Cobra-Command repair log

Received for repair this pretty rare original Cobra-Command PCB, a 2D side-scrolling helicopter shooter released by Data East in 1988  :

 

On power up I got nothing on screen, no sign of life, board was dead.I checked the RGB and SYNC pins on the JAMMA edge connector and they were silent meaning the video timing circuit was not running at all :

Tracing the signals upstream lead me to one of the two surface mounted custom ICs marked 'L7B0072 BAC 06'  (the one @11E) 

I found its pinout on Robocop and Heavy Barrel schematics :

Probing its outputs I found that most of them were floating, stuck at undefined logic voltage level of +1.44V :

This was not new to me, this custom IC is well known to be prone of faiure, you can find several repair logs on the net mentioning it, like this Robocop one on JAMMArcade :

Robocop repair log #3 - JAMMArcade

Having no spare I asked the owner of the PCB to look for a donor board and after some time he sent me a dead Midnight Resistance PCB which carries two of this 'BAC 06' custom ICs :

 I removed the suspicious bad one and soldered in the spare:

The board sprang to life but playing some games I noticed that some backgrounds were messed up and the board randomly freezed :

 

In this comparison with a MAME snapshot you can clearly notice the corruption of background graphics :

Part of tilemap graphics data are stored in four 27256 OTP ROM :

Probing them revealed some address lines were stuck LOW :

These are generated by the other 'BAC 06' custom IC but they first go through a couple of 74LS273 before reaching the ROMs :

Doing some logic analyzing of the 74LS273 @15B revealed most of outputs were amost inactive while inputs were toggling and clock present too :

 The IC was likey faulty hence I removed failing the out-of-circuit testing :

Fitted a good IC :

Background graphics were restored :

Board fully working again and another repair accomplished.