Korg SP250 Electric Keyboard - No Power - Fault Finding Advice needed

Discussion in 'Technical Repair' started by Hecla_1, Dec 5, 2017.

  1. Hecla_1

    Thread Starter New Member

    Dec 5, 2017
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    Hi All,

    First post.....looking forward to learning from the wealth of experience here.

    I have a Korg SP250 keyboard that won't power up.

    It has a seperate power supply (transformer) that I have tested and is outputting the 12 odd volts it requires, but the unit itself shows no signs of life.

    I have opened it up for a look, specifically looking at the power stage PCB and see no obvious signs of issue (no burn marks or visibly blown caps).

    The only other thing I have tested is the switch, which I bypassed to see if it was a faulty connection.....but no positive results.

    Can some knowledgable person advise me on my next move?

    Many thanks in advance!

    -------
    For context, I am an Audio Eng by trade, surface (day to day) knowledge of electronics and used to problem solving. I have fixed and built plenty of things in the past, yet lack deeper understanding and/or techniques of a regular electronics engineer. I am always willing to learn.
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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  3. Hecla_1

    Thread Starter New Member

    Dec 5, 2017
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  4. Hecla_1

    Thread Starter New Member

    Dec 5, 2017
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    Hiya,

    Just checked two regulators as per the schematic....and they both appear healthy, outputting 5v and 9v as they should. (IC4 was 5v, IC3 was 9v)

    So any further ideas would be much appreciated!
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Top right of the schematic it says
    "* The amplifier is turned on by applying approximately 2.5 V or more to the STANDBYSW pin.
    This is Pin5 on boards using the LA4708 and Pin4 on boards using the LA4628"
    Are you getting that 2.5V?
     
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  6. Hecla_1

    Thread Starter New Member

    Dec 5, 2017
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    Thanks for this Alec_T

    This is the IC you pointed out.... Just to check, am I right in saying they are counted 'left to right' from this image's angle? Many thanks! IMG_0599.jpg
     
  7. Alec_t

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    Sep 17, 2013
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    This is what the datasheet shows :
    Pinout.PNG
    Looks like pin 1 is on the left in your photo.
     
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  8. Hecla_1

    Thread Starter New Member

    Dec 5, 2017
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    Feeling slightly silly....

    Where is the best point to lay the probes? Red on pin 5 and black on...?
     
  9. Alec_t

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    Sep 17, 2013
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    Black on 4 (or any other ground point).
     
  10. Hecla_1

    Thread Starter New Member

    Dec 5, 2017
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    Cool.

    In looking at the schematic, I thought it would be Pin 4.

    So I should see 2.5v (or close to) on the meter when I run the 12v power through the board.

    I will report back. Thanks Alec_t
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Pin 5 needs at least 2.5V to un-mute the audio. It may well be higher.
     
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