29 Nov 2017

Twin Cobra - Taito 1987 (repair log)

Game was completely dead: black screen, no sound.



On this hardware many things can lead to this as it seems the self test requires a lot of things to be operational before displaying the RAM/ROM test screen.

1) Getting the board to boot

I started to probe the board and found many signals were absent on the top board. In fact they were present only a fraction of second on start-up. Long story short they were coming from various LS245s, themselves having their enable pin (/E signal) coming directly from address line A16 of the main CPU (68k). I was a bit lost so went back to the basics : RAMs.
One of them on the top board had all its data pins stuck high. I took the chance to piggyback it and game booted:


 
But with no sprites at all...

2) Fixing the graphic glitches

The concerned cicuitry is on the top board and probing the six 2148 RAMs I found one had data signals tied low. They were coming from a LS244 which in turn had inputs stuck low. This lead me to a LS74 and finally to a series of LS166 which had their shift/load pins shorted to Vcc.
Due to the corrosion of the board I wasn't able to determine the point of least resistance so I pulled the chips I identified were connected to this stuck signal:
- LS166 @ 13E pin 15
- LS166 @ 14E pin 15
- LS166 @ 16E pin 15
- LS166 @ 17E pin 15
- LS374 @ 23D pin 11
The last chip I pulled was the LS166 @ 13E and it was indeed the culprit.
Unfortunately I didn't have any in stock but I powered the board without it in hope for any improvement : sprites were back. Corrupted but back:


After I received the replacement part I put it back on the board and:



Game fixed.


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