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Bally / Sente SAC-1

Snake Pit, Hat Trick, Off the Wall, Gimme A Break. Bally/Sente's SAC-1 system (1984-1985) has a feature found nowhere else in this wiki: its sound stage rests on CEM3394 analogue synthesiser chips — the part from Sequential Circuits synths.

Identification

Field Value
Manufacturer Bally / Sente (USA)
Years 1984-1985
CPU MC6809E @ 1.25 MHz — 20 MHz ÷ 16, or 10 MHz ÷ 8 depending on the board
CRTC C6545-1 @ 1.25 MHz (10 MHz ÷ 8)
Sound CPU Z80 @ 4 MHz (8 MHz ÷ 2)
Sound 2 × AY-3-8910A · MSM5205 · 2 × Z80 CTC
Synthesis CEM3394 — analogue synthesiser voice
PCB format proprietary

The crystal is verified, the speed is not

The driver is explicit: for the MC6809E, “xtal verified but not speed”. The 20 MHz crystal is confirmed, but the division ratio is not.

And for the sound board it is worse: it carries 2 × Z80 CTC, 2 × AY-8910A, one M5205 and an 8 MHz crystal, with the note “divisor unknown for every device” — not one divider is known.

→ On this platform, clock values are not a reference. Diagnosis is by comparison against a working board.

The CEM3394 — a synthesiser in a cabinet

This is what makes the board remarkable.

The chip from the Prophets

The CEM3394 is an analogue synthesiser voice — VCO, VCF, VCA on a single chip — designed by Curtis Electromusic and used in Sequential Circuits synthesisers of the period.

Bally/Sente put it in an arcade cabinet. These games' sounds are therefore not generated digitally: they come out of an analogue circuit, with everything that implies — drift with temperature, ageing passives, settings that wander.

What emulation does not reproduce

The driver admits it: “CEM3394 emulation is not perfect”. Comparing a real board's sound against emulation therefore says nothing about the board's condition.

The SAC-1 self-test — RAM you diagnose by ear ✅

Nine beeps, and you know which bit is dead

The SAC 1 manual (Bally/Sente) describes a self-test that speaks as much as it displays. For RAM:

  • ⓘ only the first error encountered is reported — fix, rerun, start again;
  • the faulty address is displayed on screen;
  • ✅ and the error is played as nine tones: the first says whether the address is odd or even (high or low pitch), and the next eight give the data bits at that address, from bit 0 to bit 7 — ⚠️ high = ok, low = bad.

⚠️ This is a diagnosis by ear, and you have to know it before you hear the sequence: without the key, nine beeps at power-on read as a game "making noise". With it, they name one precise bit at one precise address.

The sound board test: one byte, and every bit has a name

✅ Reading it as a whole: "all 0s = ok, any 1s = bad, all 1s = bad serial communications". ⚠️ A byte of all ones therefore does not mean "six dead voices": it means the serial link itself is not answering.

Bit What it reports
0 (LSB) to 5 voices 1 to 6 — 0 = good, 1 = bad
6 and 7 0 0 = ok · 0 1 = RAM bad · 1 0 = ROM bad · 1 1 = optional RAM bad
The two option banks, and the factory setting — as printed

Two banks on the CPU board, marked H1 and G1 in figure 1, beside the cartridge connector and the 5 V indicator.

Bank 0 (H1)

Sw. 1 Sw. 2 Result
off off 1 coin, 1 play
off on 1 coin, 2 plays
on off 2 coins, 1 play
on on free play
Sw. 8 Result
on keep all high scores
off keep the top five

Bank 1 (G1)

Sw. 1 Sw. 2 Extra life
on on every 10,000
off on every 20,000
on off every 30,000
off off every 40,000
Sw. 3 Sw. 4 Lives
on on 2
off on 3
on off 4
off off 5
Sw. 7 Sw. 8
off difficulty easy off no sound in attract
on difficulty hard on attract sound on

The factory setting, as the manual lists it: 3 lives, extra life every 20,000 points, keep the top 5 high scores, attract sound on, 1 coin 1 play. And the matching positions:

Bank 1 2 3 4 5 6 7 8
H1 off off on on on on on off
G1 off on off on on on on on

✅ The two tables agree, and it is checkable without the cabinet: H1 "off, off" does give 1 coin 1 play, H1 sw. 8 "off" does give the top five, G1 "off, on" does give every 20,000, G1 "off, on" on switches 3-4 does give 3 lives, and G1 sw. 8 "on" does give attract sound on. ⓘ One point is not carried in the factory list: G1 switch 7 is on, which the other table makes difficulty hard — the manual does not mention it among the factory settings; this wiki notes it and draws nothing further.

⚠️ And a spelling from the manual, reproduced as it stands: resetting the high scores and resetting the counters are both done "by pressing Alaron button" — in option switch mode for the scores, in accounting data mode for the counters. ⓘ The word is printed twice, so it is not a scan accident; it looks like Alarm, but this wiki does not correct a source.

What the parts list says about the cabinet

ⓘ The monitor is named: 700-0160-01 — MONITOR, COLOR 19″ RGB IN-LINE, with its own harness (001-8042-01-0A) — a 19-inch in-line colour tube, not a vector display.

ⓘ The sound is a passive-radiator enclosure: two 4.5-inch, 8 Ω, 10 W speakers and two 10-inch passive radiators in the same box, plus a 12-inch fluorescent tube.

⚠️ And a note that matters for sourcing: under the coin-door exploded view, the manual states "FOR YOUR CONVENIENCE THESE ARE WICO PART NUMBERS" — that door's numbers are not Bally/Sente references but Wico's. Looking them up in a Sente catalogue is a dead end.

ⓘ Three control panels are drawn in the same document: the Snake Pit one, a trackball assembly and a Wico joystick assembly — the cabinet takes all three. The graphics, though, are referenced Snake Pit (side panels, CRT surround), which dates this copy of the manual.

Notable games

Chicken Shift (1984) · Hat Trick (1984) · Goalie Ghost (1984) · Off the Wall (1984) · Snake Pit (1984) · Gimme A Break (1985) · Grudge Match (prototype and Italian version).

Common faults

Symptom Probable cause Next step
Wrong or unstable sound CEM3394 or its analogue surroundings ⚠️ an analogue synth drifts normally — recap before concluding
No digital sound AY-3-8910A or MSM5205 Stages separate from the CEM3394
No picture MC6809E or the C6545-1 CRTC Both at 1.25 MHz
“Wrong” clocks possibly nothing ⚠️ the sound board's dividers are unknown
Erratic timer behaviour one of the Z80 CTCs Two of them on the sound board

Repair (technician)

  • Recap before judging the sound. A 1984 analogue stage drifts with its capacitors: wrong sound does not necessarily accuse the CEM3394.
  • Do not trust the frequencies: the driver itself declares unknown dividers across the whole sound board.
  • The CEM3394 has no modern replacement — it is a sought-after synthesiser chip, and expensive when it can be found at all.

→ Methods: PCB diagnosis · Recap

Procedure — a dead board, step by step ✅

What this adds

The general method lives in PCB diagnosis. This is its application to this platform: the same order, but with the values actually expected here, so you do not have to go back and forth.

Tools: multimeter, oscilloscope (or frequency counter), current-limited bench supply, desoldering braid. → Tooling

If the board is still in a cabinet

A CRT holds its charge with the machine unplugged. → CRT safety

1. Inspection, power off. Corrosion, cut tracks, oxidised EPROM legs, leaking SMD capacitors, dubious earlier repairs.

2. The connector, before anything else. A large share of "dead boards" are a contact problem, not a fault. Clean the contacts with isopropyl alcohol and reseat — not with an abrasive eraser on a gold edge, which takes the plating off and brings the problem back worse. It costs two minutes and settles the question.

3. Power, measured on the board. Aim for +5.0 V measured as close as possible to where power reaches the board — not at the PSU output: the drop is in the loom, and that is exactly what makes a board boot on the bench and not in the cabinet. Where you probe depends on this board's connector — JAMMA edge, JVS loom or a proprietary harness — plenty of makers kept one long after JAMMA arrived — see Identification above. → Wiring Check −5 V and +12 V too if the sound stage uses them. Suspected short: bring it up current-limited and find the hot spot.

4. /RESET. It must release after power-up. Held low = the CPU never starts. A 1–2 Hz loop is not the reset circuit, it is the watchdog: the CPU is crashing at boot, so look at the bus, not at the reset.

5. Clocks. There is no measurement table for this platform — MAME's sources yield nothing usable for it. Any frequency quoted under Architecture on this page serves as a marker; otherwise compare against a working board rather than against a number: an invented value condemns healthy parts. → PCB diagnosis

6. Bus. Activity on the main CPU's address lines tells you whether it is running or frozen. Frozen with good clocks and good reset → work RAM, bus buffers, ROMs.

7. It boots but the picture or the sound is wrong — that is another job: see Common faults on this page, then signal tracing.

Modernisation & mods

  • No suicide: no battery-backed protection.
  • ⚠️ Do not host ROMs — method only.

Emulation & FPGA

MAME covers the platform (bally/balsente.cpp, driver by Aaron Giles), with the declared reservation about CEM3394 emulation.

Photos / assets

(To take: the CEM3394 chips — few people have seen them inside a cabinet —, the sound board with its two Z80 CTCs, and the 20 MHz crystal.)

Archived original documents ✅

Manufacturer document archived locally, found on Internet Archive on 13 August 2026. ⚠️ The item declares no licence — kept for preservation, like the rest of the collection, and its provenance is named.

Sources & attribution

Text (synthesised here; sources listed for attribution):

  • MAME — src/mame/bally/balsente.cpp (BSD-3-Clause / GPL, driver by Aaron Giles): the SAC-1 system, MC6809E at 20 MHz ÷ 16 with the note “xtal verified but not speed” and a 10 MHz ÷ 8 variant, C6545-1 CRTC at 10 MHz ÷ 8; the sound board's make-up — 2 × Z80CTC, 2 × AY8910A, 1 × M5205, 8 MHz crystal — with the note “divisor unknown for every device”; sound Z80 at 8 MHz ÷ 2; the CEM3394 chips and the reservation “CEM3394 emulation is not perfect”; and the game list with release dates — https://github.com/mamedev/mame/blob/master/src/mame/bally/balsente.cpp

Review & corrections

How this space is used

Spotted a wrong value, an outdated procedure, a chip reference that does not match your board? Say so here, with what you observed (model, board revision, serial number, measurement). Every report is cross-checked against a source before anything changes — an unverifiable correction is published as “reported by …, not cross-checked” rather than silently applied.

Reading is open to everyone; posting requires signing in with Discord. Reports from the wiki's declared reviewers are handled first; anyone else's are read too, but go through a human before anything is changed.