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Technics KN800 MIDI keyboard no sound

Discussion in 'Electronic Repair' started by Geoff C, Sep 1, 2010.

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  1. Geoff C

    Geoff C Guest

    I was given a Technics SX-KN800 keyboard and am trying to repair it. I have
    opened it up and the six or so PCB's are easy to get at. The symptom is no
    audio. The PA thumps through the speakers on turn off and the PA has power
    supply OK so it is not a concern at the moment. The line out has no audio
    either. Of course it is possible that a crash has occured but I cannot find
    a reset so far. No audio is getting to the PA. I could not find any signal
    at the master volume either, though it may not have real signal at it.

    I believe it was made early to mid 90's. There are a lot of Technics
    proprietery chips in it. I have an op manual for a KN1000 which is similar,
    but no schematics appear anywhere I have seen.

    What I would like to know is a general question about how the audio signal
    is generated from the PCM pulse stream. It may be a simple LPF from a
    digital chip which regenerates the audio, or there may be a specialised
    audio chip to look for? If I can't go much further I will have to junk it.
  2. Geoff C

    Geoff C Guest

    I suppose the isolating relay would be on the power amp baord if it
    existed, but I can't see it. I'll have another look for it though. I have
    been using a scope for signal tracing, but your tracer idea has its
    merits too, since it will be very quick.

    Also, I have read the manual for the next model up which mentions a
    method of resetting to factory defaults, and also a MIDI out only mode.
    I'll try these too.
  3. GeoffC

    GeoffC Guest

    I have fixed it so I thought I'd follow up the fix. There was no -15 volt
    rail due to a 4.7 ohm fusible resistor on the main board being open. This
    was adjacent to the connector which received the power supplies from the
    power amp/power supply board.
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