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Digital Piano Low Volume

Discussion in 'Troubleshooting and Repair' started by zifey, May 23, 2017.

  1. zifey

    zifey

    3
    0
    May 23, 2017
    Hey guys,

    I have a Korg SP 250 that I picked up very cheaply at a pawn shop. One speaker was crackling when I bought it, so I just unplugged and continued to enjoy my cheap keyboard.

    Recently, the volume would become very quiet while playing, no matter how high the volume slider is set. After a reset, the volume would return to normal for a few seconds before rapidly becoming quiet again. The only way to get acceptable volume is to hook it up to an external amplifier.

    What do you guys think the problem may be? I feel like it would be obvious if I had more experience. How might I go about fixing this? I have opened it up and checked all the capacitors, but they are all visually fine. It seems like a problem with the amplifier board, but I can't find anybody that is still selling those for this model.

    Thanks in advance
     
  2. duke37

    duke37

    5,224
    718
    Jan 9, 2011
    A crackling output would make me suspect a capacitor. Varying volume could also be due to a capacitor in the power supply. I suggest that you do not use it until the fault is found. You do not want to take out the mains transformer.

    A photograph of the relevent area and a schematic would be a great help.
     
  3. zifey

    zifey

    3
    0
    May 23, 2017
    duke, thanks for your response. I initially suspected capacitors as well and was kind of hoping to affirm my suspicions before wasting too much time on it. The keyboard uses three boards, a main, an amp, and a control board. I've attached the schematic, and will add pictures when I can the keyboard open again.
     

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  4. 73's de Edd

    73's de Edd

    2,684
    1,088
    Aug 21, 2015
    Sir zifey . . . . .

    Your problem will very likely be the potted POWER Audo Amplifier Module that I have marked up in the top right of the schematic with points of initial interest in [YELLOW].
    Get that 250 board acessible for AC metering and initially put a continuous tone / note thru the unit and monitor the two YELLOW X Circles to see if you have a constant tone making it through the unit at each of those points.
    With a suspected level of 100's of millivolta AC on up to a volt.
    If so, you should have sound coming out of the speakers if both sections of that LA4708 IC are good.

    IF not, initially inspect the individual 18 pins of that IC/Module to see if it possibly has developed some floating pins within their solder blobs, due to a poor wave soldering of the board at manufacture. It takes a vewy-vewy bwight light to examine, and a strong magnifying glass would certainly help.

    The other markups are relevent to other signal paths, if they seem to be involved later on.
    I certainly think that IC2 or its support circuitry and components is your most likely problem.

    MARKED UP REFERENCING:
    [​IMG]
    http://i.imgur.com/FFdZWC7.png

    73's de Edd
     
    duke37 likes this.
  5. duke37

    duke37

    5,224
    718
    Jan 9, 2011
    Check that there is 12V on pin 18 of the LA4708.
    Have a good look at C38 and the voltage on it.
     
  6. zifey

    zifey

    3
    0
    May 23, 2017
    duke and 73's de Edd, thanks so much for your advice. I've had a pretty good look at LA4708 and all of the voltages are within the expected ranges. C38 is visually fine and has ~ 6V DC going to it. I do not have an oscilloscope but am tempted to purchase one in the spirit of learning.

    I did notice that the output pins have ~6V DC on them, I'm not sure if that was expected.

    I think it's worth noting that the headphone out is also very quiet. The only way I am able to get acceptable volume is by going through the headphone out to an external amplifier. This leads me to believe that the issue is prior to the LA4708 chip. What do you guys think?

    I plan to look for the expected values of CN10-6 and CN10-3 wires (circled in green in Edd's marked up diagram) and check them out.
     
  7. duke37

    duke37

    5,224
    718
    Jan 9, 2011
    The 6V on the LA4708 outputs is reasonable since there are two amplifiers working in anti-phase.

    You could use a little amplifier to trace the signal, before and after the LA4708. Note that the LA4708 has a mute connection, check again that this is above 2.5V
     
  8. Johannes v I

    Johannes v I

    1
    0
    Friday
    Did you solved the problem? I have a problem that the Piano is switching on badly. when you push the ON/OFF button after some seconds the lights come up and the piano play badly. Maybe you had some solution? I made note of the comments above and will try this weekend. someone any other suggestions, on the fault I described, please let me know. Thank you
     
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