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So the (Row) Outputs 2 AND 3 from UC9, must BOTH be going Low.
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There is another potential explanation, which is that, due to a fault on its 'A' input, the 74LS145 is always seeing a 'high' or 'low' on that input.
Whether UC7 outputs 0000 or 0001, UC9 activates the same column output, rather than consecutive outputs - so for 50% of the possible code outputs from UC7, the wrong key column output is going low on UC9. This would be why we get 'W' when 'W' is pressed (correct) but we also get 'W' when 'Q' is pressed (incorrect).
The exact nature of the problem within UC9 is only a matter of nerdy technical interest: As long as replacing it fixes the fault I think I will be happy.