Some time ago I've been sent a Gang Wars PCB for repair.For the uninitiated this game is a beat 'em up developed by Alpha Denshi and published by SNK in 1989.The board was in fair condition (a little bit dusty perhaps...)
At power up I was greeted by a static garbage screen :
Checking the program ROMs with a logic probe revealed stuck data lines :
At visual inspection I found that one the two SIL custom ICs marked 'ALPHA-INPUT87' was missing :
Time ago I made replacement of this part therefore I know that it's used for inputs and it's connected to 68000 main CPU data bus.I promptly installed the reproduction :
The board finlly booted but backgrounds had jailbars through :
Graphics data are stored onto four 4Mbit MASK ROMs :
Checking them with a logic probe revealed that pin 19 (data line D3) of the one @16J was stuck low :
I traced the pin back to the SDIP64 custom IC marked 'ALPHA-8921' :
After a quick research on the net I found that the 'ALPHA-8921' can be found also on NEO-GEO hardware but under different part name, specifically as 'PRO-CT0' on AES cartridges and 'SNK-9201' on MVS boards.Regarding the functions of this chip, it can be used as graphics muxer on Alpha68k hardware and security device on NEO-GEO hardware.
The chip on my board was likely faulty so I removed it and installed a SDIP64 socket :
This fixed the graphics so this confirmed that my chip was really bad :
The repair was done but, since 'furrtek' during his NEO-geo development made schematics of this custom IC (thanks to him for his always brilliant job!) , I decided to design a modern replacement :
The reproduction was succesfully tested on the Gang Wars PCB and Fatal Fury 2 MVS cart :
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