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PC Speakers malfunction. Need help

Discussion in 'Troubleshooting and Repair' started by Kar, Oct 15, 2014.

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  1. Kar

    Kar

    25
    2
    Oct 15, 2014
    Hello guys, I'm new here and I need help because I don't know much about this stuff.

    This problem with my speakers started a week ago. First time I turn the main speaker on, the music plays fine for a few seconds, then the volume starts randomly decreasing and going back to normal, and even the LED light does the same, until it shuts down completely and there is no sound or LED light. Next time I turn it back on, I can hear the music for just a second this time, then it does the same thing again.

    I took the transformer off just to see if I could get a replacement, but the guy in the store tested it and told me it was working fine. So what could be causing the problem?
    It's not completely dead, but instead the power is irregular and then it shuts down, probably as a safety measure.

    I need to repair it . I would appreciate any advice.
    Also if you think you can help me I could post some pics.

    I give some details below.

    The speakers are Thonet & Vander Kürbis, bought 6 or 7 years ago
    The transformer I took off reads :Input 240v~50hz Output 13.5v x 2 ~1.6A
    From the transformer 3 cables go into the board, and that's all I know.

    Thanks in advance.
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    hi ya Kar
    welcome :)

    show us some sharp and well lit pics of inside and out of the system

    it may be something as simple as smoothing capacitors in the power supply section

    Dave
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    Yes, post some good clear pics, and tell us the model number. It's not likely we'll find any publically available service information, but it's worth a try.
     
  4. Kar

    Kar

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    Oct 15, 2014
    Thanks guys, I'll post pictures tomorrow with sunlight so that the image looks clear.
     
  5. Kar

    Kar

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    Oct 15, 2014
    Ok, here's the picture. The power from the transformer is supposed to be connected right there in the bottom left corner.
    [​IMG]
    Also there is this ugly stuff below the big capacitors. I don't know if that's normal.
    [​IMG]
     
  6. davenn

    davenn Moderator

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    Sep 5, 2009
    there looks to be a lot of silicone sealer around the caps and other parts to stop them moving around

    do you have a multimeter ?
    you are not going to get far testing without one

    Dave
     
  7. Kar

    Kar

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    Oct 15, 2014
    Yes, I do have one. What should I be testing? the large capacitors? they say 4700µF and 25V
    Another question, do I need to test the components with the power on? or do I have to take them off the board and test them separately?
    I guess I'll have to place the transformer back where it belongs first.
    Thanks
     
  8. errol-smith

    errol-smith

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    Oct 14, 2014
    It sounds like a dodgy on-off volume switch. Does it have one of them ?
     
  9. davenn

    davenn Moderator

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    Sep 5, 2009
    Hi Kar

    thanks for the pics
    now one of the bottom side of the board would be great as well .. I want to see what is happening there :)

    OK first test
    for the power plug/in the lower left ... are there 2 or 3 wires in the plug ?
    1/2 guessing there's 2 with the centre pin blank
    if there are 3 then we are dealing with a "split rail power supply ( a centre tapped transformer)
    if that is the case, with the power on, I will get you to measure the AC voltage firstly, between the 2 outer pins
    then between each outer pin and the centre pin


    Dave
     
  10. Kar

    Kar

    25
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    Oct 15, 2014
    @errol-smith No, it uses a standard on/off switch directly to the transformer.

    @davenn There are 3 ,two blue wires on the sides and a black one in the center

    I replaced the two large capacitors (4700uF, 25v) but the results were not good. I turned the speakers on and the LED lit up at half of its usual brightness for a few seconds, then it died. Now it wont lit at all.
    I hope I didn't screw up.
     
  11. davenn

    davenn Moderator

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    Sep 5, 2009
    OK no probs ... thanks for the wiring confirmation :)

    now do those AC voltage measurements as I suggested ... There could easily be 20V across the 2 x blue wires so set the meter to a higher scale for a start
     
  12. Kar

    Kar

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    Oct 15, 2014
    I can't right now, I'll let you know when I have news. Thanks a lot
     
  13. Kar

    Kar

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    Oct 15, 2014
    Ok, I took a pick from the other side of the board. First one is normal and the other is mirrored, in case that makes it easier.
    [​IMG] [​IMG]

    Now about the testing. I got readings of -0.05 so, either the transformer is bad or I'm not very smart and I'm using the multimeter wrong.
    I set the multimeter on direct current and 20. The connector has 3 wires, I put the black probe into the middle hole in the connector, and the red into each side, and both of them read the same around -0.05. When i turn the speaker off it goes back to 0.00.

    That's it. I really appreciate you help
     
  14. Gryd3

    Gryd3

    4,098
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    Jun 25, 2014
    Is it at all possible for you to unclip the wires, and move the red/black pair that goes across the image?
    It would help us to better see the components on the board.
     
  15. Kar

    Kar

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    Oct 15, 2014
    I see what you mean.
    Here's a different image if it helps.
    http://postimg.org/image/vw2du0iuf/

    I can try to unclip the wires. EDIT: here it is
    [​IMG]

    I don't know if this means something but the first time I turned them on today the LED lit up for about 5 seconds then slowly went off. After that it does nothing.
     
    Last edited: Oct 16, 2014
  16. davenn

    davenn Moderator

    13,653
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    Sep 5, 2009
    I did say to you ACV ;)

    not DCV
     
  17. Kar

    Kar

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    Oct 15, 2014
    Oh, my bad. I'll try ACV.

    edit: It reads 13.7 on both sides and it's quite steady
     
    Last edited: Oct 16, 2014
  18. Kar

    Kar

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    Oct 15, 2014
    I've been reading and trying to learn some things.

    in an article titled: Four-Diode Full-Wave Bridge Rectifier I read this:

    If one diode of either of the two diode sets is faulty, an open will occur for that half-cycle and the output voltage of the bridge will drop approximately in half.

    I don't know if this is the problem, but I think I'll take off and check those four diodes, just in case.

    Also, excuse my ignorance, it's the first time I see most of this components, and I really appreciate your help.
     
  19. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    Can you measure the DC voltage across one of those two big electrolytics. It doesn't matter which. Hold the meter on them, then turn it ON and get the initial reading. Then see if it changes.

    Then measure the DC voltage across each of those electrolytics when it has died, and check the AC voltages on the 3-pin connector from the transformer - measure between the middle pin and each side pin separately.

    Can you post the markings on the output IC that's mounted onto the heatsink as well please.
     
  20. Kar

    Kar

    25
    2
    Oct 15, 2014
    @KrisBlueNZ The first capacitor ( the one closer to the edge of the board) goes from 0 to 18 when I turn the speakers on. The other one only goes up to 2. After a few seconds the voltage drops to around 13 and goes back to 18 very quickly a couple of times until at last it goes all the way down to 0. Now the LED light is not working at all, but the behaviour appears to be the same.

    The output IC reads :
    TDA7265
    CZONJ746
    VWCZ MAR

    and there is a circle with a number 3 on it above that appears to be a logo.

    By the way, the diodes work properly.
     
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