Retro Pinball: Reverse Engineering the Bally AS-2518-51 Sound Module Assembly

One of my friends has a beautiful 1980 Nitro Ground Shaker pinball machine. It's in great condition, but there is absolutely no sound output. As I was trying to figure out a way to troubleshoot the sound module, I found myself falling deeper and deeper into the 8-bit pinball rabbit hole. In this post I will try and explain the inner workings of the amazing Bally AS-2518-51 Sound Module Assembly.

Overview

The 2518-51 Sound Module is based on the famous Motorola 6800 series microprocessor (U3 in the picture and in the schematic). Sound synthesis is performed by a General Instrument AY-3-8910 (U1). There's also a 6820 Peripheral Interface Adapter (PIA) (U2) to interface the AY38910 to the MPU, a ROM (U4) that contains game-specific code and data and a RAM chip (U10). The microprocessor is clocked by a 3.58 MHz crystal. Due to the built-in clock divider the 6800 operates at 895 kHz.

The Bally AS-2518-51 Sound Module Assembly as found in Nitro Ground Shaker

The pinball system interfaces with the sound module using a 15 pin connector (J1) that provides both power and input. The only output of the sound module happens through the speaker connector (J2).