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31316re: [midatlanticretro] DEC VT131 Fault Diagnosis

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  • B. Degnan
    Jun 17, 2013
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      Jay West I believe, of cctalk fame has one
      bd



      -------- Original Message --------
      > From: "Mike Strange" <mjstrange@...>
      > Sent: Monday, June 17, 2013 8:58 AM
      > To: midatlanticretro@yahoogroups.com
      > Subject: [midatlanticretro] DEC VT131 Fault Diagnosis
      >
      > Has anyone here any experience with a DEC VT131 terminal keyboard at
      circuit level?
      >
      > I appreciate that there are probably many implementations to achieve the
      VT131 spec; I am
      > interested in one that uses an 8039 microcontroller, 2716 program store,
      CD4028 column
      > drivers and CD4051 row detection.
      >
      > We have a very odd fault whereby every letter to the left of P on each
      row intermittently
      > fails to function; we have changed the relevant column driver 4028 and
      the frequency of
      > fails reduces.
      >
      > It cannot be the row detection as this would exhibit a different set of
      symptoms; the supply
      > voltage from the display unit is fine and the drive signals to the CD4028
      are still present.
      >
      > Anyone seen this fault before?
      >
      > Regards,
      > Mike
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