FT690mk2 LCD almost dead. How to test?

Discussion in 'Radio Circuits, Repair & Performance' started by BX2ABT, Mar 8, 2021.

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  1. BX2ABT

    BX2ABT Ham Member QRZ Page

    Preparing for the new 6m season I found that my Yaesu FT690mk2 (or mkII if you prefer) has finally become unusable because of the dying LCD. Every year for the past 6 it has degraded bit by bit, so I am not surprised, only sad. At least I presume it's dying, although some parts are still lighting up perfectly.


    As you can see in the photo I already taken the set apart. Now the question is how can I check if the display is still working? Is it simply a matter of putting a certain voltage on each pin or are there other methods? I googled this, but only answers regarding mobile phones came up.

    The LCD itself is only connected to the HD61391A41 driver chip, so nothing complicated there.

    I hope someone can help me on my way with some tips or hints how to troubleshoot this. We don't have a Yaesu repair shop in Taiwan and even if we had this rig is regarded as too old to repair, so I'm on my own here. Cheers and 73 de BX2ABT
  2. G4COE

    G4COE Ham Member QRZ Page

    The LCD is not dying, just that some segments are missing with the looks, this is usually due to the connectors to the LCD screen..

    Now then, there are usually two way the connections are made-:

    1) A spongy foam material is used that has conductive strips, imagine a pile of jam sandwiches, the jam being conductive thie strip is held against the display screen by slight pressure to make contact.

    2) Some displays uses pins clipped to the display.

    3) LCD and TV screens is connected by flat ribbon cable and bonded to the display panel with special conductive epoxy (and gives lots of screen problems), we need not be too concerned about this method here.

    Simply some of the connections are not being made to the display..... the conductive sponge can be easily removed with extreme care an d cleaned with a 'soft lint free cloth' moisten with isopropanal or similar because it's fast drying and leaves no residue... avoid contact cleaner and such like.

    Now for the crimped types, should the 'fork ends' the grabs the display work loose can pose a problem..... squeeze them and the screen might crack.

    One trick that is used with monitor and TV screens, to avoid the monitor or TV being thrown in dustbin...... sometimes a cure can be had by packing the area in question with stiff foam or rubber to apply constant pressure to their connections... the fault here are lines or stripes usually running down the screen. Whether this remedy can be applied to the above faults depends on the nature of the fault.

    Although this is a clock take a look and it'll give you some idea, some display's uses one strip and some uses two, one at the top and one at the bottom..... LCD display repair. I am sure better videos can be found.

    Sorry forgot to mention, should it be foam that is used ensure you fit the foam back the right way as only two sides are conductive... and should it be crimps pins that is used, check the soldering on the pcb for badly soldered connections.

    Last edited: Mar 8, 2021
  3. BX2ABT

    BX2ABT Ham Member QRZ Page

    Hello Dave,

    Thanks for the long reply, I really appreciate you taking the time. However, in my FT690mk2 no ribbon cables or spongy materials as the controller IC is connected directly to the LCD via the PCB. You can see that here on the photo: all the soldered dots are the leads to each segment of the LCD (32 in all).

    I haven't taken the LCD apart yet, but I guess it is not of much use, so I'll leave it for now. I did some more googling and found this article: https://www.eeweb.com/liquid-crystal-display-lcd-tester/ . I don't have a 4047 or something similar lying around, so I'll go to the store and get me on, because it seems a nice little thing to make and try. To be continued.....
  4. G4COE

    G4COE Ham Member QRZ Page

    You need to look at the 'glass display panel' and see how the screen is connected - you can't solder to display panel unless the use clips to the actual glass.

    DO NOT try and resolder those connectors because of damaging the foam rubber connectors 'should' they be used - you need to find out how the display panel is connected first, if the display uses clips on the panel then you are OK to solder.... but don't overheat or the clips might conduct heat too much and damage the impregnated connectors on the display glass.

    One thing you could try is with it switch ON try applying 'slight' pressure between display front and back of the board with finger and thumb along the edges and see if the display alters the missing segments.

    Missing segments in a LCD display are often mechanical rather than component failure and should be the first thing to investigate. You can play about with components all day long and get nowhere, if the LCD isn't making connections.... it isn't going to illuminate the relevant segments.

    Taking a closer look after enlarging it looks like it uses clips - I could be wrong, the inside row, 6th down looks suspicious.... is that a good joint or not??

    Last edited: Mar 9, 2021
  5. BX2ABT

    BX2ABT Ham Member QRZ Page

    Hello Dave,

    Okay, finally understood what you meant. Disassembled the LCD and saw the "spongy foam material" a.k.a. zebra connector. I tested conductivity and it seems to be doing its job.
    I cleaned everything, but still no improvement. I got some other zebra connector (from another LCD) and the same result.
    Certain things always work, others not. LSB and CW work fine, USB sometimes, FM never. Digits are never complete, although LOW does have a dash under it now. I wonder if the traces on the glass pane are the culprit.
  6. G4COE

    G4COE Ham Member QRZ Page

    I *think* I would try to avoid rubbing the traces although I have wiped the contact area on the display glass where that pink foam rubber sits..... that is the connector strip I mentioned, with a clean lint free cloth very gently.... no harm in damping the cloth very lightly with isopropanol or similar that dry's quick and leaves no residue - that strip will pull out so you can wipe the top and bottom be sure you put it back the same way you took it out.

    If you look at the display at about 45 degrees, you should see the connectors along the inside edge a very gentle wipe there will do no harm..... I would very much think it's a connector issue rather than anything else - I could always be wrong though.... apart from cracked displays the actual rubber strip itself can cause problems especially if the rubber starts to become hard.

    I remember well, the very first display I did, a brand spanking new expensive transistor radio I repaired, I stripped the display and the boss walked in.... "No, don't do that the liquid crystal will all run out", he said looking rather anxious expecting to find a puddle on the bench. When I put it all back and he saw it working he was all smiles.

    When swapping connectors they need to have the same pitch - connector spacing. I forgot to mention clean under the rubber strip as well on the circuit board.


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