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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. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    When the unit has stabilised, what is the voltage across each electrolytic? You said one of them drops to zero, but what about the other one?

    In that state, can you also measure the AC voltages on the 3-pin connector from the transformer - measure between the middle pin and each side pin separately.
     
  2. Kar

    Kar

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    Oct 15, 2014
    The other one starts charging and reaches 2.5, then it remains steady for a few seconds and then it starts going up and down, it reaches 4 and then it drops to 0 just like the other one.

    The transformer reads 13.7v on both sides and steady, no drop at all. I also tested one of the medium sized electrolytic capacitors next to them and it reads 12 without dropping.
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    epoint 270824 caps indicated.jpg

    OK, can you post the markings on the three disc-shaped mustard-coloured devices indicated by the arrows. Also if there are any more similar-looking components nearby, post the markings on them too.

    Those photos aren't the best to work from. Can you take a photo of each side, with the camera directly over the centre of the board, with the board centered in the image and at 0° rotation. Don't use a flash. If you have a zoom lens, use it for the top side, so the big electrolytics don't obscure so much of the surrounding area. For more guidelines see https://www.electronicspoint.com/resources/how-to-take-photos-of-circuit-boards.6/ and if you have to use a cellphone camera, https://www.electronicspoint.com/resources/photos-with-phones.27/
     
  4. Kar

    Kar

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    Sorry about the image, I'll try to take a better one.
    There are only two of them, the third arrow is actually a big amount of glue of some kind.
    I marked the position of each of those disc shaped capacitors in this picture. I can't see the markings on those two from any angle ,but they are bigger than the rest. The tiny ones are marked 102, the medium sized are 104. So I assume the large ones you marked are either 106 or 108 , is it possible?
    Picture:
    [​IMG]
    Thanks
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    I'm interested in the ones that you marked with a question mark. I'm not totally sure that they are actually capacitors! I think they may be PTC resettable fuses, despite the marking on the PCB.

    If there aren't any resettable fuses, I guess it's possible that the diodes are failing under load. If there's 13VAC on each side of the AC from the transformer (measured relative to the centre pin) but no voltage on the big electrolytics, there's not much that it could be.

    With the unit in the fault state, confirm that there's AC voltage on both ends of the transformer wire, then measure the DC voltage between the stripe end (the right end) of the diode furthest from the edge of the board, and the non-stripe end (the right end) of the diode closest to the edge of the board.

    This voltage is the output from the bridge rectifier. If the diodes are OK, it should be at least 30V. Half of that voltage should appear across each electrolytic. If there is DC voltage present on the diodes, but still nothing on the electrolytics, see if you can follow the tracks from the diodes to the electrolytics and see if there's anything connected in between.

    I will be able to give you clearer instructions when you've uploaded some good quality pics.
     
  6. Kar

    Kar

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    I think they say 221 on the inner side but its hard to tell.
    Ok, the diodes output is 32v and the capacitors are not dropping to 0, they're at constant 19v now, perhaps I was mistaken the first time. I tested the rest of the electrolytic, should all of them be charged? I have a couple of capacitors with 0v.
     
  7. Kar

    Kar

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    Oct 15, 2014
  8. Kar

    Kar

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    Oct 15, 2014
    The output voltage from the connectors is 0.5v on the two-wire connector in the center, and the six-wire connector reads 0.03 on the first group of three, and 0 on the other group of three. The cable for these two connectors goes into the Volume control board inside the cabinet.
    Is the voltage low? the LED is not even lighting up. If that was actually the problem, would it help if I replaced all of the electrolytic capacitors?
     
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Those pictures are a lot better.

    Don't worry about the voltages on the six-pin connector. Those pins only carry low-level audio signals, which are AC, so there shouldn't be any DC voltage on those pins.

    What is the 2-pin connector near the middle for? Does that go to the LED? There might be a problem there.

    Can you measure some DC voltages. Plug in the transformer at the top left, but not the other three connectors. Put the multimeter's black probe to the middle pin of the transformer connector, and measure the voltages on pin 4 and pin 8 of the 8-pin IC at the right.

    epoint 270824 top side IMG_0285 cropped annotated.jpg

    ... and post the voltages you get. And answer the questions I asked earlier in this post.
     
  10. Kar

    Kar

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    Oct 15, 2014
    Ok, I understand.

    Pin 4: 12.1V
    Pin 8: 1.8V

    The center connector goes to a small board inside the cabinet with a LED and 3 knobs
    Now it reads 0.8 instead of 0.5
     
  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Are those voltages both positive? The voltage on pin 4 should be negative, so the multimeter should show a "-" sign when you measure it. Can you check them both again?
     
  12. Kar

    Kar

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    Oct 15, 2014
    They are both negative Pin 4 is -12.2 negative , and pin 8 is -0.8.

    By the way, the IC says
    JRC 4558D
     
    Last edited: Oct 18, 2014
  13. Kar

    Kar

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    Oct 15, 2014
    I think I found a relation between these capacitors and the voltage in the IC. When I measure one of these capacitors I get +12v; the other one give me -0.08v. This isexactly the same voltage I get at those pins you indicated. The difference is I'm getting negative voltage on pin 4 instead of +12v that I get when I measure the capacitor.
    Take a look:
    [​IMG]
    Does it make any sense?
    Should replace that capacitor?
     
  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    No, don't get obsessed with replacing capacitors! This isn't a switching power supply or a computer motherboard.

    I suspect the resistor I've marked with an arrow. Can you desolder one end and lift it out of the board, then measure resistance (the Ω range) across the resistor itself? And post either the colour bands, or a photo of them?

    epoint 270824 resistor marked.jpg
     
  15. Kar

    Kar

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    Oct 15, 2014
    Hehe ok, ok... no more capacitors. But is it actually good if the capacitor reads -0.8v?
    I'll try checking that resistor, and learn something about them as well. The stripes are Gold, brown, purple and yellow.
    And thank you very much for your patience.
     
  16. Gryd3

    Gryd3

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    Jun 25, 2014
    Common uses for capacitors are:
    A) Energy Storage. (Commonly used in a power supply portion of the circuit to help keep the voltage levels smooth)
    B) Filters. (Commonly used in an audio/signal portion of a circuit. Allows / Disallows specific frequency ranges to pass through a circuit.)
    C) RC Timing... (Used with things like 555 timers where the charge/discharge rate is used to time or control a circuit)

    In a sense, capacitors in power supplies are used like A and B... as they will typically allow noise to be directed to ground in addition to filling their role to provide power or absorb power during high, or low points where a transformer based power supply may operate.
    Larger capacitors allow lower frequencies to pass through, and also store more energy to allow the circuit to pull more current without causing the voltage to dip too far.
    Common failures in capacitors that I am aware of is usually quite visually apparent, as the capacitor will bulge (Electrolytic)
    They can also fail and either go open-circuit, or short-circuit.

    The steps are are taking right now with your circuit is checking if the ICs in your circuit are receiving the appropriate voltage. If these readings are not appropriate, then you can begin to back track closer to where the power enters the board to find out what part has failed.
    Kris is excellent at finding these things, so listen carefully ;)
     
  17. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    No, it's not good for a polarised capacitor to have reverse voltage across it, but 0.8V isn't going to do any harm, especially with a capacitor that's rated for 16V or more.
    Right, that's 470Ω. You read the stripes in the other direction. Yellow = 4, purple/violet = 7, and brown = 1, and for a four-band resistor you take the first two numbers, 4 and 7, and add the number of zeros specified by the third number, which is 1. So you get 4, 7, and one 0, i.e. 470. The gold band means an accuracy of ±5%. I think you'll find it's open-circuit or a lot higher than 470Ω.
    The original looks to be rated for 1W - it's larger than the other resistors on the board. By my calculations, it doesn't need to be that big, but you'd better replace it with a 1W resistor. I don't know where you're located (there's a place for this in your profile, hint hint) but a suitable resistor from Radio Shack is http://www.radioshack.com/product/index.jsp?productId=50061446.
    No problem :) I think you're getting the hang of it now.
    Right. The capacitor we're talking about is not one of the main ones; it's across a 12V zener that's supposed to provide +12V to pin 8 of the 4558 dual op-amp IC. But that rail is sitting at -0.8V so I assume the 470Ω resistor that feeds it (from the main +17V rail at one of the large electrolytics) has failed.
    Thanks Gryd :)
     
  18. Kar

    Kar

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    Oct 15, 2014
    I've checked the resistor you marked, and it reads 470. Then I checked the other light blue resistor, and the results were the same; 470. And that was fun, so I tested the 2 medium sized resistors on the top left of the board (red, red, green, green) but those wouldn't give me a reading. Then I took them off entirely to see if I was doing it wrong , but again the meter quickly went up and then back to "1." After that, and just to be sure, I tested a spare 1k ohm unused resistor I had, and the reading was 1.00.

    The top big capacitor also has negative voltage, which is strange because it used to have +2v. And the one that reads -0.8v now, was reading +1v some time ago.

    Thanks
     
  19. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    When you measure a resistor, and it shows '1', that means your meter can't display that large of a value.
    This means one of two things:
    A) The meter needs to be set to the next largest range. (Look at the resistance setting and see if there is a 10K setting, if you were set on a 1K setting)
    B) The resistor is broken and is 'open-circuit'


    The settings on a meter are the 'largest' value that can be displayed.

    Red, Red, Green, Green = 2, 2, 10^5, +-0.5%
    2,200,000Ω +-0.5% is an incredibly large resistance... so even with a bigger setting on your meter, you may not be able to measure this. This all depends on your meter though.


    This will help you learn, or look up values:
    http://www.digikey.ca/en/resources/...version-calculator-resistor-color-code-4-band
     
  20. Kar

    Kar

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    Oct 15, 2014
    Last edited: Oct 19, 2014
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