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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. Gryd3

    Gryd3

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    Jun 25, 2014
    Very well could be discoloured, but it would be strange if both resistors dies at the same time in the same way depending on how they are connected.
    If the 3rd and 4th bands are actually gold, your meter would only show about 2Ω..
    I'm not going to distract you too much though. I figured I'd post some details for the resistors to help you measure.
    If I started giving you trouble shooting tips at the same time as Kris it would make this more complicated than it needs to be.

    In any case. Good eye.
     
  2. Kar

    Kar

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    Oct 15, 2014
    @Gryd3 On the contrary, I appreciate any kind of help, specially if I can learn something from it.
    I have no idea how those two are connected, but I find it hard to believe that they would cause so much trouble.
     
  3. davenn

    davenn Moderator

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    Sep 5, 2009
    you learn through experience :) ...
    1) there is a larger gap between the 2 green bands, indicating the end green is a tolerance etc band
    2) Gn Gn Rd aka 5500 Ω ( 5.5kΩ ) isn't a valid resistor value

    so a Rd Rd Gn is a 2.2 MΩ (2.2 million Ohms)
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    You're right, those resistors should both be red-red-gold-gold, 2.2Ω. The one that's red-red-green-green doesn't look like the colour bands have changed; they look very even all the way round. My guess is that the wrong component was installed at the factory.

    Those resistors, and the two capacitors at the ends of them, are a circuit called a "Zobel network" which helps keep the amplifier from becoming unstable. It's quite possible that it will work perfectly well without them, or with the wrong value components there. You should (eventually, at least) replace those resistors with new 2.2Ω 0.5 watt resistors.

    OK, I'm a bit concerned by the fact that voltages seem to be changing every time you measure them. Here's an image showing the points to measure with your multimeter. Push the black meter probe into the solder at the black dot, and push the red meter probe into the solder at each of the red dots to measure the main positive and negative rail voltages.

    epoint 270824 underside measuring points 1.jpg

    But before you take any more measurements I think that board needs a cleanup. I also suspect that the brown goop on the top side may have corroded the link wires that connect the bridge rectifier to the electrolytics. So this is what I suggest.
    • Desolder and remove the two main electrolytic capacitors.
    • Use a flat blade screwdriver to scrape off all of the brown goop from those areas.
    • As you're cleaning around the two wire links, push them to see whether they are still intact.
    • You may need to desolder and lift one end of the red-red-green-green resistor, and one end of each bridge rectifier diode, to get at all the brown goop.
    • Clean any remaining goop off the board surface and the component leads using cotton swabs and solvent. Isopropyl alcohol (tape head cleaner) is best, but methylated spirit will do.
    • Clean the underside of the board the same way, especially the corroded area near the bridge rectifier.
    • Replace the jumper wires if they were corroded
    • Refit the electrolytics
    Then you can measure the main positive and negative rail voltages at the points marked in the picture. Make those measurements with the transformer plugged in, but initially, nothing else.
     
  5. Gryd3

    Gryd3

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    Jun 25, 2014
    I would just like to add, for the op's sake to please make note of the polarity of the electrolytic capacitors before you solder them back in so you don't accidentally hook one up backwards.
    It may not be immediately apparent which is negative on circuits with a positive and negative voltage rail ;)
     
  6. Kar

    Kar

    25
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    Oct 15, 2014
    It looks like you're right.
    [​IMG]

    I'll try to find a replacement for that. Is it a specific wire I have to buy? or is it the same material as the "legs" on the resistors? could I cut an unused LED and use the wire?
     
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    You can use any wire that's not too thin. Yes, a resistor or LED lead is fine. Or you could use hookup wire. If it's stranded, you can strip it and twist it tightly so it won't touch anything else, or leave the insulation on the main part.

    Make sure you clean the board really well first though.
     
    Gryd3 likes this.
  8. Kar

    Kar

    25
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    Oct 15, 2014
    :) :) :)
    Works beautifully! Thank you very much for your help, specially KrisBlue, it still amazes me how you knew exactly what was going on below that capacitor and the glue.

    The wire was falling apart, it even broke when I attempted to clean it . I replaced the rusty wires with some new ones I took from the 2.2ohms resistors I bought, which I replaced as well.

    I'm glad that I can have my music back, and I'm also glad that I could learn a few things from you.
    Again, thank you so much.
     
    KrisBlueNZ and Gryd3 like this.
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Cool! You're welcome :)
     
    Kar likes this.
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