[Coco] CoCo 64K (26-3126) Keyboard Issues Follow-up

Melanie and John Mark Mobley johnmarkmelanie at gmail.com
Mon Dec 11 08:26:44 EST 2017


First we need to determine if the problem is with the keyboard or the computer.  Apparently some ROMs have better key de-bouncing firmware than others.

Locate the problem to:
  1) Firmware
  2) Hardware PIA
  3) Keyboard connector
  4) Keyboard

Assuming there is a short across two pins...
It may be possible to locate the short using a suction cup type telephone inductive pickup coil.

  1) Create an AC signal that works with the telephone pickup coil.  You can use a 60Hz to 1kHz sine wave, square wave, (full-bridge rectifier hocked to a mains power AC transformer,) You can use music and an audio amplifier.  This signal should be current limited to 100mA or so depending on the sensitivity of the telephone pickup coil.
  1) Place the AC signal across the shouted pins.
  2) Connect the telephone pickup coil to an audio amplifier and speaker.
  3) Move the telephone pickup coil around until you hear the AC signal.
  4) The telephone pickup coil is directional like a directional arena.  It picks up at 0 degrees and 180 degrees from the suction cup.
  5) trace the short using the telephone pickup coil like a stethoscope.

Assuming there is a short across two pins...
It may be possible to clear the short.
Locating the short is sometimes very difficult so the idea is to treat the short as a fuse and deliberately blow the fuse with high power.
If you damage the keyboard in the process then it is not a problem because it was bad anyway.

  1) unplug the keyboard from the computer.
  2) Get a power supply and set the voltage to 5 volts and set the current limit to 1-7 amps.
  3) Set up a camera to record the flash of light.
  4) Momentarily place the power supply across the two shorted pins and look for a flash of light.
  5) Place the keyboard back in the computer and see if it is now fixed.

-John Mark Mobley

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