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caius

Grand Master
Joined
Nov 21, 2016
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Italy
Recently I've been contacted by a couple of arcade collectors, both had a not working 'Prehistoric Isle in 1930' PCB (a shooter game originally developed and published by SNK in 1989) with a problem regarding a custom IC marked 'SNK IO'.In one PCB the chip was faulty, in the other it was totally missing, likely removed to fix another board (picture from 'tekno retro') :

SNK%20IO_missing.jpg

They asked me to take a look at this custom IC in order to study a replacement.Since I have a fully working Prehistoric Isle PCB I gladly accepted the challenge.

The chip come in a plastic DIP42 package, as said it's marked as 'SNK IO' (numeric string does not matter as I think it's a simple production/lot code that changes from time to time) but silkscreening says 'SNK I/O'.Besides Prehistoric Isle it's used on the 'The Next Space' (and perhaps other SNK boards, further info are welcome)

SNK_IO_original.jpg

As part name suggests, this custom IC handles the inputs, specifically the player 1 controls and dispwitches.As always I do, I looked for and found a couple of bootlegs so I studied how the functions of this custom IC were reversed.With the help of the user 'tekno retro' who did some check on his board I was able to figure out the full equivalent circuit of the 'SNK I/O' so I routed it to a proper board layout :

SNK%20IO_repro_preview.jpg


After some time I got the blank boards from the manufacturer and assembled one :

SNK%20IO_reproduction_2.jpg

The testing was successful, the replacement worked flawlessly at first try :


Another custom IC reproduced, thanks again to 'tekno retro' for his help.
 
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