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caius

Grand Master
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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...)

Gang%20Wars%20PCB.jpg

At power up I was greeted by a static garbage screen :

garbage.jpg


Checking the program ROMs with a logic probe revealed stuck data lines :

stuck_data_lines.jpg


At visual inspection I found that one the two SIL custom ICs marked 'ALPHA-INPUT87' was missing :

missing_ALPHA-INPUT87.jpg


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 :

ALPHA-INPUT87_repro_Installed.jpg


The board finlly booted but backgrounds had jailbars through :

jailbars_backgrounds.jpg



Graphics data are stored onto four 4Mbit MASK ROMs :

GFX_MASK_ROMs.jpg


Checking them with a logic probe revealed that pin 19 (data line D3) of the one @16J was stuck low :

16J_stuck_low.jpg


I traced the pin back to the SDIP64 custom IC marked 'ALPHA-8921' :

ALPHA-8921_2.jpg


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 :

SDIP64_reworking.jpg

I borrowed a 'SNK-9201' from a Fatal Fury 2 MVS cart :

SNK-9201_installed.jpg

This fixed the graphics so this confirmed that my chip was really bad :

fixed.jpg


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 :

ALPHA-8921_repro.jpg


The reproduction was succesfully tested on the Gang Wars PCB and Fatal Fury 2 MVS cart :


 
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