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Fuse confusion?

Discussion in 'Troubleshooting and Repair' started by Nerdydude101, Nov 22, 2014.

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  1. Nerdydude101

    Nerdydude101

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    Jul 28, 2014
    i had a small tv with a built in DVD player that hangs on a wall. Recently we believe there was a power surge while we weren't home because it won't turn on and the fuse broke. We couldent read the fuses rating so we took it to radio shack and the gave us some replacements after thinking they figured it out. However as soon as we put it in, the fuse role again. So I ipened the tv up and found the fuse holder and it says next to it "100-120v:4A" which we have a 125v 4A fuse, but what confuses me us that right underneath the "100-120v:4A" on the board, it says "125v or 250v" and it's in the same boxed off area as their her but, so what does that mean? Do I need a different fuse?
     
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hello
    Where do you live? What is the voltage of your mains?
    Adam
     
  3. duke37

    duke37

    5,364
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    Jan 9, 2011
    The important fuse rating is the current (4A) but the rate of blowing should also be specified. (slow blow or fast blow)
    Fuses do not fail for the fun of it so you will need to find the fault. Do you have many power surges in your area?
     
  4. Arouse1973

    Arouse1973 Adam

    5,164
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    Dec 18, 2013
    And voltage also
    Adam
     
  5. Arouse1973

    Arouse1973 Adam

    5,164
    1,081
    Dec 18, 2013
    Ok so as you have not replied in an hour of our replies. As long as you make sure the voltage rating of the fuse is the same or greater than your mains voltage you will be fine. The voltage rating of a fuse comes into play when the fuse blows and is its plasma tolerance.
    Adam
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    It's common for switching power supplies to fail due to a voltage surge. The fuse then blows, and replacement fuses also blow immediately. This is because semiconductor components in the power supply have failed and must be replaced. The failed component may be the bridge rectifier, but in my experience it's more often the switching component(s), which are usually MOSFETs nowadays. Assuming you have a switching power supply this is the most likely explanation for the problem you have.

    Upload a photo of the power supply board and we can advise you further. Also, do you have a multimeter?
     
  7. Nerdydude101

    Nerdydude101

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    Jul 28, 2014
    Sorry, I wasn't receiving the emails for some reason, i live in America for the first question, as for fuse ratings I'm not sure, my parents decided to throw away the packaging and all it says on the fuse is what I have put in the main question and no, surged are not commin here I don't have a multimeter but I have a neighbor who does, however my parents are ready to just toss it. I opened it because I was curious about electronics and like to open things up and see what's goig on, though my knowledge is limited I still make some sense of things. U fortunately I doubt my parents would like me opening it up (I did it while they weren't home) however the producer is todhiba and here is the info in the back:
     

    Attached Files:

  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Hi Nerdydude,

    One thing to look at is how the fuse blew.

    Sometimes a fuse will blow and you might see a bit of wire remaining, or even if not, the glass envelope is clean. Other times you will see that the inside of the glass envelope is coated with black and/or shiny stuff.

    In the former case the fuse blew due to a long term slight overload (like a 4A fuse being asked to carry 8A) causing it to melt and break. In the latter case a significant fault has caused a very large current (perhaps 40A for that poor 4A fuse) which vaporizes the wire and deposits it on the glass envelope.

    I suggest you will have the latter type.
     
  9. Nerdydude101

    Nerdydude101

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    Jul 28, 2014
    No wire left and the glass is clean, so what's recommended?
     
  10. Nerdydude101

    Nerdydude101

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    Jul 28, 2014
    I take that back, there is a little by of wire
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
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    Jan 21, 2010
    OK, that;s interesting. When the fuse blows, does any part of the TV come alive briefly before the fuse blows?

    Whilst this is not a 100% certain test (the fuse could be rather too generously rated) a failure like this often means that there is an overload (often a significant one) rather than a direct short across the mains (which is essentially how many SMPS faults behave)

    Do you have the original fuse? It might be wise to double-check Radio Shack's advice (not meaning to be rude, but going to them for electronics advice is like going to McDonalds for advice on molecular gastronomy).
     
  12. Nerdydude101

    Nerdydude101

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    Jul 28, 2014
    Haha that's true, unfortunately no, I don't have the original fuse. However, the tv does come on for a few seconds before we hear the pop and the fuse goes out. Nothing appears on the screen but I think I know why that is (to find the fuse my dad disconnected some wires and just put them back together willy nilly, so if we pair them up correct that should work) but again, yes te LEDs to tell you it's on stay on for a brief moment and then go out.
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
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    Jan 21, 2010
    OK, that's very useful. (Other than not having the fuse).

    How is this device tuned on? Is it turned on by a physical switch, or by a remote? It sounds like the device can be connected to power without it failing -- do you get any standby indication?

    And yes, it would be far better if you can connect everything up correctly.

    It is also possible that the fault is in the power supply for the screen backlight. Is the device described as "LCD" or "LED" anywhere on the bezel or otherwise? (In the image you have provided it says LCD. This implies it has a CCFL backlight. These have their own power supply and often are not powered on immediately. They generally have pairs (often 2 or 4) of "soft" wires that are often coloured white and some pastel pink or blue which plug into the inverter board near some (often) brownish coloured transformers that are about the side of 2 SD cards side by side and about 10mm tall. These can be plugged in in any order.

    Please try to address all questions :)
     
    KrisBlueNZ likes this.
  14. Nerdydude101

    Nerdydude101

    12
    0
    Jul 28, 2014
    For the issue being the backlight, I have no idea, I suppose it could be possible but would tat cause te fuse to blow if ts a seperate power supply? As for the "LCD" or "LED" all the info I have is in that picture, the front says the brand, has a LED to tell you if it's on and that's pretty much it. There are some smaller stickers that say things like it has HDMI compatibility or computer display compatibility but nothing about LED or LCD. Those wires you described sound EXACTLYD like the ones my dad unhooked and re-connected, though I don't see any sd cards, perhaps I just missed tem or they were under one of the other boards I didntth get a look at. Hope that helped some.
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
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    Jan 21, 2010
    OK, don't worry about the 2 x SD CARD *sized* things -- you've found the wires that are important.

    That's possible, especially since it turns on (often) a short time after the device powers up. That's one thing we'll look into.

    The image you posted says LCD, and you've confirmed the presence of the pairs of wires which indicate it has CCFL. so we're good there.

    OK, this is important.

    When does the fuse blow?
    1. Can you plug it in and turn on the power at the wall without the fuse blowing?
    2. If so, is there any indication of power (my TV shows a red light on the front even when it's turned off)
    3. *HOW* do you turn it on? Is it a clicky switch with two obvious positions for on and off, or is it via a remote control, or maybe by some other "soft-power on" button.
    4. If you can't answer (3), do you know if the device would turn itself off if you removed the power, or would it stay "turned on" the next time you plugged it in?
    5. Your description sounds like the power fails here, i.e. after you try to turn the TV on at its power switch (whatever that is)
    It may be useful if you can post some pictures of the power supply board where the fuse is located and also the board that the CCFL wires (white and pink/blue) that were disconnected and reconnected). Also take photos of anything else that you think might be useful.

    Note that to be useful the images need to be sharp and we need to be able to read the writing on the board and (preferably) also on the components. We have some resources which describe how to take good photos that you might want to read.
     
    KrisBlueNZ likes this.
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