Faulty motherboard

Discussion in 'General Electronics Chat' started by shirreff, Apr 7, 2012.

  1. shirreff

    Thread Starter Member

    Sep 6, 2011
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    I have a number of 1980's music keyboards all Korg M1's.
    I also have quite a few spares mostly motherboards for the M1.
    The problem I have on one board is that it just hisses like a radio that is not tuned in instead of playing the keyboard sounds.
    Another motherboard I have just does not produce any sound.
    I have tried the boards in more than one keyboard and swapped all the other internal boards over so I am confident that I have narrowed it down to the motherboards.
    On the one that hisses I have swapped a couple of capacitors and one transistor but it was purely guess work and it made no difference.
    The board that produces no sound I have not touched.
    Any ideas?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    How much experience do you have building or fixing electronics and computer systems?
    Do you have access to an oscilloscope?
    Do you have the service manual and schematics?
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    My son bought a used laptop computer at an auction. It was full of the latest music and many oldies because it was owned by a DJ.
    It took me about 3 years to hear it all.
     
  4. PaulEE

    Member

    Dec 23, 2011
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    That was nearly as useful as the OP! ;)
     
  5. shirreff

    Thread Starter Member

    Sep 6, 2011
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    I can solder but I don't have an oscilloscope. I have a multimeter. I have 5 mother boards and I have one which is cracked which I am using for parts so I have plenty of spares.
    I don't have loads of experience though, mostly guess work..
    M1 diagram compressed.jpg
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Pick one board ans let's start with that.

    I can give u pointers on where to look.
     
  7. shirreff

    Thread Starter Member

    Sep 6, 2011
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    Hi,
    Ok I have picked one, all functions work visually and the sound works but when I play a note it kind of plays it twice. Almost like there is an effect on it.
    Difficult to describe, imagine pressing one note on the keyboard then pressing it again straight after that is what happens when you only press the key once.
    what sound does come out is pretty clear.
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Show me the works.
     
  9. shirreff

    Thread Starter Member

    Sep 6, 2011
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    What do you need to see? Do you want some images?
     
  10. shirreff

    Thread Starter Member

    Sep 6, 2011
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    Looking closely I can see a couple of very small resistors that are visibly damaged, only very slightly but that may be a good place to look. I will swap them over tomorrow and see what happens.
     
  11. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I cannot repair what I can't see.
     
  12. shirreff

    Thread Starter Member

    Sep 6, 2011
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    Ok here you go, quite a few pictures which I hope helps.
    1.main.jpg

    2.inputs from keyboard and outputs to audio board.jpg

    3.inputs from control panel.jpg

    4.chips.jpg

    5. resistors 21 and 22 which have been changed.jpg

    6.more resistors.jpg
    Now if I press a note quickly it plays perfectly but if I press and hold a note it plays it twice. I have tried the motherboard in a different keyboard and the problem still happens so I am sure it is contained to the motherboard.
     
    Last edited: Apr 9, 2012
  13. shirreff

    Thread Starter Member

    Sep 6, 2011
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    and some more images and the service manual.
    7. and more.jpg

    8.diode which apears to work.jpg

    9.don't know what the thing with tape on is.jpg

    10.lots or transistors.jpg

    11.don't know what these are.jpg
    View attachment m1 service manual.pdf
    I don't know how much you know about musical keyboards but they have instruments which play long continuous notes like an organ and short notes like a piano, then there are touch sensitive keys which when hit hard play loud and when hit softly play quite and then of course you can play the notes fast or slow.
    Playing fast seems to be fine like at the speed of a single beat but if you hold the note it repeats once. If you play lots of notes together it crackles very slightly which may be due to the notes trying to repeat.
    If you need any more info or images I will gladly send them.
     
  14. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Really should think twice before blabbering...:D

    I was thinking of computer mother boards.

    Oh well! A new challenge.
    Now lessee.!

    This is new. so I always starts with basic. Follow me. Get a digital multimeter, and set it up to measure DC voltages.

    First thing to do is how the power supplies fluctuates with the load.

    Looking at the schematic, page 9 according to pdf reader. I am looking at the PSU diagram.

    It's a standard SMPS. Hook it up so you can play sound. Connect the negative to GND indicated at connector CN9 B4B-EHX2.

    Provide a secure connection to the probe, do what u gotta do to make sure than black probe is connected to the GND.

    Power is up, keep ur hands away from the supply. One hand to play the note the other hand holding the red probe of the DMM.

    Measure the Voltages ±12V & 5V. The unit needs to be powered up without playing anything. Check whether the ±12V falls with in 5% of the rated 12V and the 5V should not go below 4.8VDC

    If there is a difference as stated above then u need to replace the secondary caps. If not continue below.

    Measuring the DC 5V, play a note and tell me if it fluctuates.
    Do the same for ±12V.

    Post ur results.

    PS ±12V means a positive rail (PIN 1)and a negative rail (PIN 4) of CN12
     
  15. shirreff

    Thread Starter Member

    Sep 6, 2011
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    Oh God I think I can just about manage that, the board I will be testing is the power supply board klm 1269 which to my knowledge is fine because I have 3 of these and the motherboard gives the same results with all three power boards.
    I will give it a go in the morning as it is getting late.
    Where are the secondary caps located?

    The motherboard is klm 1266.
    I also spotted something sticky around this area
    4.chips.jpg This could mean something spilled on the board.
    Thanks for the help so far, it is so close to working but that probably means it is going to be harder to fix.
    I hope I don't give you too much hassle.:)
     
  16. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    can we assume that you can access the full program parameters and are fimiliar with each of the parameters?
     
  17. shirreff

    Thread Starter Member

    Sep 6, 2011
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    Yes ,
    that side of things is not a problem, I have tried turning all the effects off and also re-loading all the sounds etc. I have done this many times in the past so the slightest difference stands out.
    I can also route the sound through 4 different outputs and the result is the same.
     
  18. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    what I'm thinking is that one or more of the parameters are 'out of bounds', occuring from a corrupted or stuck bit/value. We can then assume that you've inspected the parameters and input controls to confirm that they produce some affect, even over the error, or does the error/malfunction inhibit effects/parameters? Are you aware of any third party patches, software upgrades, or is it original?

    beyond the logic side, you have input and output. My approach, rather than hit and miss components, unless something as obvious, is logical deduction of the problem, expressing the controls to measure, confirm, then isolate.
     
    Sparky49 likes this.
  19. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I know u would say that.

    Are u gonna do it or not?
    If you have done it things would be easier for me.
     
  20. shirreff

    Thread Starter Member

    Sep 6, 2011
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    The motherboard is totally standard, no modifications. I have loaded in. The factory sounds supplied by korg and all parameters are set as they should be. Altering effects, parameters etc has no effect on the outcome.
    I have left it switched on overnight to see if that will make any difference because the board has only been on for short periods.
    I will test the voltage this morning and post the results.
     
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