Rolling Thunder repair log #3

Received from Spain this faulty Rolling Thunder PCB for repair:

Board was not booting stuck on a static garbabe screen with some background sound noise (which seems a common issue on this PCB)

After ruled out all the custom ICs, few was left to test.The background sound noise very likely  suggested a problem in the digital sound circuit that on this hardware must be working for proper operation of the whole system.Probing this circuit with a logic comparator revealed nothing abnormal until I tested a 74LS257 @7C, I was warned about a problem on its output pin 12:

 

Also comparing with a scope this ouput with the one of the reference IC gave discrepancies, signal was not properly toggling (good on left of the below picture)

Looking at schematics pin 12 of this 74LS257 outputs the address line ‘A0′ to a 6264 static RAM @3F tied to the custom ’30’ which controls the sound system:

Sure enough I removed the TTL which failed the out-of-circuit exactly in pin 12:

After replaced the TTL the board successfully booted, no further issue were found so 100% fixed.