How the sound works

Two processors, one byte

The main processor is a 6809. The sound processor is a Z80. They share no memory at all - just one byte, written by the game at 3A00 and read by the sound CPU at C800 in its own entirely separate address space.

One byte, one direction, no acknowledgement. The game says "play sound 7" and carries on without waiting, which is exactly why a sound fault can never hang the game.

The sound CPU has its own ROM and its own 2K of RAM, and drives two YM2203s at 1.5MHz. Each YM2203 is really two instruments in one package - three FM channels and three square-wave PSG channels - so the pair gives six of each. That is a generous voice count for 1985 and it is why the Ghosts'n Goblins theme carries a real melodic line over a hard percussive bed.

The module, and why it matters for repair

Here is the part that makes this board different to work on. The sound Z80 is not a bare chip socketed on the board. It sits inside a Capcom custom module at 13K.

For Capcom that is convenient packaging and a small obstacle to copyists. For anyone holding a dead board forty years later it is a genuine problem: you cannot simply swap a Z80, you cannot easily scope its internal signals, and if the module has failed your options narrow to finding another one or rebuilding its function on a daughterboard.

Exactly what else is inside that module is one of the open questions on this board, and the schematic sheet is the best evidence available. That is one of the things the extraction pass over sheet 4/8 is trying to establish.

The analogue end

The last stretch is the only part of the machine with no logic in it. Each YM2203 hands its FM output to a companion YM3014 DAC while its PSG channels leave as raw analogue; the two are summed with resistors, buffered through an LM324, set by a mixing pot and a volume pot, and driven into the cabinet speaker by an HA13001 power amplifier.

It is drawn in a completely different idiom from every other sheet - component values instead of pin numbers - and it reads like a different era of engineering, because it is one.

The drawings behind this story

Each one opens the interactive schematic for that sheet.

Every chip in this story

11 chips across 1 sheet. Click any to open the schematic with that chip selected.