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Troubleshooting LCD (alphanum) display problem

Discussion in 'Electronic Design' started by Don Y, Oct 8, 2011.

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  1. Don Y

    Don Y Guest

    Hi,

    I have a NAVITEK LAN/cable tester (for want of a better name).
    <http://www.tequipment.net/Ideal33-846.asp>

    It includes a two line alphanumeric (plus icons) display to
    present prompts and results to the user.

    The display has developed faults -- similar to "dropped pixels".
    I'm trying to figure out the best way to sort out where the
    problem might lie -- in the display connection *or* in the
    (serial) interface to the display PCB.

    And, of course, ideas as to how it might be repaired. While
    the device wasn't inexpensive ($500), it's long obsolete so
    probably not worth sending it in for service (I will contact
    the manufacturer just to see how outrageous their service
    prices are :> ).

    OTOH, it's *really* handy when I need it.

    I.e., fixed it would be wonderful. Even "as is", it has some
    value (though it requires patience to determine what the display
    is saying as the prompts cycle by).

    I suspect the first step would be to see if the individual
    "displays" (prompts?) are consistent each time they appear
    or if they exhibit some variation.

    Then, determine if there are any patterns in the faults.

    Finally, mechanically stress the device to see if these patterns
    change.

    Any other things I might want to try or observe?

    Thx,
    --don
     
  2. Don Y

    Don Y Guest

    Opened a brand new lithium 9V just before my post
    (after seeing the problem with the *installed* battery,
    previously)
    That is my current thinking. But, I will have to drag out
    a movie camera to capture the various displays (it cycles
    through various displays every second or two as it runs
    its tests -- no way to "pause" it anywhere but at the start
    of the test sequence). Then, I can look to see if certain
    pixels are missing in *all* characters in a particular
    display position -- or, just a particular character
    (i.e., the font may be defective, assuming pixels are
    being transferred to the display instead of characters)

    There doesn't *seem* to be any change in any *particular*
    "display". I just haven't been able to correlate those
    deficiencies with *other* "displays"
    The "display board" interfaces to the "processor board" via an
    8-pin connector fabricated similar to the way LCD glass mates to
    an elastomeric connector (i.e., just "touching" surfaces, not
    "engaging" pins)

    [this complicates troubleshooting as opening the package
    causes the display board to lose its connection to the
    main board -- I'd have to find some suitable vias and
    attach pigtails]

    So, I suspect the display interface is serial -- it doesn't make
    sense to interpose a serializer to a parallel display.

    The display has icons as well as the graphic area for the 2 lines
    of text so I imagine this might complicate locating a COTS replacement.
    If this is indeed the case, then being able to objectively
    review images of the various "screens"/displays should allow
    me to verify those pixels are, indeed, dead. Given that
    the missing pixels don't seem to change (e.g., when flexing
    the device), any other result would suggest a defect in the
    data being sent *to* the display (i.e., font defined in rom)
     
  3. Joerg

    Joerg Guest

    Yes: Open it up and see if the LCD contacts the circuit board or a
    sub-module via the usual two rubber strips. They look like thin
    mechnical buffers but in reality they contain contact fibers. Over time
    this can dry out a bit and then some segments lose contact between LCD
    glass and the board below. This is also true in most stand-alone LCD
    modules except that you often can't see the rubbers because they are
    behind a thin aluminum frame. Such conducting rubber strips can be bought.

    A typical sign is that when you have the unit apart and then gently
    press on the glass sides where the strips are the fault pattern changes.
    But not always.

    Of course there's always a chance that something else happened, like on
    one of our large atomic wall clocks: One fine days the leading "1" had
    vanished. So it would show 1:00 instead of 11:00. Opened it up but
    turned out the driver was shot. Since that was under a blob of tar that
    was the end of the clock.
     
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