Repairing a watch without sound

Comprobando altavoz piezoelectrico con el modulo de la tapa trasera

In this article we will see how to repair a clock that has no sound, checking the functioning of the piezoelectric speaker and replacing it with a new one

Some time ago I bought a second-hand Casio DW-240 on ebay. This model is a classic of the brand, with almost 30 years behind it.

Among its most outstanding features, and unfortunately not found in current Casio models are:

  • Module 690
  • Multiple functions: stopwatch, countdown, alarm, time signal
  • REM function. This function allows us to activate an indicator on the screen of the clock that will remind us to remember something. This feature is typical of the modules of the 80's or 90's, and is no longer seen in the current models.
  • Resin box with internal steel reinforcement
  • 5-year battery cr2016

It has the typical ailments of age and use, but after a button cleaning, a battery change and a "reset" of the module it came back to life again without problems.

The problem was in checking the alarms: they didn't go off.

The animation of the alarm symbol on the display was correct, and the behavior was normal in the module, but it did not sound.

So, I had to open the clock again to check the connections of the module with the back cover and the speaker.

Index

    The piezoelectric speaker

    As we can see in most Casio watches, on the back of the watch, we can see a metallic circle of different color.

    This circle is a piezoelectric speaker that allows the watch to "beep" on time signals and alarms. In addition, if we look at the module itself, it has a couple of tabs, to connect the power of the module with the speaker, so it can "beep".

    Connecting the speaker to the module

    Caja Casio DW-240 con rosca de tornillo a reparar
    Casio DW-240 box with screw thread to be repaired

    Since the watch didn't "beep", I could only think of two possible causes:

    • The piezoelectric is broken, so even if it made good contact with the module's eyelashes, it would be impossible for the clock to beep.
    • The module does not make proper contact with the piezoelectric. If the tabs on the module that contact the back cover and the piezoelectric are crooked, they may not make contact in their correct position, and the speaker may not be powered.
    Tapa trasera con el altavoz
    Rear cover with piezoelectric speaker

    As we can see in the photo above, the piezoelectric has a small mark at the bottom that indicates where the module's power supply tab makes contact. Obviously the other tab makes contact with the back cover itself, thus receiving the necessary power to operate.

    The best way to know what the problem was is to check the speaker. We will use a pair of wires with crocodile teeth to, on one side, connect each one to each tab of the module and connect on the other end of the wire to the cover and the piezoelectric.

    Comprobando altavoz piezoelectrico con el modulo de la tapa trasera
    Checking the piezoelectric speaker with the

    And here we see how we make the connections on the back cover. The negative pole of the module is connected to the back cover of the clock and the negative pole is connected to the top of the piezo.

    Comprobando altavoz piezoelectrico de la tapa trasera
    Checking back cover piezoelectric speaker

    Now, with the assembly done, we must press a button on the clock to check if the clock "beeps". In this case, unfortunately there was no way the piezo would "beep", so we can conclude that it is broken and we will have to make a speaker change to recover the sound in the clock.

    Fortunately, I had another piezo from a watch that drowned at sea one unfortunate day. We can see the different parts that I explained before. The negative pole is the one that will be connected to the golden part, while the positive one will be connected to the grey part.

    Altavoz piezoelectrico
    New piezoelectric speaker

    After checking that the new piezo does work (with the cables and as we did before), we need to perform a series of fundamental tests.

    I mean to know in which position on the back cover we must place the piezo so that it makes correct contact with the tabs of the module and recover the sound of the clock.

     

    Tapa trasera con el nuevo altavoz
    Back cover with the new piezoelectric speaker

    If we are lucky that the piezo is similar in size to the previous one, we can be guided by the position of the original speaker on the cover, but if not, we will have to use the trial/error method. We will guide ourselves by the position of the module's tabs on the back cover to connect correctly to the piezo's poles.

    In this case, as the piezo is not the same size, we can check in the following picture that we have to move it a little bit up and to the left so that the contact with the module is correct.

    Tapa trasera con el nuevo piezoelectrico colocado
    Rear cover with the new piezo in its correct position

    Once the correct position has been checked, we can attach the piezo to the rear cover. It is important that the piezo is in contact with the cover, otherwise even if it is in the correct position, it will not make contact and will not work.

    In this case, the Casio DW-240 with the new piezo, already sounds its various alarms correctly.

     

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