Without much detail, I believe this game was pre-auxiliary board. So, I'm assuming the fuse is on the power supply itself. I see where the schematics say 7A slow blow, but the picture says 2.5A. We'll worry about the value later. Either way, if the schematics are correct, this is for the -12v that will appear on connector J6 and goes to the CPU board and the Sound board. -12v is almost always for sound amplifiers only. This is no different. The -12v only goes do the op-amps in the sound circuits. There are 2 sets of sound circuits. The external sound board for background sounds and the main sounds on the CPU board. Disconnect your sound board power J3. The -12v only goes to a few components. U9, U1, C6 and C1 and C2. Check for shorts on those components to ground. I'm guessing it's not the caps, because they are always connected in circuit. You can try to start a game w/ this connector disconnected. You won't have background sounds. But if the fuse doesn't blow, you've narrowed it down.
On the CPU board, disconnect J17 and check for shorts to ground on the op-amps U4, U5 and U1? I can't read the schematic. I'm guessing you probably have a bad op-amp on one of these boards because you say it only happens when a game starts.