Today I had to work on a Stargate by Williams. The complaint was that it was stuck in the rug pattern. Taking a look at the back, I saw that all voltage LEDs were lit on the power supply. I also noticed that I was getting a RAM error code: 1-3-1.
1-3-1 is a RAM error code that is supposed to indicated that the first RAM in bank 3 has failed. It is also the first RAM that get's checked, awfully suspicious. First thing that I checked was the voltages, all were within 5% so I figured that it was good. Next step I did was just swap the RAM chip. All the RAM chips on the main board are socketed and I had one on hand. So it was an easy check. Still not working.
The next thing that I checked was the 74153 MUXs, they are responsible for addressing of the RAMs and after doing some reading they are common failures. Checking all the outputs, everything looked correct except for the output of pin 7 on 3E.
On the oscilloscope, it was looking like either just noise or that it wasn't going low enough to register as a logic low. Regardless of that, it looked faulty. Pulling it out of circuit and testing it, it seems to have failed.
Now that it has been replaced, it seems like the game is working just fine.