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Digital Piano Low Volume

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

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  1. zifey


    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


    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


    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.

    Attached Files:

    • test.pdf
      File size:
      608.8 KB
  4. 73's de Edd

    73's de Edd

    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.


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


    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


    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


    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

    Oct 11, 2019
    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
  9. StevenVerlinden


    Nov 27, 2020
    It is mentioned above but do check your supply voltage. I had exact the same problem as Zifey describes: piano working but at a very low volume with slider all the way up. It turned out that my adapter was faulty and produced only about 8V, although the power indicator led was on. The digital parts is fully functional because the 8V input is enough for the power module to generate the internal 5V supply (IC4). For the amplifiers the 8V resulted in my case in the very low volume. At even lower values, several parts of the circuit will gradually fail, which might result in any strange result. If you checked your adapter and it is fine, I recommend to take the following steps;

    1. Check the supply voltage on the board. You can access (board top side) them without removing the board (you'de have to open the keyboard obviously).
    - Ground: e.g. JMP36 (bare metal wire, somewhere in the middle), your 0V reference.
    - 12V : The cathode end of D14 (big component, close to the power connection)
    - 5V: JMP23
    - 9V: JMP16
    - V/2 (6V, the analog ground): R2 terminal facing the middle of the board

    If 12V is not there, check the anode end of D14 (the voltage here should be about 0.7V higher), although this component is unlikely to fail. If there is no 12V then you most likely have a problem in the on/off switch or in the power input plug.
    If 12V is ok, then continue;

    If 5V is not there, the problem is most likely in IC4 (7805).
    If 9V is not there, the problem is most likely in IC3 (7809).
    V/2 is derived from 12V by a resistive voltage divider: it is very unlikely to fail but if you have the 12V and not the 6V for V/2, then you could try replacing C1 or even C1,R1 and R2

    2. If all supply voltage are ok and still no good sound
    - check MUTE: you can measure this signal on the R62 terminal closest to the edge of the board. It should be close to 0V (certainly less then 0.5V). If the voltage is higher then about 0.6V (most likely it will be above 5V in that case), the signal from the synthesizer board is blocked.
    - check AMP_MUTE: you can measure this signal n the R10 terminal closest to the middle of the board. It should be above 2.5V.

    If any of these 2 signals is not ok I would strongly suspect your headphones plug. It has an internal switch that may be faulty. Try to plug in and unplug the jack (several times). you should measure 12V at JMP17 when the headphones are unplugged. If that is not the case (even after a few trials), you should replace the headphone jacks.

    If both signals are ok, then go ahead and check the signal paths decribed above by 73's de Edd.
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