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SiriusHardware · 12th Feb 2020, 9:45 pm

It is by no means certain that the column driver IC is to blame although the nature of the fault, in which column N and N+1 of a row produce the same letters, then column N+2 and Column N+3 produce the same letters, then column N+4 and N+5 produce the same letters, etc, makes that an obvious thing to eliminate at the earliest opportunity.

It sounds like the 'KEY A' (ones bit) line between the 6520 and the 74LS145 is either stuck in one state or being read as such by the 74LS145, resulting in the column select line advancing by two, not one, and only on every second step of the four bit count being output by the 6520.

We can only wait and see.

12th Feb 2020, 9:45 pm	#103
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,575	Re: Commodore PET 3016 It is by no means certain that the column driver IC is to blame although the nature of the fault, in which column N and N+1 of a row produce the same letters, then column N+2 and Column N+3 produce the same letters, then column N+4 and N+5 produce the same letters, etc, makes that an obvious thing to eliminate at the earliest opportunity. It sounds like the 'KEY A' (ones bit) line between the 6520 and the 74LS145 is either stuck in one state or being read as such by the 74LS145, resulting in the column select line advancing by two, not one, and only on every second step of the four bit count being output by the 6520. We can only wait and see.