie81
    Registered Users Posts: 1,412 ✭✭✭


    Hello,

    I have a 28' CRT widescreen over four years in living room, well this morning when I switched it on I got discolouration on both sides now it happened before but it vanished nearly straight away but not this time, how long will I wait until I actually need to take action is their any way I can fix it without having to pay a tech atleast €50 to repair which would be pointless considering am due to purchase a flat screen for my room shortly?

    Thanks,
    Francie


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    #2
    ZENER
    Registered Users Posts: 6,163 ✭✭✭ZENER


    Sounds like a degaussing issue. Over time - or by accident, suddenly - the shadow mask behind the glass at the front of the tube becomes magnetised. This is a normal occurrence and the TV set has a circuit built in to deal with it. This circuit is fired up when the set is turned on. A brief decaying magnetic field is applied via the degaussing coils and a VDR.

    On most modern sets the circuit will fire when the set is turned on both by waking it from standby and switching the power switch on.

    These circuits can go wrong of course. They're subjected to heat and a large current for short periods so can fail like any other component in the TV.

    The cause of the discoloration may have been caused by this circuit failing or may be as a result of some nearby magnetic source - a loudspeaker or other electrical device in close proximity or by the kids playing silly beggars with magnetised items and loving the fact that it gives Rihanna a cool purple or Green complexion !!

    In the first instance, the only way to fix it if it's gone faulty is to return it to a TV engineer. The latter faults can be usually cured by power cycling the set a few times. However doing this quickly wont work, you must allow at least a few minutes between cycles for the VDR to cool down so that the full magnetic field is achieved the next time it's used.

    So unplug the set for about 20 minutes, then power it back up and see if there is any improvement at all. If there is at least some improvement then repeat the step a couple more times allowing time for it to cool as mentioned above. If the circuit is working then you should see the discoloration clear up after a few tries.

    If this doesn't work then there are 3 possibilities:

    1. The Degaussing Circuit is faulty requiring repair by a qualified repair shop.

    2. The magnetic field causing the discoloration was so strong that the circuit cant counteract it. In this case an engineer will apply a stronger field to the front of the tube to clear it.

    3. The magnetic field causing the problem is still present !! Move any nearby electrical equipment and or loudspeaker away from the set by a few feet and repeat the procedure.

    Let me know how you get on !

    Ken


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    #3
    francie81
    Registered Users Posts: 1,412 ✭✭✭francie81


    ZENER wrote: »
    Sounds like a degaussing issue. Over time - or by accident, suddenly - the shadow mask behind the glass at the front of the tube becomes magnetised. This is a normal occurrence and the TV set has a circuit built in to deal with it. This circuit is fired up when the set is turned on. A brief decaying magnetic field is applied via the degaussing coils and a VDR.

    On most modern sets the circuit will fire when the set is turned on both by waking it from standby and switching the power switch on.

    These circuits can go wrong of course. They're subjected to heat and a large current for short periods so can fail like any other component in the TV.

    The cause of the discoloration may have been caused by this circuit failing or may be as a result of some nearby magnetic source - a loudspeaker or other electrical device in close proximity or by the kids playing silly beggars with magnetised items and loving the fact that it gives Rihanna a cool purple or Green complexion !!

    In the first instance, the only way to fix it if it's gone faulty is to return it to a TV engineer. The latter faults can be usually cured by power cycling the set a few times. However doing this quickly wont work, you must allow at least a few minutes between cycles for the VDR to cool down so that the full magnetic field is achieved the next time it's used.

    So unplug the set for about 20 minutes, then power it back up and see if there is any improvement at all. If there is at least some improvement then repeat the step a couple more times allowing time for it to cool as mentioned above. If the circuit is working then you should see the discoloration clear up after a few tries.

    If this doesn't work then there are 3 possibilities:

    1. The Degaussing Circuit is faulty requiring repair by a qualified repair shop.

    2. The magnetic field causing the discoloration was so strong that the circuit cant counteract it. In this case an engineer will apply a stronger field to the front of the tube to clear it.

    3. The magnetic field causing the problem is still present !! Move any nearby electrical equipment and or loudspeaker away from the set by a few feet and repeat the procedure.

    Let me know how you get on !

    Ken

    Thanks for that ken you seem to know your stuff fairplay.

    I have it in for repair with a guy who use to rent to us and he tells me he has it back up and running that a thing called a 'ptc board' failed did you ever hear of that and is it related to degaussing?

    Francie


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    #4
    ZENER
    Registered Users Posts: 6,163 ✭✭✭ZENER


    Hopefully it didn't cost you too much ? I presume what he replaced was a board containing the VDR. PTC or "Positive Temperature Coefficient" is the main characteristic of the VDR that makes it work in the degaussing circuit. Though it's usually a small component on the main circuit board it's not unusual to find them on a separate board of their own.

    The idea is that as the device - which is easier to think of as a variable resistor - changes characteristic as it warms up. In this application we need a magnetic field that decays slowly, so the resistance of the VDR increases as it heats up. This reduces gradually (usually within a couple of seconds) the current in the degauss coils resulting in a decaying magnetic field ! . . . Simples :)

    Anyway glad you got the set sorted !! Enjoy !

    By the way, many CRT type TVs and computer monitors have an option in the menus to trigger the degausser manually. As the circuit only performs during power up it has no effect afterwards until the set is turned off and on again. Fine most of the time but if the screen is on for a long period of time discoloration can gradually creep in. The menu option allows you to correct the purity of the colors without powering off/on.

    Ken


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