Atari Gauntlet 2 arcade pcb repair
Game worked but had artifacts on certain sprites in certain locations – usually vertical lines down the screen on every other scanline.
My first guess was failing object (sprite) RAM, which are the 4 RAM chips in the corner of the board. All 4 were socketed and replaced and at first I thought the problem was fixed. However this was more a case of the different brand of RAM acting a bit differently – the vertical lines were mostly fixed but certain sprites still had ‘sparkly’ artifacts. After checks on the IC’s that drive the data and address lines I found a physical problem – a hairline crack on a pullup resistor pack. Despite the crack it tested fine (470ohms on each pin) but the sister pack seemed to have an internal failure – 4 of the 8 pins showed 960ohms. Both resistor packs were replaced and the sprites were properly fixed.
The purpose of the pullup resistor packs is provide an all high data signal to ‘erase’ pixels in the line buffer when no sprite data is active. With the higher resistance the voltage on the data pins was lower and likely in the intermediate range between low and high. So the sparkles were certain pixels randomly being regarded as low instead of high.