Konami Simpsons arcade pcb repair
Board would not boot most of the time – when it did the check screen showed an EEPROM error as well as a ROM error. The background of the check screen was also red when it should be black.
The EEPROMs on Konami games are known for failing, but a check with a logic probe and multimeter here showed that 5 of the 8 pins weren’t even connected – trace damage where the chip met the pcb. With that repaired it made absolutely no difference to the boot failures or bad check screen. In fact the logic probe showed the software wasn’t even trying to read the EEPROM as the chip select or clock pins never pulsed. After checking the chips involved with reading the EEPROM data (shown in the schematics as EEPDI, EEPCLK, EEPCS) were fine I suspected the PAL chips used to form the memory map may have failed in a subtle way and were responsible for both the EEPROM error as well as the ROM one. This was a fault I had encountered in the previous Konami Vendetta repair – the PAL worked enough to boot the game to the check screen but failed in some kind of edge case that would give bad output over certain memory regions. Simpsons has two PALs like this – marked as 55394 and 55395. 55395 is the one that directly enables the EEPROM, but replacing this made no difference. Replacing 55394 though finally enabled the game to boot properly with no errors – so the fault was most likely that it was activating regions at the wrong time.
The game still had a fault with the red channel stuck on, as well as not booting most of the time. As I knew the palette RAM was ok (because of the game self test) I started with the custom 052535 DAC IC. Replacing this made no change, I should have paid more attention to the schematics as there was also a LS273 chip in between the palette RAM and the DAC and this had simply failed with outputs stuck high.
Inconsistent boot faults can be much harder to track down though – in this case I got a clue it was a physical problem when I found it would boot if I flexed the PCB a certain way. Reflowing the solder on the surface mount custom ICs made no difference but removing all the ROMs and cleaning the ROM sockets did – one program ROM socket had a strangely dirty pin even though all the others were fine. Game now booted and ran 100% of the time.