I was inspired to engineer this article based on my recent experiences with going through 9 ADPCM sound boards (and others from locations), and some other past exploits with the CVSDs as well. The information contained within is based on my interpretation of the schematics, so I suppose there could be a margin of error. None of this information should risk any harm if it is incorrect; the listed parts I know are correct. My intention here is covering the sound boards used in Williams/Midway Y-unit and T-unit games, but I also feature some of the pre-CVSD hardware from System 11 pinball as well.
ADPCM Sound Board
The ADPCM sound board was introduced with Terminator 2: Judgment Day in 1991. ADPCM stands for “adaptive differential pulse-code modulation”, or in other words a more sophisticated means for digital sound playback. With its improved digitizer by way of the Oki MSM-6295 these sound boards had better sound quality compared to the previous generation CVSD (which I was told by a sound designer was like 1940s walkie talkie technology!)
The ADPCM sound board is controlled by a Motorola 6809E CPU. It uses a Mosel 2064 SRAM, with 3 ROMs containing the contents of the audio code. Communications are handled by standard TTL (2x 26LS31 and 26LS32). The FM synthesis module is a Yamaha YM2151 with YM3012. The DAC is an Analog Devices AD7524.
Unlike the CVSD which featured 2 separate amplifier circuits for 2 channel FM and DAC/Digitizer, the ADPCM funnels the audio through two separate TL084 op amps. The AD7524 DAC passes through U29 LM324 first before passing through U33. One channel FM (cited in schematics as YAM1-OUT) goes through U33, while the other channel FM (cited in schematics as YAM2-OUT) and Oki MSM-6295 go through U32. The outputs of U32 and U33 are then combined to form MIX-OUT through an electronic attenuator U30 MC3340 before reaching the amplifier.
Depending on which revision ADPCM board is used there will be different amplifiers (all other electronics save for some capacitors are the same). The older model board (Terminator 2: Judgment Day/Total Carnage era) uses U31 Hitachi HA13116. Newer model boards (Mortal Kombat/NBA Jam) use a U31 Fujitsu MB3731.
MB3731:
HA13116
**These boards are subject to capacitor failure and potential leakage which can destroy nearby components. It is strongly encouraged you replace the capacitors as soon as possible.**
Troubleshooting:
Scratchy or distorted sound:
Failing capacitors or a dirty/failing volume pot can cause the sound to sound scratchy or distorted. Otherwise replace TL084 op amps at U32 and U33.
SOUND IRQ NOT DETECT:
Ensure the ribbon cable between the CPU and sound boards is connected correctly. Both boards will have number markings next to the 20-pin headers denoting 1/2 and 19/20. The easy and reliable way to hook up the ribbon cable is to align the red stripe of the ribbon cable with pins 1/2 on both the CPU and sound boards.
If the ribbon cable was installed correctly and communication to the sound board still doesn’t work, inspect the 26LS31 and 26LS32 chips on the sound board in the vicinity of the amplifier. Leaking/outgassing capacitors in the area may have gunked up the chips, inspect the chip legs for any corrosion. Cleaning up a capacitor mishap such as this is not unlike cleaning up battery leak/outgassing (a total mess and involved process).
Error codes:
The sound board will play tones indicating either normal operation or a component failure. 1 tone indicates normal operation, it’s expected to hear it on power up and it may play again once the game has completed the power on test. The legend from the General Test under Sound Diagnostics is 2 tones indicate a RAM error and 3 tones indicate a ROM error U3. U3 apparently contains the necessary code to run the sound board and potentially all the FM music, the other ROMs at U12/U13 contain voice samples that don’t directly interface with the CPU but rather the Oki digitizer. I can speak from experience about a RAM failure that it didn’t just play just 2 tones, but 2 tones and then 1 more after. I suspect it was generating the RAM failure code and then the sound board reset after, but regardless it was a bad Mosel 2064 SRAM that prevented the sound board from playing game sounds at all. 0 tones will imply either a power failure to the sound board or perhaps total amplifier failure. The sound board should play 1 tone even if the ribbon cable is unplugged or plugged in backwards. If you suspect something may be wrong with communications between the CPU and sound boards enter Test Mode (via the slide switch above coin door) and go to Diagnostics -> Sound Board Test.
CVSD Sound Board
CVSD stands for “continuously variable slope delta modulation”, a form of delta modulation with variable step size with analog-to-digital and digital-to-analog conversions. The digitizer is a Harris HC55536, which is a newer revision of the 55516 that was utilized in older Williams Pinball System 7/9 and Rev. 1 video game hardware applications for games such as Gorgar, Black Knight, Space Shuttle, and Sinistar. (Some Williams game schematics may cite the part as a 55536.)
The CVSD sound board is controlled by a Motorola 6809E CPU. It uses a 6116 SRAM, with 3 ROMs containing the contents of the audio code. Communications are handled by a 6821 PIA.
What sets the CVSD hardware apart from its old System 7 Pinball predecessor is it features FM synthesis with the Yamaha YM2151 and YM3012, 1408 DAC, and 55536 speech digitizer. The CVSD Sound Board uses 4 MC1458 op amps and 2 TDA2002 amplifiers. YM2151 output passes through op amps U9 and U10 MC1458s. The 1408 DAC passes through op amp U17 MC1458. The 55536 speech digitizer passes through op amps U18 MC1458. The two TDA2002 amplifiers output the sound in conjunction to the speakers.
Troubleshooting:
Wrong Sound Playback:
If a CVSD sound board is playing wrong sounds the 6821 PIA chip could have failed. I’ve had a Smash TV sound board in the past that would play music constantly when there was supposed to be no music playing and it wound up being something with the seating of the chips in the sockets. Older CVSD boards came with single wipe sockets, if any chip legs are bent or crooked they may not be making good connection in the sockets.
Distorted Sound Playback:
If sound playback is distorted on CVSD sound boards, try replacing all the electrolytic capacitors first. The 1000 uf/16V (x2) and 470 uf/16V (x2) capacitors directly in front of the amplifier heatsinks are particularly prone to drying out due to the heat from the amplifiers, in addition to 30+ years of age. There are additional 100 uf/16V (x3) capacitors around the board too. If replacing the capacitors doesn’t help at all, listen closely for which sound section is the most distorted. The MC1458 op amp locations are as follows: YM2151 FM audio (MIDI/synth) at U9 and U10, 1408 DAC (often explosions or drums) at U17, and 55536 at U18. If replacing the op amps doesn’t help, then it could be in the TDA2002 amplifiers. You can sub a TDA2003 in place of TDA2002, I’m told these are a little more robust.
Error Codes:
The sound board will play tones indicating either normal operation or a component failure. 1 tone indicates normal operation, it’s expected to hear it on power up and it may play again once the game has completed the power on test. The legend from the General Test under Sound Diagnostics is 2 tones indicate a RAM error. ROM errors will play 3 tones for U4 error, 4 tones for U19 error, or 5 tones for U20 error. 0 tones will imply either a power failure to the sound board or perhaps total amplifier failure. The sound board should play 1 tone even if the ribbon cable is unplugged or plugged in backwards. If you suspect something may be wrong with communications between the CPU and sound boards enter Test Mode (via the slide switch above coin door) and go to Diagnostics -> Sound Board Test.
Evolution of Williams Sound Boards:
High Speed Background Sound Board
There’s another variety of sound board which to my understanding was only used on High Speed. Only the final variant of System 11 called 11C used the separate CVSD sound board for all sound functions. System 11, 11A, and 11B variants had the digitizer on the MPU with its own amplifier and the external sound board had FM synthesis and DAC built in.
High Speed’s Background Sound Board only features DAC, which was used for the background music and probably other sound effects. I’m assuming all the other sounds in the game were strictly digitized as there is no YM2151 FM synthesis component in the entire game. The Background Sound Board has no amplifier, it beams back to the MPU for amplification which I believe is backwards to all other System 11 games with CVSD sound boards, but I’m a little rusty on my pinball fu lately. If I obtain correct intelligence I will make the necessary changes then.
The Background Sound board is controlled by a Motorola 6809E CPU and uses a 2016 SRAM (pictured is a SYS128-4 module, but the parts sheet calls for a 2016). It uses a single rom and communications are handled by a 6821 PIA. The only sound generation on the board is from a 1408 DAC, which has one 1458 op amp to pass through before outputting back to the MPU for amplification.
Road Kings Background Music Board
Road Kings, the 2nd of the original 3 System 11 games, employed a Background Music Board which added YM2151 FM synthesis along with the DAC. The amplifier section of the board is unstuffed, which means the System 11 MPU is used for amplification.
Other revisions of the boards followed, some using different ROM capacity and hardware configurations which will not be covered here.