Schematics explanation needed (UMK3/Wolf Unit)

[RenierD]

Hi all

I'm working on a Midway UMK3 PCB. The self test identified U113 as bad. The CE, OE and all other lines seem to toggle on a logic probe and I've verified the contents on an EPROM reader. All fine.

My next step is to trace the chip back towards the CPU. I think there might be RAM issues. I found a scan of the Wolf Unit schematics online, but I have almost no experience with schematics.

I've identified the ROM location and the schematics refer to IADDR (21,00) - see attached. All the other graphic ROMs also refer to that location. I have no idea what that means. It is a pin, diagram, page number, bus number? Please help me understand.

Thank you in advance.

 

[twistedsymphony]

U113 is a graphics ROM and graphics ROMs are all on the same bus. so you can check continuity of each of the address and data lines with an adjacent graphics ROM
U121 or U125 are good ones to test against. The only pins that should differ would be the CE or OE and everything else should have continuity.

if not you found your dead trace.

 

[RenierD]

U113 is a graphics ROM and graphics ROMs are all on the same bus. so you can check continuity of each of the address and data lines with an adjacent graphics ROM
U121 or U125 are good ones to test against. The only pins that should differ would be the CE or OE and everything else should have continuity.

if not you found your dead trace.

Thank you. That helped a lot.

I also figured out what IADDR (21,00) means. It continues on sheet 3 of the schematics.

Just to prevent confusion for other Noobs, all the address lines are connected as advised, but the data lines will only show continuity with data pins of ROMs in the same position of each other bank, not within banks. In other words, the data lines of the first ROM in bank 1 won't show continuity with the second ROM in bank 1. It will show continuity with the first ROM in bank 2 and the first ROM in bank 3, etc.

I almost missed that.

For me, it looks like pin11 (A1/IADDR01) was the culprit. IADDR01 traces back to the U24 (74F373) (called a flip-flop?). Every time I probe around it the game resets. So I've ordered a new one...