I am repairing a WPC89 CPU board, that has a strange problem: Everything related to CPU data line D5 is stuck high. So I have lamp row and col #5 stuck on, three coils controlled by D5 stuck on, ground short at switch row 5 and switch col 5 always high.
Now the strange thing is, that the board boots fine, so D5 is OK in ROM and RAM. First I thought the problem is in U12 driving the driver board, but in that case switch matrix would not be affected.
The board had some slight battery leak damage, and so far I have replace U11, U12, U13, U14, U15, U17 and U20. Also tried with another WPC ASIC. Checking continuity of D5 from CPU to all chips is OK. Removing U12 results in lamps and coils not being stuck on anymore, so the driver board is OK.
Something keeps D5 high, but what - and why it does not affect ROM and RAM read/write?