Started diving into my new machine, which is actually the one Pinballer22 was discussing a couple pages back.
The issue is most likely with the CPU board. There are a few old repairs on the clocking circuit at IC13/14/15 that could be the culprit's. Typical missing pads, crusty brown flux, a few jumper wires. It's partially booting, and does kinda respond to dip switch #4, but ends up just flashing zeros on all the displays. The good news is these repairs all got sockets, though not the best soldering job.
Interestingly the CPU, sound, and driver boards all look to be conformal coated at some point. Never worked on a Zac before, but never seen this on a pinball board from any other manufacturer. Normally something that's only done if there are environmental concerns. Are they all like this? Seems to be an acrylic coating too, as isopropyl alcohol is taking it off easily. Could explain the reputation for these boards to be difficult to solder on.
Neat way of doing their schematics, but also hard to follow in some regards. Wish there was a map for the test points somewhere.
-Hans