Skip to content

Williams 6809 (Defender, Robotron, Joust…)

The great American early-80s arcade platform: the Motorola 6809-based Williams hardware, with its bitmap display (framebuffer, no tiles) and its speech synthesis (CVSD). Defender, Stargate, Robotron: 2084, Joust, Sinistar, Bubbles, Splat!, Blaster. No DIP switches: everything is set in the operator menu (CMOS).

Identification

Field Value
Manufacturer Williams Electronics
Years 1980 (Defender) → 1983 (Blaster); Joust 2 1986
Type ROM-board platform; bitmap display
Connector type Williams harness (not JAMMA)
Settings CMOS/NVRAM operator menu (coin door) — no DIP

Architecture

Block Chip Clock Role
CPU Motorola MC6809E 1 MHz (12 MHz / 12) game logic
Sound CPU Motorola M6808 (separate sound board) ~0.9 MHz (then 3.12 MHz) drives the sound
Sound MC1408 DAC + HC55516 (CVSD) music/SFX via DAC + digitised speech (Sinistar "I hunger!", Robotron…)
Video bitmap framebuffer in RAM (bank-switched with ROM) pixel ~8 MHz ~292×240, 16 colours, no classic hardware tiles/sprites
Accelerator "Special Chip" (DMA blitter) — SC1/SC2 fast block copies (software sprites); absent on Defender & Stargate
I/O 6821 PIA inputs, sound, counters

Specificity: Williams works in bitmap (the CPU/blitter writes pixels directly into video RAM), whereas most hardware of the era is tile + sprite based. Hence the so characteristic look (Robotron, Sinistar) and the high CPU load — the "Special Chip" blitter precisely offloads those copies (except on the first two, Defender/Stargate, which don't have one).

Notable games

Defender (1980), Stargate / Defender II (1981), Robotron: 2084 (1982), Joust (1982), Sinistar (1982), Bubbles (1982), Splat! (1982), Blaster (1983), Joust 2: Survival of the Fittest (1986, late variant).

Settings: the operator menu (no DIP)

No DIP switches — everything is in CMOS

The Williams 6809 hardware has no DIP bank: coinage, difficulty, lives, free play, bookkeeping and high scores live in a CMOS/NVRAM set via the operator menu (accessed with the coin-door buttons). It's the equivalent of a soft-dip (like on Neo-Geo MVS or Cave's EEPROM menu). See the general DIP convention.

Common faults & preservation

Symptom Likely cause Lead
Lost settings / scores, "factory settings" Flat / leaking CMOS battery Replace the battery; clean the corrosion (possible trace damage)
Frozen graphics / missing blitter Dead Special Chip (SC1/SC2) Blitter diagnosis; community substitutes (Special Chip repro)
No speech (SFX OK) CVSD HC55516 stage Sound board diagnosis
Corrupted bitmap image Video RAM / bank switching RAM test, contacts
No image Power, 6809, harness Base diagnosis

CMOS battery leak = corrosion

As on many battery-backed boards of this era, a leaking CMOS battery eats the traces. Removing / relocating the battery (holder or external coin cell) is a good preservation practice.

  • Emulation: MAME (williams/williams.cpp).
  • FPGA / MiSTer: cores exist for the Williams 6809 games (Defender, Robotron: 2084, Joust, Sinistar, Stargate, Bubbles, Splat!, Blaster) — a hardware rebuild of the 6809 + framebuffer + Special Chip blitter.

The +5 V is set by soldering resistors ✅

Joust — R10 and R24, not a potentiometer

The Joust instruction manual (Williams, ref. 16P-3006-101, September 1982, 11 pp., for upright games) documents a supply adjustment this wiki had nowhere: ⚠️ the +5 V is not turned, it is wired.

Depending on what the supply already carries → neither R10 only R24 only R24 and R10
To raise the voltage, if under 4.75 V add R24 remove R10 — remove R10
To lower it, if over 5.25 V add R10 — remove R24 or add R10 remove R24 or add R10

⚠️ And a check that comes before the adjustment: "Before adjusting the voltage output, always check at the output of the supply for AC hum. This hum should never rise above .005 V on the +5VDC supply." Hum beyond 5 mV means you are not in the right place — the manual then sends you to the schematic, measuring first on DC, then a second time on AC.

ⓘ With a named test point and two thresholds: at TP5, on top of capacitor C10, a voltage below +11 V or a ripple above 700 mV rms condemns the capacitor.

Self-diagnostics, and the two switches that drive them ✅

ⓘ Everything runs from the coin door: the AUTO-UP / MANUAL-DOWN switch and the ADVANCE pushbutton. On MANUAL-DOWN + ADVANCE, the board enters diagnostic mode and runs the ROM test; each further press of ADVANCE brings the RAM test, then CMOS, sound, switches, colour RAM and the monitor test patterns.

ⓘ A mirror is built into the cabinet for this. "For ease in monitor adjustments, the monitor may be slid back and the screen viewed in the CRT mirror provided on the inside-top of the cabinet." Remove two bolts, slide the monitor back, and lock it in the extended position by putting the two bolts through holes provided for that purpose.

⚠️ The auto cycle mode has a condition you would not guess: it loops ROM, RAM and CMOS, but "the coin door must be open during the Auto Cycle test". A detected error stops the cycle and displays the failure; to leave it, switch off and on.

What the error messages ask you to measure — and a divergence with this page
On-screen message What the manual prescribes
"ROM ERROR" + chip no. power off, replace the named chip
"RAM ERROR" + bank and chip no. ⓘ first check the voltages on the named chip: −5 V pin 1, +12 V pin 8, +5 V pin 9 · then power off and replace · ⚠️ "with multiple RAM failures always check power supply"
"CMOS RAM ERROR OR WRITE PROTECT FAILURE" with the power off, measure pin 22 of the CMOS RAM

✅ The three RAM voltages match exactly what this page already publishes — a confirmation from a second Williams document.

⚠️ But the CMOS figure diverges, and this wiki does not arbitrate. The Joust manual writes 3.2 V DC minimum on pin 22; the Williams document read further up this page gives +3.8 V. ⓘ A minimum threshold and a nominal value are not the same thing, and nothing in either text lets you say so. Both are published with their source.

ⓘ The rest of the CMOS chain, as written: voltage present → replace the CMOS chip 1C · voltage absent → replace the CPU board's AA alkaline cells · still absent with fresh cells → replace diodes D9 and D10 · message persisting after a restart → logic probe on the CMOS memory circuit and the address decoding. → Backup batteries

⚠️ And the acceptance criterion for a supply, if you must compare one to another: "check +5 V, −5 V and +12 V outputs […]. Each MUST BE within 2 % of rated output with less than 0.1 % AC hum."

ⓘ The sound board diagnoses itself elsewhere: a DIAGNOSTIC pushbutton on the bottom of the sound board tests its ROM and produces sounds if the check is good. ⚠️ If sounds come out there but not in the game's test 4, look for a ROM board PIA output stuck low; if nothing comes out at all, it is the sound ROM, IC12, the input power or the rest of the board.

Conversions

  • Same hardware, swappable games: the collector scene does ROM swaps between games on the same base (e.g. Robotron ↔ Joust ↔ Stargate), even multigame boards. Check the presence/compatibility of the Special Chip for the target game (blitter games won't run on a board without an SC).
  • No suicide or encrypted protection: the CMOS only holds settings.
  • ⚠️ Do not host ROMs — method only.

EPROM repair

6809 program in 2532 / 2716 EPROMs (2 KB), many of them (Defender, Robotron, Joust, Stargate, Sinistar, Bubbles). The blitter (Special Chip) and decode PROMs are part of the hardware. Period components (1980-82).

Modernisation & mods

The common ground for every JAMMA board

Video, power, control panel and upscalers depend on the cabinet rather than the board: everything is centralised in Modernising a cabinet. To run the board outside a cabinet (at home, on a display): supergun.

  • Battery 🟢: a save battery is fitted (settings/counters/RTC). ⚠️ On this platform, a dead battery does NOT kill the game — it is a leaking battery that corrodes traces. Check and replace it before it leaks → save batteries.
  • Storage 🟡: the game lives in EPROM/mask ROM on the board. Changing or restoring a game means reprogramming the EPROMs (and, for a mask ROM, using a pinout adapter) → EPROM method.
  • Media preservation: replacing the original storage, multicarts, ODEs and modern options by family → PCB preservation hub.
  • Scan rate 📺: a TMS34010-based board: the screen configuration is programmed by the game in the graphics controller and does not appear in the driver — not measured here. In practice these boards stay at standard ~15 kHz. (Derived from the architecture documented above.) → tri-sync and upscalers.
  • FPGA / emulation: see the Emulation & FPGA section below.

Repair (technician)

  • "Special Chip" (SC1/SC2 DMA blitter): slow/corrupt graphics = the blitter (absent on Defender/Stargate) → test it; a rare part (donor / repro).
  • Separate sound board (6808 + MC1408 DAC + HC55516 CVSD): silent digitised voice (Sinistar "I hunger!", Robotron) = the CVSD stage; music = the DAC.
  • Watchdog: these boards reset if the CPU crashes → a reset loop = 6809 / RAM.
  • CMOS battery (high scores) that leaks → remove/replace before it corrodes (backup batteries). Capacitors: recap.

→ Methods: PCB fault-finding · Recap · EPROM · Backup batteries

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.

Photos / assets

(To add: Williams ROM board, sound board, Special Chip, CMOS battery.)

The Williams self-diagnostic, taken from the Robotron manual ✅

Everything below is transcribed from manual TM 16P-3005-101 (March 1982), Self Diagnostics section. It holds for the family: the Stargate and Joust manuals announce the same four procedures — operation, auditing, adjustment, diagnostics.

Entering diagnostic mode — set AUTO-UP/MANUAL-DOWN to MANUAL-DOWN in the coin door, then press ADVANCE. The ROM test starts. Each further press of ADVANCE steps through: RAM, CMOS, sound, switches, colour RAM, then the monitor test patterns.

The ROM board's LEDs speak before the screen does — and that is what saves a game with no picture:

LED What it means What the manual prescribes
0 tests passed —
1 RAM error check the voltages on the indicated RAM: −5 V pin 1, +12 V pin 8, +5 V pin 9. Power off, replace. ⚠️ Multiple RAM failures → check the power supply first.
2 ROM error the LEDs show the chip's two-digit number. Power off, replace.
3 CMOS error measure +3.8 V DC on pin 22 of the CMOS RAM

⚠️ The CMOS chain, in the manual's order: +3.8 V present → replace CMOS 1C. Absent → replace the AA alkaline cells. Still absent with fresh cells → replace diodes D9 and D10. Message persisting after a power cycle → replace memory protect gates 6E, IC1, Q1 and address decoder 4G/6J.

⚠️ AA cells in a 1982 machine. That is this family's preservation point → save batteries.

Two visual diagnoses the manual gives outright:

  • a thick vertical band = colour RAM fault → replace 1B, 2B or the colour analogue circuit; ⚠️ if both RAMs look bad, it may actually be flip-flop 1A;
  • a scanning rug pattern, a blank screen instead of INITIAL TESTS INDICATE: OPERATIONAL, an empty high-score table, a blank intro or a crash → special chip. The manual says to replace the two special chips one at a time with known-good ones, re-running the power-up tests after each swap.

What the Stargate manual adds — and it is more precise

The Stargate manual (16P-3002-101, October 1981) describes the same arrangement a year earlier, and names better what the Robotron one calls "the LEDs":

✅ It is a 7-segment display, on the ROM board — 7-segment Diagnostic Display. It does not flash a code, it spells out a sequence:

Fault What the display shows, in order
RAM 1, then the bank number (1, 2 or 3), then the chip within the bank (1 to 8) · pause · the sequence repeats · then the CRT writes RAM ERROR XY (X = bank, Y = chip)
ROM 2, then two digits = ROM chip number (01 to 12) · pause · repeats · then the CRT writes ROM ERROR X
Tests passed 0, then a few seconds later INITIAL TEST INDICATES on screen

✅ And the manoeuvre that saves a game with no picture: "if there is no CRT display at power-up", press the Reset push-button on the CPU/Video board and read the ROM board's 7-segment display. The faulty chip names itself, screen dead.

⚠️ Before any of that, the fuses: five on the power supply, two on the sound board, plus the power panel's 3 A line fuse. And check the connectors are seated — 3 on the power supply, 4 on the sound board, 5 on the CPU/Video, 3 on the ROM board.

⚠️ Mains voltage: "The line voltage must agree with that specified on the back of the cabinet or serious damage to the machine could occur." For low-line supplies (105 V or 200 V), the manual refers to its power wiring diagram.

AUTO CYCLE mode loops ROM, RAM and CMOS repeatedly with the coin door open; it stops at the first fault and displays it. To leave it: turn the game off and on.

Defender — Theory Of Operation: the Williams hardware explained circuit by circuit ✅

Two editions archived below, 38 and 37 pages. Where the Robotron, Stargate and Joust manuals give the procedures, this one gives the theory — what each circuit does and why.

Its table of contents is its value: after a Block Diagram Theory, twenty-six sections of Detailed Theory — reset circuit, watchdog, MPU, clock generator, E and Q generator, video address generator, video RAM control / timing / address multiplexer, video RAM, video shift registers, colour RAM circuit, monitor sync generator, blanking circuit, page 0 decoder, CMOS RAM, vertical count buffer, count 240, ROM board memory scheme, section/page select decoder, ROM section/half decoder, ROM board PIA/decoder, interface board assembly, and power supply.

✅ And a plate that answers the question you ask with a board in your hands: "Identification of early system boards", with photographs —

Board How to recognise it, per the document
CPU/Video a single decoder ROM, batteries at upper right
ROM board 14- and 16-pin ICs mounted parallel to the ROM ICs
Interface board four chips in total, plus a resistor pack

⚠️ Two editions, and the wiki does not arbitrate: the archive titles them Theory Of Operation (1) and (2), one "early", one "later". Both are kept — on a family whose board revisions are told apart by eye, the early version is not an outdated duplicate.

Archived original documents ✅

Manufacturer documents archived locally, found on Internet Archive on 13 August 2026 and identified from their title page. ⚠️ The items declare no licence — kept for preservation, provenance named.

Sources & attribution

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.