A small envelope arrived in the post Today. A tube of 74LS00 and 74LS393 to replace the ones identified as broken by my chip tester. The chip tester says that the new 393s are also faulty so I suspect the tester is a little flaky. Anyway, on with the show.

There has been some improvement but not a fix. I also reseated the chips that were not tested (ROMs, DRAMs etc.). I moved the 4116s around too to try and flush out the problem.

Back to square one?

It looks a bit like it. I think this might be a clue.