this is seriously where a logic probe would be beneficial. easiest place to start is the reset line on the CPU. it should be high. pulsing means it's resetting (watchdog), low means it's not functioning at all. (or at least that's how I interpret this)
as with just about any old game, there could be a formation of corrosion on the legs of the chips. if they're anything but shiny silver, you have corrosion. Dig Dug being of Namco descent has custom chips (the solid black ones with a number in the corner) that have notoriously fragile legs, so cleaning them requires a lot of finesse. otherwise you'll be getting some practice time in fixing the legs you broke off.I've only ever sanded them, I guess that opens them up to additional corrosion again later, but even with the plating on them it happened in the first place, so what's the difference?
theoretically couldn't we just dip the things in CLR or something?
the other problem you can encounter is the sockets for the chips. I think Dig Dug had single wipe sockets, which aren't the bane of all evil like people make them out to be, but the way the pins are inside there's always a chance those could rust or corrode and snap right off. I've seen it a ton on Williams boards. obviously your socket is then hosed. the plastic housings for those lift right off, so you could open up the CPU sockets and just verify all the pins are uniform and intact too.
if the game was working before and now it isn't I would doubt that's the problem though. you have to physically move things around to break those.
I'll be trying all of the above once I get a chance. Real life gets in the way of running an arcade most of the time.
It's getting to the point I'll need a logic probe. I've eclipsed 80 machines at this point, with 40 running in my arcade. It's to the point I need some better troubleshooting tools. I've gotten by with my base knowledge and help from klov. But I need to take the next step. I'm very mechanically inclined and fix about 80% of my own stuff. I could prob get to 90%+ if I picked up some more advanced tools.
