Quoted from gdonovan:
While I agree in principle (look in the area where you last worked), System 80B will boot up and run without a 5101 even being installed.
A trace run may go in order 6502 to 5101 to EPROM. If the trace is ripped out at the 5101 it also disconnects everything downstream from it and if a ROM has an open data/address it will lock the CPU.
Quoted from MaxAsh:
barakandl thank you for that info, I'm game to run through what you said, but I'll be honest, I got a little lost in there. It sounds like you were suggesting two things:
1) Check continuity between the 5101 and the 6502 / Piggyback chip. I was trying to figure out where from the schematic section, but I'll be honest, it's a little confusing.
For example, I see A0 - A7 on the 5101 as pins 4, 3, 2, 1, 21, 5, 6, 7. They seem to be labeled as a group "A", which goes over to AB0-AB10, BAB11? Which then heads over to the 6502 A0 - A11, pins 9-20? I'm a little confused there, my apologies. I'm pretty good with schematics usually, but chip-to-chip stuff and the logic involved isn't as in my wheelhouse sometimes. I'm sure I'm just missing something simple.
2) Remove ROMs (not sure which), turn the game on, and poke around with a logic probe? I'm a little unsure of what to do there. I have a probe, but I've only used it a handful of times on arcade games in the past. Happy to perform whatever testing you suggest, just a little more explanation/guidance would be awesome, if you have time.
I am saying check continuity from the CPU chip to the most likely problems which is 5101 since you just reworked there and the usually problematic piggyback. I think the B thrown in stands for "bus". Its a bit confusing they used multiple designations for things that are electrically connected but AB0 = A0 and etc.
D0 = 6502 P33 = 5101 P9&P10 = PIGGYBACK's 2764 socket P11
A0 = 6502 P9 = 5191 P4 = PIGGYBACK's 2764 socket P10
The BAB10 through BAB13 for upper address piggypack socket is shown on the PROM selection section and connect back to Z10 or Z12.