Silencing a small oven

Discussion in 'Electronics Repair' started by Jason Roberts, Jan 6, 2017.

  1. Jason Roberts

    Jason Roberts

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    I bought a small oven, the Ktichenaid KCO275. It has an internal speaker that loudly sounds for any input you give it. To silence it I took it apart. As I was unsure where the speaker was I plugged it in to listen. While twisting an input knob the control board shifted and made contact with the frame. My guess is that this created a short as I heard a fuse snap and the unit will no longer power on.

    Now I need to find and replace the fuse. Then find and remove the speaker.

    I have no idea where the fuse is, my guess for the speaker is the round black element on the control board labeled BZ?

    [​IMG]
     
    Jason Roberts, Jan 6, 2017
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  2. Jason Roberts

    davenn Moderator

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    hi, welcome :)

    yup, that's it
     
    davenn, Jan 6, 2017
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  3. Jason Roberts

    Jason Roberts

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    So, the speaker is found. My plan is to just use the solder gun to ease it out of the board. For other beginners I found this page helpful in identifying components.

    http://www.uchobby.com/index.php/2007/07/15/identifying-electronic-components/

    If this does not have a fuse I wonder what happened. It must have been physical as I heard the "snap" sound and the board went dark. There was no smell and I can see no burn marks or discoloration anywhere. The heating elements were not even on, I was just using the controls to adjust settings when the board touched the frame.

    I get continuity on all the power inputs at the board to the end of the power cord where it plugs into the outlet.
     
    Jason Roberts, Jan 6, 2017
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  4. Jason Roberts

    davenn Moderator

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    sounds like a plan .... specifically, it's a piezo buzzer ... down the page a bit in that link


    oops ... you really need to be extremely careful and shouldn't really power a device up when it's in pieces unless you are really sure of what you are doing
    .... lesson learnt ;)

    are there any components on the other side of the display/control board ?

    I don't see any obvious fuses on either board, which is very surprising

    what is the black thing arrowed, beside the wire terminal. Does it have a part number ??

    GvuL3Kl.jpg



    Dave
     
    davenn, Jan 6, 2017
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  5. Jason Roberts

    Jason Roberts

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    This may end up as a learning event. I spent 260 on this thing so I hope I find a way out, I only used it once so at least I got something out of it.

    Item with arrow is labeled T1

    [​IMG]

    Back of control board

    [​IMG]

    Back of power board

    [​IMG]
     
    Jason Roberts, Jan 6, 2017
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  6. Jason Roberts

    davenn Moderator

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    OK, a transistor

    still don't see any obvious fusing ... VERY unusual

    nor any obviously popped components, not sure what to suggest next .... do you know what part/area of the board contacted the metal casing ?
     
    davenn, Jan 6, 2017
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  7. Jason Roberts

    Jason Roberts

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    After dis-assembly I held the board clear and plugged it in. It powered up and the screen lit up as usual. Then I went to twist the dial settings which causes the buzzer to sound. Here is where I let the board move and it touched the frame.

    [​IMG]
     
    Jason Roberts, Jan 6, 2017
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  8. Jason Roberts

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Please don't tell us that you powered on a mains appliance whilst disassembled and holding a potentially live part of the circuit.

    Aaaaagh!
     
    (*steve*), Jan 6, 2017
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  9. Jason Roberts

    Jason Roberts

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    Yes, this was a mistake. This was dangerous and I should have been more careful.

    That said, the picture is a bit misleading as the board was in its holder so I was holding and touching only the controls as you would in everyday use. As the control section was un-screwed I allowed it to move after locating the sound of the beep and that is when it contacted the frame.

    Re-creation of crude death wish move:

    [​IMG]
     
    Jason Roberts, Jan 6, 2017
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  10. Jason Roberts

    Jason Roberts

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    I found some help from betwixt on this.

    "The AC passes through the 47 Ohm resistor (R2) then two series diodes (D1 & D2) to a Pi filter made of L1 and the two brown electrolytics. From there is goes directly to the drain of the Viper12 (8-pin IC is a switching voltage regulator). That means the output is taken from the source pins and the feedback is 'lifted' above ground as it should never be more than 1V above source voltage on that device."

    However, the root cause was slightly simpler. The outlet this is plugged into has in integrated GFCI. This must have been tripped and as it was right near the unit it created the sound I heard seeming to come from the power board. After re-setting the GFCI the unit powers up.

    So it seems I am not even capable of frying myself even while trying. Just going to cover the speaker element to silence it instead of soldering and carefully re-assemble the unit.
     
    Jason Roberts, Jan 6, 2017
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