Digital Piano acting strange.

Discussion in 'The Projects Forum' started by mogmat, Oct 4, 2015.

  1. mogmat

    Thread Starter New Member

    Nov 7, 2013
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    Hello,

    I've got an old Casio CDP-100 that's been acting up. One block of eight keys has been only working intermittently for awhile. However, after I accidentally left the piano turned on overnight, they stopped working altogether. I took it apart and found a weird quirk. Each key is connected to two diodes and if I touch them whilst pushing the affected keys, suddenly they all start working again for a short time. I'm guessing there's some sort of grounding issue, but I have no clue how to find and fix it. Any ideas?

    Cheers,
    Morgan
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,779
    1,103
    Welcome to AAC!
    Without a schematic we'd only be guessing as to the cause of the problem.
    My guess is that the driver circuit which scans the keys has gone south, or has a poor connection, and static in your body was enough to charge up some capacitor for a while. If the keyboard has any plug/socket connectors, make sure they are seated properly.
     
  3. mogmat

    Thread Starter New Member

    Nov 7, 2013
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  4. mogmat

    Thread Starter New Member

    Nov 7, 2013
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    From the key matrix I can see that the problem is just with SI4. Beyond that, I'm completely lost.
     
  5. absf

    Senior Member

    Dec 29, 2010
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    Since the problem is along the SI4 trace, you should measure the continuity of this line all the way from the key contact PCB till the MCU chip. Along the way there are capacitors and resistors like C43, RM3 and RM12 should also be checked. ALso make sure the the female connectors and flat cable is making proper contacts.

    If just a few B/W keys from different groups are having intermittent problems or no sound, then the problem could be due to the rubber contacts that was lying on the surface of the keyboard PCB.

    I've never repair Casio D.P. before but it should be similar to the other brands like Yamaha and Technics.

    Allen
     
  6. absf

    Senior Member

    Dec 29, 2010
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    If you have an oscilloscope or a logic probe, it would be helpful to look at the SI4 line and see if it is pulsing with low duty cycles. You can compare the SI4 pulses with other SIx lines to see how the normal line pulses.

    Allen
     
  7. mogmat

    Thread Starter New Member

    Nov 7, 2013
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    Thanks Allen,

    Unfortunately I don't have any test gear at the moment (I'm just getting back into electronics), but it's obviously time to buy some. For the meantime though, I've got an idea. It's occurred to me that the only component I'm probably going to be able to replace is C43. Everything's surface mounted and I just ain't got the skills, but C43 is the first connection from the ribbon cable and then it just goes straight to ground. It should be easy to jerry rig something without having to mess with the main circuit board at all. C43 also seems to be a likely culprit. If it's not connecting the circuit to ground properly, my touching the circuit would fix that for a short time. Does that seem plausible?

    Cheers,
    Morgan
     
  8. absf

    Senior Member

    Dec 29, 2010
    1,492
    371
    It's still too early to say. Anyway it's a good start to change C43 and have our fingers crossed. If you have a multi-meter, you can measure the 2 resistors I mentioned above even though they are SMDs. The value of the resistance should be around 1KΩ for RM3 and 220KΩ for RM12, as per the schematic.

    The resistors are in groups of 4 and there should be 4 pins on each side, which is easy to identify.

    I only hope that it is not one of the pins (P119) of the IC7 COB going south, or else you'd need to buy a new main board to make it work again.

    Allen
     
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