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31314DEC VT131 Fault Diagnosis

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  • Mike Strange
    Jun 17, 2013
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      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?


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