Error Spotlight: Space Firebird
Time for another error spotlight! This time, we are going to look at the arcade game Space Firebird, by Nintendo and licensed to Sega/Gremlin. This error can be found in the schematics for the sound board in the manual published by Gremlin. This snippet zooms in on sections D5 and D6, we are going to be looking at how IC2, IC9, IC10, and IC12 interact with each other.
To start, lets focus on the first problem, IC10 pin 12 and IC9 pin 4. IC10 is a 74ls74 dual flip flop and pin 12 is the data input for it. IC9 is a 74ls30 8-input AND gate and pin 4 is an input. The problem is that two inputs tied together does absolutely nothing, you are just left with a floating line which can cause problems.
The other part of the error has to do with the outputs of IC12 pin 13 and IC2 pin 2. It is not good practice regardless of the circuit to have two TTL outputs tied together this way as it can cause conflicts. They are going into yet another input of the 8-input AND gate. Two outputs into one and zero outputs into another raises a huge red flag.
After confirming with an owner of an original sound board, it was confirmed that these were incorrect. IC12 pin 13 was supposed to connect the inputs IC10 pin 12 and IC9 pin 4. By moving this connection over, it solves both the conflict issue with the two TTL outputs and the missing input. Shown below is a corrected version of the schematic