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HDTV backlight turn off after 10-15 minutes, with some t-shooting

My Polaroid TV is having power supply problem. After 10 minutes of so, backlight turns off. I am electrical engineer, so I looked at circuit board.. Component NTC1 (Thermistor) Is getting to 90C after short time.. After blacklight cut off, part cools significantly.. Was wondering if this is abnormal operation for this In Rush Protection NTC Thermistor, and if so how can I find specs, as this is unmarked part..

Power Supply Circuit Board is: Proview IPOS 250 A40
TV is Polaroid TLX-04240B
When blacklight goes out, rest of TV (sound, and LCD rendering is working normal) Checked LCD with SPOTLIGHT.....

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  • jwangelinjr Sep 10, 2009

    NTC component is in the AC feed to the TV for In-Rush Current Suppression.. It looks like 5-10ohms at powerup, and as it conducts and gets hot, the resistance goes to near zero. The spec I read on the NTC is that its normal operating temp is 150-200F..

    As for the Burn switch, that has also worked for me.. My thoughts are that since my backlight was going out every 3 hours then deteriorated down to <5min, that there must be a protection device on the HV backlight supply that has been degrading over time.. I did measure initially that there was an interrupt being received at the mpc, and it was sending a poweroff signal to the inverter chip.. The Burn switch must ignore this interrupt.. I still see it being driven to the mpc, however now the turn off signal is not coming out of the mpc, to the inverter chip..

    For now, I may leave the set as is, but if time permits, I will look at the protection components, and do some part swapping to see if I can fix it to run in regular mode..

  • mrkh May 11, 2010

    Thanks for the updates jwangelinjr. Have you any idea what other protection components are in the circuit other than the NTC? Were you able to probe where the interrupt to the microcontroller was coming from? There are analog inputs on this microcontroller, so perhaps it measures the backlight current (this could explain the 40" and 46" DIP switches - different current limits), but I can't see any resistors small enough to be for current sensing. There are three resistors under the top of the righthand heatsink I can't see the values of, however. Cheers, Ken

  • mrkh May 11, 2010

    I've opened up my TLX-04240B to flip the BURN switch, and just for the info of anyone else here, the microcontroller on the power supply board is a NEC uPD78F9222 http://www.am.necel.com/micro/product/su... and the PWM controller driving the inverter is a ST Microelectronics SG3525 http://www.st.com/stonline/products/lite... As far as I understand from jwangelinjr's comment, it's this microcontroller that sends the shutdown signal to the PWM controller based on an interrupt it receives.

  • mrkh May 11, 2010

    Jwangelinjr -

    Did you have any success finding out about the NTC thermistor? Did you
    try cooling it with a fan or similar to see if that delayed or
    prevented the backlight turning off?

    cdmc -

    Is your TV still working well after flipping the BURN switch? I'm not
    sure if you read in the other threads that there is an advanced/service
    menu for this tv... http://www.avsforum.com/avs-vb/showthrea...

    On this menu, I noticed a "Burn" option - I wonder is this somehow connected with the switch on the control board?


    As for why flipping the BURN switch seems to fix this problem, it's
    hard to know. I'd speculate this switch is related to burn-in testing
    (note that this is not related to pattern burn-in on a CRT or LCD due
    to displaying the same picture for a long time) at the factory where
    they run all tvs for a period of time to weed out any that are going to
    fail immediately. Speculating further, I'd guess that that the burn
    switch runs the tv in a more stressful mode (e.g. higher backlight
    intensity and similar things) to help weed out problems. Connecting the
    dots with pure speculation, I'd say that the tv normally has some
    safety shutoffs (perhaps upon hitting a temperature limit or a current
    limit), but that these are disabled in the burn mode.

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I have read everything I can find on the net about this problem for Polaroid TLX 04240b TVs (most, like mine, bought from Wal-Mart on Black Friday after Thanksgiving). I thought I was on a hot lead when I changed a 220uF 450v capacitor, but I still had the same problem. I saw a short post from somewhere else on this Fixya web site (sometimes I can find it again and sometimes I have trouble locating it) that said someone flipped a tiny switch on their circuit board and the problem was gone. I did the same thing and my set has been working fine. They had said their set was on for over 8 hours two days in a row when they posted. The other night my daughter left our set on over night for 20 hours and the picture still was working great. If you look through the top of the set when the picture is on you can see some white light at several locations, when the screen goes blank the white lights go out and a red LED light can be seen. If you take off the plastic cover on the back (3 screws) and then take off the metal cover (3 screws) this red LED light is on the upper circuit board in the lower left corner. Just a little to the right of the LED light is a small switch block that has 4 tiny white switches. I turned my set off and took the bottom switch (#4 and had the letters BURN by it) to the right "ON" position, turned the set back on and, so far so good. PLEASE NOTE!!! I know almost nothing about how transistor circuits work. I have no idea what the switches do or what long term effect this might have on the set. The set was only good as a paper weight before so even if this only fixes the set for a short time it was at least worth a try for me. Does anyone know what those switches are supposed to be for????

Posted on Aug 13, 2009

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  • phantomlord_
    phantomlord_ Apr 15, 2013

    I Tried this fix, it works like a charm. thank you so very much.

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Also could be just a few bad capacitors on the power supply, if you want you can do a visual of the power board caps will appear swollen on top and are easy to solder in cost about $1 each
look at the individual flouresent bulbs if one is dark or unlit before they all go off the panel is bad

Posted on Aug 01, 2009

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