This game has a specific 44 pin connector and flat cables connecting the 2 boards were missing.
First thing to do was to build a converter to JAMMA and pull the missing cables from a known working game:
The game displayed OOOO on screen, colours were bad and there was no sound:
Comparing the ROMs with the MAME romset I found 3 of them bad (n°23, 24 and 26).
After replacing them sound came back to life.
Then I found a dead LS74 @ 11A of the top board. This changed the colours:
There wasn't any activity on one of the data lines of the RAM IC52. I found a dead LS157 @ 16B on the bottom board. Sprites were restored:
No activity on address lines A0 to A7 of EPROMs IC89 and 91 (27C64 type). I found a dead LS273 @ 6E on the bottom board. Background reappeared:
(The label was tied to the top board when I got the game. It says "Karate, no back ground just OOOO, works". I guess sign of life = works in the evil world of arcade operators)