Connect with us

Help with LCD 4 digit display

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

Scroll to continue with content
  1. BobWa3

    BobWa3

    11
    0
    Dec 22, 2020
    I am trying to fix a 4 digit frequency display on a Dittel FSG 60M avionics transceiver. The radio is 25 years old and works perfectly, but last summer the frequency display began
    to occasionally show all 8s instead of the tuned frequency. Gradually this got worse to the point where it now only shows.8's (although randomly it will occasionally show the correct frequency,
    and if you drop the operating voltage on the radio below 5 volts the correct frequency will appear). I have a maintenance manual and a complete schematic for the radio.
    The display board is relatively simple with very few components. I have measured the logic inputs on all of the driver chips and they are correct. All resistors on the board check out.
    I don't have a good way to check the capacitors. The multivibrator chip is putting out a 5 volt square wave at oscillator pin(13) and at Q+ and Q- pins(10&11).
    The Q outputs are half the frequency of the oscillator output. The LCD display is a mystery to me. I have no data sheet for it but the connection to the PCB is through elastomeric connectors.
    I have attached a copy of the pertinent part of the schematic and pictures of the physical board.

    Any help appreciated.

    Unfortunately my photos cannot be uploaded for some reason

    [edited by a moderator ... Please DONT do underlining like that again !
    Also broke up the long string of text into something easier to read]
     

    Attached Files:

    Last edited by a moderator: Dec 23, 2020
  2. Bluejets

    Bluejets

    5,025
    1,048
    Oct 5, 2014
    Underlined text makes for difficult reading....o_O
     
    davenn likes this.
  3. davenn

    davenn Moderator

    13,877
    1,964
    Sep 5, 2009

    I have edited it :)
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    11,702
    2,717
    Nov 17, 2011
    Possibly the contrast voltage is too high as indicated by a correct display at lower supply voltage. The chips are all standard components. They are likely all good. Can you provide a better resolved image of the connector on the lower right? This section:
    upload_2020-12-23_10-6-6.png
    Also check the supply voltages for correct voltage and low ripple. After 25 years the electrolytic capacitors are probably no longer doing their duty. Simply replacing these may help.
     
  5. BobWa3

    BobWa3

    11
    0
    Dec 22, 2020
    Sorry about the underlining, I thought the program selected it.
    The radio is operated from a 12 volt battery supply. There are no electrolytic capacitors on the display board itself.
    I have looked at the supply voltages coming into the board with a primitive oscilloscope and they appear clean. I have included a better picture of the connector to the circuit board. Connections are labeled:
    20-Ground
    19-Heating
    18-no label
    17-Dimmer
    16-Ground
    15-Blanking(digits flash when supply voltage drops below 11V)this works
    14-14volt+
    13-10volts+
    1-12 are various frequency logics and they are correct
    There is a lighting circuit which is not powered up in my application. There is also a heating circuit to warm the LCD display in cold temperatures.
    How is the contrast set on an LCD? I really don't understand how to properly drive an IMG_1156 (1).jpg LCD.
    What is the reason for the square wave input to the LCD? Thanks for the reply.
    Bob
     
  6. bertus

    bertus Moderator

    2,054
    776
    Nov 8, 2019
    Hello,

    Have a look at this video:


    Bertus
     
    Martaine2005 likes this.
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    11,702
    2,717
    Nov 17, 2011
    There is no explicit contrast voltage on this display, The LCD and the drivers are powered by 10 V (connector P2003, pin 13). CHeck the value of this voltage vs. GND. There is also a series resistor R2006 in the 10 V path. Check its value. Next to R2006 there is an electrolytic capacitor C2003. Check this or simply replace it. R2006 and C2003 form a low pass filter for the 10 V supply. If the filter doesn't work correctly, noise on the 10 V supply may have a negative influence on the operation of the display.
     
    Martaine2005 likes this.
  8. BobWa3

    BobWa3

    11
    0
    Dec 22, 2020
    Thank you for this link. I had looked all over the net for an explanation of how to drive an LCD but everything I found was about LEDs. The answer lies in this video but it will take me a while to fully digest it.


    Great video. I believe the answer to my problem lies in fully understanding the information in this video. That may take a few more viewings, some additional research and some new measurement on the actual board. The 4047 chip on my board is putting out complementary 10 volt square waves from pin 10 and 11 which are fed to the common connection and the P connection on the LCD board which is also connected to a pin labeled "1". The first digit on the display is fixed at one. I have included a low resolution photo of the board. Thanks for your help. I may have some more questions after I have done a bit more reading.
     

    Attached Files:

    Last edited by a moderator: Dec 23, 2020
  9. BobWa3

    BobWa3

    11
    0
    Dec 22, 2020
    I have checked resistor 2006 and it is precisely 100 ohms. The capacitor is not shorted but I haven't measured the actual value. I have included a picture of the actual board with a capacitor C2003 circled. Is this an electrolytic cap?
     

    Attached Files:

  10. BobWa3

    BobWa3

    11
    0
    Dec 22, 2020
     
  11. Martaine2005

    Martaine2005

    3,561
    968
    May 12, 2015
    I have never seen one before.
    Could it be a dipped tantalum?.

    Martin
     
  12. Harald Kapp

    Harald Kapp Moderator Moderator

    11,702
    2,717
    Nov 17, 2011
    I agree, looks like a tantalum. Also supported by the comparatively low value of 1 µF only.
     
  13. Martaine2005

    Martaine2005

    3,561
    968
    May 12, 2015
    @Harald Kapp can I ask how you get 1μF?
    Brown, black, green and blue?
    Does green and blue have no significance? Thanks.

    Martin
     
  14. Harald Kapp

    Harald Kapp Moderator Moderator

    11,702
    2,717
    Nov 17, 2011
    Of course you may ask. Do you expect an answer;)?
    I took the value from the schematic:
    upload_2020-12-24_14-27-30.png
    Where do you see blue?
    Tantalum capacitor color markings are a bit different from e.g. resistors, see this example which incidentally is for a 1 µF capacitor.
     
    Martaine2005 likes this.
  15. bertus

    bertus Moderator

    2,054
    776
    Nov 8, 2019
    Martaine2005 likes this.
  16. BobWa3

    BobWa3

    11
    0
    Dec 22, 2020
    Could someone explain the purpose of capacitor C2003 in this circuit, possible failure modes, and consequences of the failure modes? What measurements could I make to confirm the failure? Thanks again, the responses have been quite helpful.
     
  17. bertus

    bertus Moderator

    2,054
    776
    Nov 8, 2019
    Hello,

    It looks like that C2003 is a decoupling capacitor.
    One of the failure modes of a tantalium capacitor can be a short circuit.
    Measure the voltage over the capacitor and see if it is around the 10 Volts that is coming in via the 100 Ohms resistor.
    LCD driver pcb.png
    You can replace the capacitor with a tantalium capacitor again or an other electrolitic capacitor.

    bertus
     
  18. Martaine2005

    Martaine2005

    3,561
    968
    May 12, 2015
    Hehe, sneaky!.
    I never looked at the schematic. It is blurry when enlarged on my phone.
    It looks like blue by the leads.
    And I thought green was the multiplier and blue the tolerance.
    Thanks for clearing that up.

    Thanks @bertus , I have read that before.
    Never sunk in about the extra colour through first and second colour. Pink in this case, 35v.

    Martin
     
    Last edited: Dec 24, 2020
  19. Harald Kapp

    Harald Kapp Moderator Moderator

    11,702
    2,717
    Nov 17, 2011
    See my post #7.
     
    davenn likes this.
  20. BobWa3

    BobWa3

    11
    0
    Dec 22, 2020
    From post #7 "If the filter doesn't work correctly, noise on the 10 V supply may have a negative influence on the operation of the display." What would the noise look like, what would the negative effect be, how can I test for it?
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Similar Threads
There are no similar threads yet.
Loading...
Electronics Point Logo
Continue to site
Quote of the day

-