SQUEAK CHECK!

What it is

Squeak Check stands in for the game. It supplies +5 V and +12 V, drives the four sound-select lines and the strobe the game's CPU board would drive, holds or releases RESET, gives the board its volume pot, and brings the audio out to two RCA jacks for an amp.

• Size: 98 × 80 mm, 2 layers, all through-hole, about $20 in parts. The biggest single part is the $7 hex dial.
• Controls: a 16-position hex dial (sound 0–F), a PLAY button, a RESET button and an on-board 1k volume pot.
• Power in: a 4-pin KK-396 header (+5 V, GND, GND, +12 V) from any bench supply; 5 V at 1 A and 12 V at 0.5 A is plenty.
• Protection and indicators: resettable fuses on both rails, and a green LED on each rail after its fuse. A dark LED means no supply or a tripped fuse.
• Out to the sound board: three wire harnesses, J1, J2 and J3, soldered into the tester. Their other ends carry the plugs that go on the sound board.
• Audio out: line level on both RCA jacks. Feed it to an amp or powered speaker, not a bare speaker.

Each harness wire passes through two holes. It goes up through a bare hole with its insulation on, then the stripped end solders into the plated hole next to it. The wire is held by its jacket, so bending it doesn't stress the solder joint.

How the sound board works in a game

The Cheap Squeak Deluxe and Sounds Good are computers in their own right. The game's CPU board puts a 4-bit number on J1 and pulses a strobe line, and the sound board's own 68000 plays whatever that number means in its ROMs. Some games also have other sound hardware alongside it; in Spy Hunter the Super Sound I/O board makes the effects (see below).

The chain is the same on the Cheap Squeak Deluxe and the Sounds Good; MAME's source notes the two boards are almost identical.

• Command in: the four bits land on the 6821 PIA's port B. The strobe (SIRQ) hits the PIA's CA1 input, which interrupts the 68000 so it reads the nibble.
• Status out: two PIA outputs come back to the game as STAT0 and STAT1 on J1, so the game can tell what the sound board is doing.
• Sound out: the 68000 writes 10-bit samples to an AD7533 DAC, split 8 bits on PIA port A and 2 on port B. Three LM359 low-pass stages smooth it, and an MC3340 attenuator sets the volume from the cabinet pot's wiper voltage.
• Reset: J2's RESET pins are active-low. Pulling them to ground holds the 68000 in reset; the game can use this to restart the sound board.

A short history of Midway's sound boards

Midway's early MCR video games (1981–84) put sound on a board that also handled the game's inputs and outputs, hence the names Sound I/O and Super Sound I/O (SSIO). Each had a Z80 driving two AY-3-8910 tone chips, and they carried Kick, Solar Fox, Tron, Journey and Tapper. For speech, Midway borrowed Bally's pinball Squawk & Talk board (6802 CPU plus a TMS5200 speech chip) for Discs of Tron and NFL Football.

From 1983 the sound boards got their own microprocessors and DACs and were built to take 4-bit commands from the game. The pinball Cheap Squeak (a 6803, simple tones, no speech) was the budget board. Its 68000-based video-game cousin, the Cheap Squeak Deluxe, went into Spy Hunter. The 6809 Turbo Cheap Squeak and the 68000 Sounds Good followed, and the Sounds Good built on the CSD's design and carried most of Midway's late MCR and MCR-68k games. After WMS bought Midway in 1988, games like Arch Rivals and Pigskin 621AD moved to Williams sound boards instead.

Years are the release years of the games listed in MAME and PinWiki, not official production dates.

Spy Hunter & Xenophobe

Spy Hunter (1983): an add-on to the Super Sound I/O

Spy Hunter runs on Midway's MCR-III system: a Z80 CPU board, a video board, and a Super Sound I/O board with a Z80 and two AY-3-8910 sound chips (the sit-down cabinet uses a Sound I/O variant with stereo panning). The SSIO makes all the game-play sounds: everything except the music.

The Cheap Squeak Deluxe (A080-91671) was added for one job: playing the Peter Gunn theme. Owners with a dead CSD report a game that plays normally with every effect but no music, and pressing the red TEST button on a working CSD plays the music on its own. The music has its own amplifier channel in the cabinet. The CSD has its own 68000, 32 KB of sound ROM (U7, U8, U17, U18) and a 16 MHz oscillator.

The main Z80 talks to it through output port 4. In MAME's driver, bits 0–3 of that port are the CSD's sound number and bit 4 is SIRQ. The same port also drives the lamp driver board and the analog-input mux, so the CSD shares its wires with other hardware. The CSD's two status bits come back on input port 1.

Xenophobe (1987): the only sound board in the game

Xenophobe runs on the MCR-68k system, whose main CPU is itself a 68000. There is no SSIO and no second sound board: the Sounds Good makes every sound in the game. MAME's Xenophobe driver has the Sounds Good as its only sound device, and owners with a failed Sounds Good report a completely silent game.

The Sounds Good is built on the same A080-91671 bare board as the Cheap Squeak Deluxe, with a larger memory map: up to 256 KB of sound ROM, which Xenophobe fills. The main 68000 writes a control word where bits 0–3 are the sound number and bit 4 is the strobe, the same four-bits-plus-strobe interface Spy Hunter uses.

The interface: J1, J2 and J3

The three harnesses on Squeak Check carry every pin of the sound board's J1, J2 and J3 in the same order and under the same names, including duplicates and unused pins, so each wire lands where the original harness had it.

The volume pot is wired so clockwise is louder. The MC3340 attenuates more as its control voltage rises, so turning toward the +12 V end (J3-12) turns the sound down. A full pin-by-pin list with hole coordinates is in Harness_Pin_Map.csv.

Hooking up a Cheap Squeak Deluxe

J1 and J3 sit side by side on the bottom edge of the board and J2 runs along the top edge, so the three harness plugs go on from opposite sides. The Sounds Good uses the same bare board, so the connectors are in the same places.

The board has no connector housings, just rows of bare tinned square posts: 0.025 in posts on 0.100 in pitch for J1 and J3, and 0.045 in posts on 0.156 in pitch for J2. Midway's harnesses plug onto them with insulation-displacement (IDC) connectors; TE's MTA-100 and MTA-156 families fit these posts. Each plug has one keyed position so it can't go on shifted: J1 pin 7, J2 pin 8 and J3 pin 9.

• Pin 1: J1 and J3 count from their left-hand ends; J2 counts from its right-hand end.
• Harness plugs for Squeak Check: TE MTA-100 9-position (3-640440-9) for J1, MTA-100 12-position (4-640440-2) for J3, and MTA-156 12-position for 18 AWG (4-640599-2) for J2, with an MTA-156 key (640629-1) in J2's key position.
• Board variants: the Spy Hunter manual lists the CSD as A080-91671; MAME names a Spy Hunter board A080-91671-E000.

Using it

Power up and count the sound board's LED flashes first; only a board that passes its self-test is worth dialing sounds on.

1. Plug J1, J2 and J3 onto the sound board, connect the RCA jacks to an amp, and turn the volume knob fully counter-clockwise.
2. Apply power. Both green rail LEDs on Squeak Check should light; if one stays dark, check the supply or let a tripped fuse cool. Then watch the sound board: its LED flashes once per passed test: flashes 1–4 are the ROMs (U7, U8, U17, U18), 5 is RAM (U6, U16) and 6 is the PIA (U9). Where it stops is the failed step.
3. Dial a sound 0–F and press PLAY. Turn the volume up.
4. Press RESET to restart the sound board at any time. It holds RESET low only while pressed.

One press sends the 4-bit number on the dial. What each number does lives in the game's sound ROMs and isn't documented in MAME, so expect some exploring. On a Spy Hunter Cheap Squeak Deluxe only the two low bits matter: 1 plays the full Peter Gunn theme, 2 the short intro and 3 the ending that plays when the player dies. Send the ending while the theme is playing. Other games' ROMs may expect a sequence of presses.

The red TEST button on the sound board itself fires a level-7 interrupt on the 68000. What happens next is up to the game's ROMs; on a Spy Hunter CSD it plays the music.

Build it

Everything needed to order boards and parts is in the fab folder, along with a schematic and KiCad files. The KiCad files were generated from the board data rather than drawn in KiCad, so open them and refill the ground zone (B) before relying on them; the Gerbers are the fab source. Any 2-layer PCB fab can make it from the Gerbers: FR-4, 1.6 mm, 1 oz copper, any mask colour with white silk, HASL or ENIG, 98 × 80 mm.

Design notes

Everything on the board exists to fake the game's side of the harness safely; there are no active parts.

• Command lines: each SOUND bit has a 1k series resistor and a 10k pull-down, so an open dial position reads 0 and a mis-plugged harness can't dump +5 V straight into a sound-board pin.
• Hex dial: a Grayhill 94HBB16T coded rotary switch. Its common is on +5 V and its 1-2-4-8 outputs drive SOUND0–SOUND3. Fit it with the 0 on its dial toward the top of the board, next to the "0" on the silkscreen. The switch drops into its holes either way round, and turned 180° it swaps SOUND0 with SOUND1 and SOUND2 with SOUND3.
• SIRQ: a 10k pull-up holds it high; PLAY pulls it to ground through 1k. A 1 µF cap across the button (about 10 ms with the 10k) stops contact bounce from firing several commands.
• RESET: 10k pull-up to +5 V, button to ground. An earlier revision held RESET low permanently, which would have kept the sound board in reset.
• E-GND (J2-7): tied to ground. The Sounds Good grounds it on the board; the CSD uses it as the return for its input filter caps.
• Volume: a board-mounted 1k linear pot with a knurled shaft (Bourns PTV09A-4020U-B102), replacing Midway's 0–1k cabinet pot.
• Fuses: Bourns MF-R135 on +5 V and MF-R050 on +12 V, right after the power header, so a shorted sound board trips the fuse instead of cooking the tester. They reset when power is removed. The +5 V fuse drops about 0.1 V at 1 A; set the supply to 5.1 V if you want exactly 5.0 V at the board.
• Rail LEDs: one green LED per rail, after the fuses, at about 3 mA each (1k on +5 V, 3.3k on +12 V).
• Mounting: the 3.2 mm corner holes take M3 or #4-40 screws. Keep pan heads to 5.6 mm or smaller; the two right-hand holes sit close to the volume pot and a harness hole.
• Strain relief: every harness wire goes through a bare hole with its jacket on, then solders into the plated hole beside it.
• Layout: all traces use rounded corners, with a ground pour on the back. Design-rule and connectivity checks pass with no shorts, opens or copper islands, and the Gerbers were re-read with an independent parser to confirm they match.

Compatibility

Three Midway boards share this J1/J2/J3 pinout and command interface, so Squeak Check works with all of them.

Not compatible: the Super Sound I/O (SSIO), and the Williams System 11 sound boards in Arch Rivals, Pigskin 621AD and Tri-Sports. The pinball-era Bally boards (Cheap Squeak, Sounds Deluxe, pinball TCS) use different harnesses and weren't checked.

Squeak Tap: record the sound digitally

Squeak Tap is a companion project. It plugs into the Cheap Squeak Deluxe's DAC socket and records the exact 10-bit numbers the 68000 sends to the DAC, so you get a bit-perfect digital copy of a sound, such as the Spy Hunter theme, instead of a recording of the analog output.

• DAC adapter: lift the AD7533 out of U10 and plug the adapter in. The DAC goes into a socket on the adapter, 12.7 mm to one side, and still drives the board's audio. The 10 data bits and ground go out on a 16-way ribbon. It draws no power from the sound board.
• Capture board: two 74LVC245 buffers turn the 5 V logic into 3.3 V, and a Raspberry Pi Pico 2 samples all 10 bits 960,000 times a second. It sends only the changes to a PC over USB. With the Pico unplugged, the buffers don't load the sound board.
• PC tool: squeak_tap.py saves the stream and turns it into a WAV: at the CSD's own update rate (one sample per DAC write, lossless), at 44.1 or 48 kHz, or with the CSD's analog filter added so it sounds like the cabinet.

Why 960 kHz?

The sound board has no sample clock to tap: the 68000 writes a new value whenever its code gets there, and each value takes two writes, the top 8 bits from PIA port A and the bottom 2 from port B. Watching the bus every microsecond catches the exact moment of each write. That gives the true update rate for the lossless copy, and lets the tool remove the half-written value between the two writes, which would otherwise be a click. Only changes are sent, so the USB traffic follows the board's write rate, not 960,000 a second.

Recording the Spy Hunter theme

1. Flash the Pico with the .uf2 file, fit the adapter in U10 and the AD7533 in the adapter, and connect the ribbon.
2. Run python squeak_tap.py capture <port> spyhunter.sqt on the PC.
3. With Squeak Check driving the sound board, dial 1, press MARK on the capture board, then press PLAY. Press Ctrl-C when the theme ends.
4. Run python squeak_tap.py wav spyhunter.sqt theme.wav --rate native --from-mark 1 for the lossless copy, or leave out --rate native for 48 kHz.

To order, upload the Gerber zip as one panel. On JLCPCB set Different Design to 2 and Delivery Format to Panel by Customer; other fabs have the same options under other names. The tabs follow JLCPCB's mouse-bite guide.

The DAC wiring comes from Midway's schematic, kept in this repo as reference/Cheap_Squeak_Deluxe_Schematic.pdf.

Sources

• Spy Hunter Parts and Operating Manual (Bally Midway), Internet Archive: Cheap Squeak Deluxe schematic, assembly drawing and parts list
• MAME source, mcr3.cpp: Spy Hunter's MCR-III hardware and output port 4
• MAME source, mcr68.cpp: Xenophobe's MCR-68k hardware and Sounds Good control word
• MAME source, csd.cpp and midway_sound.cpp: Cheap Squeak Deluxe and Sounds Good emulation
• MAME source, mcr.cpp: early MCR boards, Sound I/O and Super Sound I/O
• Forum: Spy Hunter Peter Gunn sound missing and Zdziarski, Arcade Hardware Hacking Part III: the CSD plays only the Peter Gunn theme
• Forum: Xenophobe no sound: a failed Sounds Good leaves Xenophobe silent
• Forum: Spy Hunter harness J1 connector: the original harness's IDC connectors
• PinWiki: Bally/Stern and PinRepair: Bally 6803 games: Bally pinball sound boards and the games that used them
• TE Connectivity MTA-100 and MTA-156 connectors