Taito Rastan arcade pcb repair
After working on pcbs with no self-tests for a while it was quite refreshing for this one to say exactly what was wrong with it – ‘COLOR RAM ERROR’. The two color RAMs are located next to the PC040DA customs which are the digital to analog convertors for each of the RGB channels. A logic probe showed that the lower RAM had stuck outputs. Replaced this chip and the board was perfect.
Note that although the board has 2018-35 silkscreened on it (35ns response time RAM) the RAM installed was actually 45ns which works fine.