I was told the game was dead. I was full of apprehension, this hardware isn't really simple, triple 68k CPU based, loads of custom chips...
Anyway, after a quick look at the boards I could see someone previously attempted a repair. Grrr... I HATE that. Some wires had been soldered directly on the solder side of the connectors, one of the custom chips on the bottom board had the word "SYNC" carved on it (and indeed it was the one generating the sync signal), the 315-5242 colour encoder had a 'X' carved on it (X for dead/faulty?), ground trace on the video had vaporised, blue signal was shorted to what used to be ground before the trace got burnt and lastly an other custom chip had been reflowed. Well, well, well...
I plugged it without much hope and to my surprise there was no image (no RGB signals or sync at all) but game logic was working with sound and inputs were responsive. Not as bad as expected. Maybe.
I probe the RGB signals and they were all low, no activity at all. They were coming directly from the colour encoder. I continued to probe the board and found SPACE signal (could be considered as the video blanking signal) was always active (low). That could surely explain why the RGB signals had no activity. Following the path of that signal lead me to the 315-5325 PAL chip. In fact the BLANK signal coming from that same custom chip generating the sync signal was always active too (low). I feared the custom chip was faulty but it was the PAl that had its input internally shorted to ground. Since it seemed easy enough to guess how the SPACE signal was generated from the BLANK and KILL signal (a simple AND) I rebuilt it externally using a LS08. Full of hope I tried my bodgy fix and:
I obtained an image with wrong colours. To me blue colour was missing and of course it was confirmed by probing at the video connector. I went back to the 315-5242 colour encoder and probed the transistor responsible of the last stage of amplification for the blue colour : it was shorted. I replaced it and blue reappeared:
But colour were still wrong, a sign that the 315-5325 PAL had more internal damage than the shorted input involved in the SPACE signal generation. I couldn't find the jed file to program a fresh one online so my last hope was Mark Haysman from retroclinic but unfortunately he wouldn't share the file with me but would accept to sell me a replacement chip (as said before this is the same since from the chip I can reverse the equations). For those who follow this blog you already know that this is how I started to reverse those Sega PLD chips.
A couple of weeks later I was lucky enough to have someone (member Trol) accepting to send me the chip from his working board for me to reverse it and fix my game. At the end he even found a spare 315-5325 he had spared from a part board. Great! It confirmed it was the last issue the board had:
And of course I reversed the chip:
IC134 bottom board, ref. 315-5325 (PAL16R4) reversed in a GAL16V8 (many thanks to member @Trol for sending me a replacement chip):
Easy repair, isn't it?