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TV Set 'Fails' Now and Then

Discussion in 'Repairing Electronics' started by MrAl, May 23, 2014.

  1. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    In the past i have owned CRT televisions but i went to an LCD with fluorescent backlight several years ago.

    Up to now it has worked ok but a little strange sometimes, but it was never as bad as it seems now because now sometimes it will not turn on. That is, the display will not light up. I still get sound, but the display does not light up until after i disconnect power from the power plug rather than turn it off with the remote, and then plug it back in and turn it back on. It seems to need a few minutes 'rest' between running on standby to running on full screen power.
    It runs on standby because when i turn it off it stays plugged in, and the power light lights up as orange, but when it is turned on fully it lights up blue.
    After i disconnect it entirely and let it rest for a minute it will turn back on.

    This is my first LCD TV even though i had monitors much longer. Is this a standard failure mode for this kind of television set? Maybe something is going inside that can be repaired?

    Thanks for any ideas.
     
  2. fezder

    fezder Well-Known Member

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    I'm not an expert in reparing anything, just luck that i'we been able to fix anything....But, have you checked 'lytic caps? they seem to be one of major faults
    and more experienced members, correct if i'm wrong

    and beware shock-hazard, fl-driver contains HV-stuff

    minor edit: i meant fl, not cfl, but that should'nt matter much thought :)
     
    Last edited: May 23, 2014
  3. Mickster

    Mickster Well-Known Member

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  4. dave

    Dave New Member

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  5. nsaspook

    nsaspook Well-Known Member

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    It could be bad caps or bad solder joints. Had a TV with much the same problem. My problem was poor solder joints in the switching power supply heating the leads and pad causing an open circuit at times.

    https://flic.kr/p/cYCECN
    https://flic.kr/p/cYCEFy

    bad solder joints
     
  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Actually it should be CCFL anyway :D
     
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  7. tunedwolf

    tunedwolf Well-Known Member

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  8. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    No i have not tested any electrolytic caps yet but yes that sounds like it could be the problem. If i have to change the electrolytic caps in one more product i have purchased in the past though i am going to scream. :)

    I'd have to take it apart to test as i have not attempted that yet either. I wonder how much of a pain that is going to be.

    It's an AOC tv and i would not recommend this brand to anyone, and not just because of this problem but because it's a pretty sucky tv to begin with. The contrast is poor, and when i press the volume UP or DOWN button the tv input source changes, which means it switches from HDMI input to maybe TV input or some other input, which means i loose the picture until i reset that setting back to HDMI. I've had it for 5 years and 1 month and 21 days now to maybe it is time for a new one. It's only 22 inch anyway. Might still be interesting to look inside though.
     
  9. fezder

    fezder Well-Known Member

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    at least it's easy repair if it's caps fault :D
     
  10. fezder

    fezder Well-Known Member

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    lol, so there is cfl and ccfl, good to know haha :D...
     
  11. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Cold Cathode Flourescent Lights are what LCD's used before LED's, hence CCFL.
     
  12. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Yes, unless they are really packed in tight. Not sure if i will attempt this yet or not, still thinking about it.
     
  13. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Do you have an ESR meter?, it's pretty well essential for finding duff electrolytics.

    However, assuming that is the problem, often they visibly bulge when faulty (but not always).
     
  14. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi Nigel,

    Well i never got around to getting an ESR meter, but i have checked electro caps before using the method in the attachment (using a scope and waveform generator). The voltage across the cap tells the tale of what the ESR estimate would be (note the big difference in vertical scales in the waveforms in the attachment).
    One of these days i might get a meter but not sure when.
     

    Attached Files:

  15. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    As long as the set powers up you don't need a waveform generator, simply scope the outputs on the supply rails for excessive ripple - the rectifiers are often through-hole and mounted on the top, so it's easy to scope their outputs :D

    Not so easy when they use SM rectifiers though :mad: which is where ESR meters come in handy.

    However, apart from caps it's possible you have a duff CCFL tube?, if it's intermittent it can cause the set to shut down intermittently, depending on the design of the set it might shutdown totally (and possibly flash the power LED), or just shut down the picture. It's often difficult to diagnose, but by downloading the datasheet for the invert IC you can often figure out how to disable the safety shut down, but ONLY for diagnostic purposes, you MUST NOT leave it disconnected.
     
  16. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi Nigel,

    Yes that's a good idea, provided i have the guts to poke around in there with the power still turned on <he he> :)

    The way i did a computer power supply was i simply pulled the main caps and checked every one of them with the waveform gen and scope. I was very surprised to find that when they go like that they just dont change capacitance by a small amount or even 50 percent, but by a very large amount making them almost useless in the application. Same for the ESR, it went way way up. Nice thing is, it's easy to detect that much of a change.

    If i do get around to opening this thing up (i probably will eventually even if just to examine it inside, just not sure when) i'll jump back here and post some notes and maybe some pics too. The more we talk about it the more curious i tend to get about what it looks like inside. It have taken many many many a CRT TV and scopes and the like apart over the years but this would be my first LCD TV.

    Think it would be hard to get back together once the back is taken off, or do you anticipate any other problems that might come up considering i'll just be examining the internal contents (and some pics) and maybe not doing any testing yet ?
     
  17. tunedwolf

    tunedwolf Well-Known Member

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    Don't be frightened of it...just get in there, pull the power supply board out replace all the electrolytic caps shotgun style. Stick the board back in and fire it up. If your problem was just that, you should be rewarded with a good picture again and if not, look at the inverters and tubes. As Nigel already said, defeat the safety shutdown and see if the tubes are struggling. If they are, and you want to go that far, there are companies out there that can supply replacements, and if not, then bin it and buy yourself a nice new TV :)
     
  18. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Thanks, but i am still kicking around the idea of going with a slightly larger size screen too, as a sort of upgrade.
    I see the prices have come down quite a bit since i last bought one which was a long time ago now.
    I also dont like the way this set handles the multiple program content sizes that i get piped in from the content provider. Sometimes the picture gets very small, smaller than the frame of the TV. It's also not 1080p, it's only 720p which isnt as good.
     
  19. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Generally (and I've checked hundreds, if not thousands) the capacitance barely drops, and is almost always well within tolerance - the problem is solely the ESR, which is why an ESR meter is essential, and a capacitance meter useless. Using a scope to check the ripple is effectively testing the ESR, not the capacitance - although in the unlikely event it was low capacitance it would show that as well, and with a similar result. However, if it was low capacitance, it's automatically high ESR as well :D

    Interestingly I've seen many PSU's where the reservoir capacitor is completely O/C, yet the supplies still work - apart from a few where the PSU destroys itself under those conditions. You also need to be aware that high voltage caps (such as reservoirs) have high ESR anyway, and rarely fail due to that - because they only work at low frequencies (100/120Hz).
     
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  20. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,


    Well the caps in my power supply went down in capacitance by 10 times, from something like 2000uf to 200uf, as well as the ESR going high.

    Because after all even with low capacitance and high ESR it is still just an RC circuit, i can tell by the waveform if the ESR is high. It's just easier with a pulse rather than with a low frequency sine so i used a waveform generator. The part where the voltage rises suddenly is the ESR indicator, and the part that slopes upward slowly is due to the capacitance part. If the fast rising voltage change is V while the current is I then the ESR must be V/I, so that part is pretty simple, so i didnt need an ESR meter for that. Ditto for the capacitance but using C=i*dt/dv as the rule. Note also that i removed the caps from the board to test them.

    The caps i had been talking about where all relatively low voltage types like 25v or less. The test in post 13 had shown a cap that changed in value by a factor of 71.4 as per the measurements. I wondered if this could have been one of those fake caps that came in from China a while back.
     
    Last edited: May 25, 2014
  21. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Was this a switch-mode PSU?.
     

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