segments failure in tactel display board.(dual digital display)

Discussion in 'General Electronics Chat' started by xchcui, Oct 23, 2015.

  1. xchcui

    Thread Starter Member

    May 12, 2014
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    Hi.

    Several segments of a 7 dual digital display are not working.
    The 7 segments display is mounted in a socket that soldered to the PCB(tactel display board).
    This part ,as shown in the photo is connected to the controller board of the air conditioner.
    The tactel display unit looks like that: tactel display unit.jpg (i don't have the pcb photo).
    And the dual seven digital display is the one that marked with red square at the following data sheet(LC 4022-11):

    LC4022-11-723x1024.jpg

    The air conditioner is about 15 years old.
    What could be the reason that several of the segments don't work?
    If i will pull out the failed dual display from its socket and put a new one,will it solve the problem or
    it might be something else that,actually,prevent the current reach the segments rather the segment are failing?
    Do those kind of led segments are actually prone to fail themself(and how?)or in most cases the problem is outside the seven segments display?

    Thanks in advance.
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Could be the LEDs or the drive circuitry. No one can tell you what failed or caused the failure without knowing what failed and being able to dissect the failed part(s).

    Since the LED display is socketed, you can remove it and test the individual segments. If some are no longer functioning, replacing it may fix the problem; possibly temporarily. If the display tests good then, obviously, the problem is elsewhere...
     
  3. xchcui

    Thread Starter Member

    May 12, 2014
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    I am familiar with how to test the led display,but the problem is that the air-conditioner is not mine and i can't work on it more than once and i don't want to buy the part and find out that the old one is ok.
    So i mentioned the age of the air conditoner,the data sheet of the led display and a photo of the unit in order that some one might know,from experience with that kind of unit,what component,usually,fails in those unit.
    I assume that without checking all the circuit,it is hard to help through the web,but i rely on your experience.
    By the way,why did you say:
    ?
    Why temporarily?what might cause it fail again,assuming that the led display is the fail part?
     
  4. JohnInTX

    Moderator

    Jun 26, 2012
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    This display does not have to be multiplexed so its harder to tell from a distance - especially without a schematic.
    If it IS multiplexed and its missing segments from only ONE digit, its likely not the driver but the display or connections.
    If it is NOT multiplexed (each segment driven completely independently such as from a 7447), then it could be anything along the line.

    Since its in a socket, my first thought was a bad socket, corrosion etc. Bring some contact cleaner.

    You are at a disadvantage getting only one shot at it - when I have only that option, I bring lots of parts and equipment.

    Good luck!
     
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  5. dl324

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    Mar 30, 2015
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    If the display failed due to a driver failure, replacing the display could be a temporary solution. If the display failed on it's own, replacing it would be a "permanent" fix.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Did the failed segments all go out at the same time?

    If the display is in a socket, remove and reinstall it. 15 years is a long time for a component pin in a socket to remain free of corrosion, especially in an air conditioner.

    ak
     
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  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    You answered your own question..
     
  8. xchcui

    Thread Starter Member

    May 12, 2014
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    I know that the segments failed before a long time,but i don't know if the failed segments all go out the same time.
    I assume that when the leds are not failing at the same time,it can be attributed more to a corrosion problem.Am i right?
    I was trying to avoid of removing the display until i will find a replacement,because i was worry that one of the legs will stuck or break(after so long time).
    Will it be enough to spray a contact cleaner on the display legs while it still connected to the socket or i better disconnect the display?and spray contact cleaner,also, into the socket's holes?
    On the data sheet of the dual seven segment(LA40X2-11/LC40X2-11)is shown that this particularly displays with the 16 pins are direct drive.Doesn't it mean that it is not multiplexed?
    While the other displays(LD40X2-11/LE40X2-11) are mentioned as a duplex drivers,so we can understand for sure that they are multiplexed,since they have to lit two digit with only 10 pins.
    why does multiplexed display that is missing segments from only one digit is not likly because of the driver?
    If the microprocessor is used 2 decoder(one for each digit),can't one decoder be failed and caused only one digit(the one that it is connect to)to be failed?
     
  9. JohnInTX

    Moderator

    Jun 26, 2012
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    It could be direct drive OR multiplexed. You would have to see the schematic or inspect the board. To use this particular display in a multiplexed circuit, all you have to do is tie corresponding segments together on the PC board i.e. connect pins 10 and 15 together for segment A, pins 12 and 13 for segment B and so on. To use the SAME display in a direct driven circuit, each set of digit segments would be driven by a decoder (7447), individual pins from the microprocessor or something similar. I use displays like this in multiplexed setups all the time.
    Because the segment driver is common to both digits.
    Yes but you indicated that individual segments were missing, not entire digits. If you have one correctly operating digit and one completely dark then yes, I would suspect the digit driver.

    But I still think that the socket is the most likely problem.
     
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  10. xchcui

    Thread Starter Member

    May 12, 2014
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    Thanks for your explanations.
    It will help me alot to inspect the board and try to solve the problem.
    I assume that if the problem,as you mentioned,is the socket,removing and installing the display(as AnalogKid suggested)will mechanically remove the corrosive/oxide metal and maybe will solve the problem.
    If i will find out that this was the problem,how do you suggest to clean/remove the corrosive/oxide display pins and the display socket?(in spite of the fact that the mechanical action is going to scretch some of it)
    I usually clean PCB after soldering with isopropyl alcohol.
    Should i use it also for this purpose or i should use other kind of substance/spray?
     
  11. JohnInTX

    Moderator

    Jun 26, 2012
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    I use PureTronics Extra Strength Contact Cleaner but there are lots of others. Look at #6600 FluxRemover to clean boards after soldering if you want. I also use a Flux Brush with the bristles cut short and stiff to clean other junk from around contacts, switches etc.
    Good luck.
     
  12. xchcui

    Thread Starter Member

    May 12, 2014
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    Hi again.
    Finally i had a chance to open the part again.
    I pull out the 7 segments display from its socket,check each pin with DMM
    and found out that some of the segments didn't turn on.
    So,i guess that the display is the failed component.
    I don't know if i will have another chance to fix the problem,
    but it is important to know,anyway,the cause of that fault.
    I meant to say that dl324 said:
    So,how can i know if the segments burn out as a naturally process due to aging reason or
    the segments failed due to a driver failure or something like that?
    It wil be useless to put a new display if after short time the driver will damage it again.
    I had noticed that one digit of the display a-g pins is connected to one drive
    and the second one to another driver.Them both the same type-HC373(D type latches).
    BTW,what can cause the driver to ruin the display?
     
  13. dl324

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    Mar 30, 2015
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    74HC logic shouldn't be able to damage LEDs. Lifetime is in the 10's of thousands of hours when being driven at 10mA/segment; driving at lower currents increases lifetime significantly. Can you provide a schematic of the LED drive circuit? Was there any corrosion on the pins of the failed segments?

    Could also have been a random failure. The semiconductor device wearout curve is like a tub. High failure rate initially (infant mortality), low failure rate for 10's of thousands of hours (random failures), then high failure rate (wearout); overstress failures can happen anytime. Operation at high temperature/humidity and/or temperature cycling will accelerate failure.
     
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  14. xchcui

    Thread Starter Member

    May 12, 2014
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    I was able to pull the 7 segments display from its socket easily,while there was no corrosion on the pins.
    I rubbed the pins,anyway,until they become shiny,especially the ones that connected to the failure segments,but the segments didn't turned on when checked secondly with the DMM.The visual condition of all the chips on the pcb looks very clean and good.
    I don't have a schematic of the led drive circuit or a photo since
    The air conditioner,as i mentioned before,is not mine and i had a limited time to collect information and follow the circuit,but i managed to write the main chips that it was contained:There are 5 drivers-type hc373(as i mentioned before,two of them related to the 7 segments display).
    There is one MCU:type 68HC05P9.And an under circuit voltage unit-TYPE-MC34064.
    BTW,all those time between the first time i checked the display and second one,i try to find a
    replacement dual 7 segment display(LC 4022-11),but i didn't find it anywhere.
    Except in the"PLANET ONE COMPONENTS"site and in the LEDTECH company site,but with no reponse,
    likely since i told them that i need only 1-2 pieces,while i assume they sell only big quantities.
    Is it possible to fix that 7 segments display?(since i assume that i wont find a replacment)
    It seems that it is impossible,since it sealed on its back with a kind of epoxy.
    But maybe is there a way?
     
  15. JohnInTX

    Moderator

    Jun 26, 2012
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    Last edited: Nov 28, 2015
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  16. dl324

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    No way. The encapsulation protects the die from corrosion and bond wires from mechanical stress. Even if you could remove it to access the LEDs, you'd need to be able to remove and attach another die, then attach gold bond wires (which themselves are very thin and fragile). Then you'd need to be able to reconstruct the light pipes when you resealed.
     
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  17. xchcui

    Thread Starter Member

    May 12, 2014
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    .
    I was already sure that i will not be able to find it after a lot of searching on the net.
    So,thanks for the link.
    I noticed that the color of the segments is red(oppose to the green segments in my failed one),but besides that
    all the other specification looks the same,anyway,the color is less important if all the rest are identical.
    Even if i was deciding to try and fix it,after your explanation i was absolutely avoid to do that.
    You gave enough reasons why i should not try.
    BTW,i understand that 74HC logic shouldn't be able to damage LEDs(such in my case),but to what situation
    did you mean when you said:
    Which driver can damage the segments and how?
     
  18. dl324

    Distinguished Member

    Mar 30, 2015
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    Didn't know at the time that the drivers were 74HC. Or many other particulars for that matter.

    If displays are multiplexed, it's common to drive the LEDs at a higher peak current to increase average brightness. If something happens to the driving circuit, it's possible to subject the LEDs to high DC currents that would decrease their lifetimes.

    Then there are the circuits that are poorly designed to begin with...
     
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  19. JohnInTX

    Moderator

    Jun 26, 2012
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    Red would work fine. Mouser also stocks Yellow. Other distributors may stock the series in the green that you want. Search on Kingbright DC04-GWA for the green one.
     
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  20. xchcui

    Thread Starter Member

    May 12, 2014
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    Thank you very much for all the responses.
    It was a great help.
    Thanks.:)
     
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