Atari Rampart arcade pcb repair
Game would show an initial flash of garbage on 1st power up then get stuck on a black screen and continually watchdog reset. The watchdog can be disabled by shorting the jumper next to the test switch and with this done it was clear the CPU was running fine. In fact it seemed to intentionally be clearing the screen to black and then just sitting in a loop somewhere in code. A logic probe on the IPL (IRQ) pins showed it was responding to interrupts correctly which pretty much also proved the main RAM was working fine (if the RAM was bad then garbage would have been pulled from the stack at the end of the IRQ which would have caused a crash/bus error).
The game uses a selection of GAL chips mainly for memory addressing – I burned replacements for each one with no difference. The ROMs tested fine of course and all address & data lines had continuity from CPU to ROM to RAM (the latter via a pair of LS245s).
The culprit turned out to be the 2816 parallel eeprom used to store the game settings – when tested it showed a just a couple of bad bits that would randomly change on each read – if it had fully failed it would have been an obvious fault. When replaced the game worked fine, so whatever bad pattern this eeprom gave out managed to confuse the program into getting stuck.