[cmjones01]

Congratulations on getting it going. The experience of working on a microprocessor circuit like this one will stand you in good stead. Simply knowing what the various bus and control signals are supposed to look like can tell you so much.

I do a certain amount of repair and restoration on 1970s and 1980s video arcade games, and they are mostly based around standard microprocessors with memories and acres of logic for generating graphics. Poking around with an analogue scope on the pins of the microprocessor and memories can often reveal problems: data bus pins with odd-looking waveforms, pins stuck at indeterminate logic levels, missing clocks, and so on. Though I'm armed to the teeth with logic analysis gear, I've never actually used it on vintage boards. Logic analysers are great for debugging a design, but less so, I find, for working on faulty hardware. The faults tend to be loose connections and faulty chips, which show up better in the analogue domain.

A useful trick, especially with RAM chips (the popular 2114-type 1k x 4 RAM turns up in lots of equipment and is notoriously unreliable) is 'piggybacking' - simply putting a known-good chip on top of the suspect device so all the pins are in contact. It also works surprisingly well with TTL logic and buffers. You can do it while the power's on. If nothing changes, the chip on the board is likely to be OK. If something changes, it was almost certainly faulty.

Chris