Quote:
Originally Posted by Dave Moll
I must admit that my thought was the same - that a scrap ZX81 is most probably so because of a dead ULA.
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Maybe not, actually - the keypads are common failures on them (which is why it was worth someone's while to get some remanufactured). It usually manifests itself as dead indivdual keys or a whole key row out.
That would have been enough reason for them to be discarded or 'lofted' at one time, especially in the years between when the machines were originally made and when the new keypads started to be made. The keypad/membrane was a unique part, nearly impossible to repair once it started to fail and for a long time, impossible to replace with like for like.
Finding an '81 which does or does not have a working ULA is not the only issue, because ideally you want one with a ULA2C210, the version on which the video back porch issue was finally fixed.