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Help with LCD 4 digit display

Discussion in 'Radio and Wireless' started by BobWa3, Dec 22, 2020.

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  1. Martaine2005

    Martaine2005

    3,322
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    May 12, 2015
    Merry xmas Bobwa3,
    Filter caps can have many different results depending on circuit design and of course the filters.
    Ranges from ‘non turn on’ to ‘hum’ to Out right short circuit.
    All of which should have been accounted for.
    Most are pretty obvious in amplifiers.
    But a scope would show excessive ripple.
    I would tack a known good cap in circuit and see whether it makes an immediate difference.


    Martin
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,276
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    Nov 17, 2011
    Noise = ripple, peaks and valleys on top of the DC.
    Negative effect = poor display.
     
  3. BobWa3

    BobWa3

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    Dec 22, 2020
    Merry Christmas and thanks to all who have responded.
    I have looked at the 10 volt dc supply and it looks clean on my scope. However, I plan to replace the 1µ farad capacitor with a new tantalum capacitor as soon as it arrives. I ordered new capacitors as the ones I have are quite old. Do tantalum capacitors age on the shelf?
    The circuit I am dealing with is very similar to the one described in the video that Bertus posted. The oscillator chip is providing out of phase 10 volt dc through pins 10 and 11 to drive the display. The frequency is about 94.
    P and 1 on the display board, (I am guessing), are hard wired for the digit "1" and the decimal point which seem to be functioning correctly.
    The logic on the driver chips is correct, but for some reason all of the segments are turned on. The driver chips are not inverting the appropriate output pins. Why?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    It is in my experoence unusual that all driver chips develop the same defect at the same time. But unusual doesn't mean impossible. There may have been an external event (e.g. an ESD shock) that caused this defect. replacing them should be comparatively easy: in a unit that old these are certainly through hole DIP chips.
    Before you do that, hck the phse input (pin 6) of the decoders. Does the square wave from the oscillator arrive there? A broken trace may cause the phase input to be static which could cause the same effect. Although this wouldn't easily explain why at lower supply voltage the phenomenon disappears.
    Also check the blank input (pin 7). It should be static low.
    And Latch enable (pin 1) should be static high.
    Verify that the logic levels do comply with the supply voltage. If the voltage for a logic high level is too low, this could explain the behavior as with a reduction in supply voltage this input voltage would then be detected as high where with a higher supply voltage it might be detected as low.
    Otherwise I'm sorry, I'm running out of ideas.
     
  5. BobWa3

    BobWa3

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    Dec 22, 2020
    Thanks very much for your thoughts. Pin 7 is static low, pin 1 is static high until the voltage drops below 11 and then it becomes a 10 volt square wave causing the display to blink. This function is by design to warn when the battery is getting low. I don't understand how it works but it does.
    The oscillator chip appears to be working properly. It is generating out of phase 10 volt square waves at pins 10 and 11. Pin 11's signal arrives at the common pin of the LCD display and pin 10's signal is seen at the "1" and "P" pin of the display and pin 6 of all of the driver chips.
    On the driver chips the logic voltages are a positive 10 and appropriate for the frequency tuned.
    The problem came on gradually over a period of months with the display fading into 8s and then returning to the proper display. Initially, I thought the problem was my polarized sunglasses. Just prior to presenting the problem on this blog, the circuit began to work while I was probing one of the resistors in the heating circuit. It worked for about 4 hours and then returned to its current state. Repeated attempts to reproduce this event were unsuccessful.
    The tantalum capacitor is one element subject to failure that is common to all the driver chips. I can't imagine how it would cause this problem but when I get the new ones, I will replace it.
    I have also ordered the parts necessary to bread board the entire circuit and will see what I can learn that way.
    Any additional thoughts appreciated. If I learn anything new, I will definitely post.
    [​IMG]

    [mod edit: removed trash]
     
    Last edited by a moderator: Dec 27, 2020
  6. rsneha

    rsneha

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    Jan 12, 2019
    Also check this Electrostatic Capacitance Converter .
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Huh? What do you want to say?
     
  8. BobWa3

    BobWa3

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    Dec 22, 2020
    Could some one explain the purpose of capacitor C2002? It appears to be polarized capacitor based on it's schematic symbol but the capacitance value is listed as 0.033 microfarads. Why would such a low capacitance value require an electrolytic capacitor and do they make such a thing? Would it be OK to replace this capacitor with a ceramic capacitor? Again, thanks for any help.
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

    11,276
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    Nov 17, 2011
    Possibly there to filter the on/off signal for the heating to avoid constantly switching.
    A schematic error. I'm not aware of electrolytics that small.
    Of course.
     
  10. BobWa3

    BobWa3

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    Dec 22, 2020
    Again thanks to all who have responded. The link to the Australian electronics engineer's description of how LCDs work was most helpful. After extensive testing the only conclusion I could reach was that the LCD display itself was defective and the back plane was not functioning even though an appropriate AC signal was being applied at the correct pad on the PCB. I tried cleaning the pad and switching the elastomeric connectors(there are four) but to no avail. My final solution was to ground pin 10 on the multivibrator chip thus driving the LCD with a DC voltage at the segment pins. This solution will theoretically destroy the display over time but it is not working anyway. I have attached a picture of the display working and one of it out of the unit. As you can see the part number on the display has been smeared. If anyone has a clue where I could get a replacement LCD that would be very helpful.
     

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