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Thread: Help with interesting C128 keyboard fault ?

  1. #1
    Join Date
    Sep 2011
    Location
    Melbourne, Australia
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    Default Help with interesting C128 keyboard fault ?

    Hi everyone, hello from Tandyland!

    I have an interesting problem with my C128... I own two... one runs the other doesn't.

    On the machine that runs, they keyboard has a problem where some keys don't work, some do, and the rest print the wrong character when pressed. I figured it would be the keyboard, so I substituted the keyboard from my non-running C128, and what do you know... the same keys give the same result as the other keyboard.

    This makes me think there is a problem with the keyboard decoder (CIA U1) and tried another, and have the same problem.

    Can anyone else think of what would cause the results I am getting?

    Thanks in advance,

    Ian.
    *There is never a charge for Tech Support even if you don't purchase from me - We are Enthusiasts Helping Other Enthusiast and that is just the way it is

  2. Default

    Check the connections at connector CN5. Are they all making a good connection? There may be a bit of corrosion/oxidation on the pins; thereby, preventing a completed circuit to the CIA.

    Service Manual Schematics Index:

    http://www.zimmers.net/anonftp/pub/c...ual/index.html

    More specifically I/O Control Port connections (control ports, keyboard, user port, serial port, etc.)

    http://www.zimmers.net/anonftp/pub/c...manual/66r.gif

    The Keyboard:

    http://www.zimmers.net/anonftp/pub/c.../manual/51.gif

  3. #3

    Default

    Quote Originally Posted by TRS-Ian View Post
    Hi everyone, hello from Tandyland!

    I have an interesting problem with my C128... I own two... one runs the other doesn't.

    On the machine that runs, they keyboard has a problem where some keys don't work, some do, and the rest print the wrong character when pressed. I figured it would be the keyboard, so I substituted the keyboard from my non-running C128, and what do you know... the same keys give the same result as the other keyboard.

    This makes me think there is a problem with the keyboard decoder (CIA U1) and tried another, and have the same problem.

    Can anyone else think of what would cause the results I am getting?

    Thanks in advance,

    Ian.
    Check the EMI filters on each keyboard line (they usually look like three-legged ceramic capacitors). See the schematic. Also inspect solder joints on the CIA's socket.

    Frank IZ8DWF

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