Dragon 32 / Dragon 64 Machine Hub
Welsh 6809 powerhouse workbench reference — Dragon 32 & 64 board revisions, MC6883 SAM & MC6847 VDG diagnostics, DRAM rail checks, PSU/DIN pinouts and EPROM substitutions.
Split-Board Power Supply & Early Dragon 32 Triple-Rail DRAM Warning
Unlike single-PCB computers, the Dragon 32 and Dragon 64 use a two-board stacked layout: the upper analogue/power regulator board (which houses the 7805/7812/7905 voltage regulators, smoothing capacitors, and Astec RF modulator) connects to the lower digital motherboard via a 10-pin/12-pin inter-board header. Early Dragon 32 boards populated with 16× 4116 (or half-good 4116) DRAMs require +5 V, +12 V, and −5 V — if the −5 V rail drops out due to a cracked solder joint on the PSU header or a failed 7905 regulator, the 4116 RAM bank will rapidly overheat and fail short!
1 Check the External AC Transformer
- The external Dragon brick outputs low-voltage AC only (28 V AC center-tapped as 14-0-14 V AC, plus an 8.5 V AC tap). All rectification and DC regulation happen inside the computer on the top PSU PCB.
- Never connect a DC power supply to the rear Dragon power socket without verifying the internal bridge rectifiers and regulator dissipation.
- Inspect the rear 5-pin or 6-pin power socket solder joints on the upper board — heavy transformer cables frequently crack the PCB pads, causing intermittent resets or missing −5 V/+12 V rails.
2 Verify All Three DC Rails First
| Rail | Regulator | Target | Used By |
|---|---|---|---|
| +5 V DC | LM340T5 / 7805 (on large heatsink) | 4.85–5.15 V | 6809E, SAM, VDG, PIAs, ROMs, 4164/4116 VCC |
| +12 V DC | 7812 / discrete zener stage | 11.5–12.5 V | LM1889 video encoder, 4116 VDD (pin 8), audio op-amp |
| −5 V DC | 7905 / 79L05 negative reg | −4.75 to −5.25 V | 4116 DRAM VBB (pin 1) & RS-232 level shifting (D64) |
Unplug the inter-board power Molex connector from the lower logic board before powering up an unknown machine for the first time!
3 Recap the Top Power/Video Board
- Main 4700 μF 16 V / 2200 μF 25 V axial smoothing caps: Located directly next to the boiling-hot 7805 heatsink. After 40+ years these bulge, leak electrolyte, and cause severe 100 Hz hum bars rolling up the TV display.
- Thermal compound refresh: Drill out or unscrew the rivets holding the 7805 regulator to the aluminium heatsink plate, clean off the chalky dried 1982 silicone paste, and apply fresh thermal compound with an M3 machine screw and nyloc nut.
Model Comparison & Revision Selector
Toggle between the Dragon 32 and Dragon 64 (plus Tano / Eurohard variants) to inspect board differences, RAM configs, and ROM layouts.
Dragon 32 1982–1984 • Cream Case • 32 KB RAM
Manufactured by Dragon Data Ltd in Port Talbot, Wales (and later by Eurohard in Spain). Built around the Motorola MC6809E 8/16-bit microprocessor running at 0.895 MHz alongside the MC6883 Synchronous Address Multiplexer (SAM) and MC6847 Video Display Generator (VDG). Features a full-travel mechanical keyboard, Parallel Centronics printer port, and right-hand cartridge expansion slot.
Key ICs & Architecture
- CPU: Motorola MC6809E / Hitachi HD6809E @ 0.894886 MHz
- SAM: Motorola MC6883 / SN74LS783 (40-pin DIP) — clock generator, DRAM refresh & memory map controller
- VDG: Motorola MC6847P (40-pin DIP) — 256×192 graphics & 32×16 semigraphics/text
- PIA 0 & PIA 1: 2× MC6821P (40-pin DIP) — keyboard matrix, joystick comparators, DAC sound, cassette & parallel printer
- BASIC ROM: 16 KB Microsoft Extended Color BASIC (2× 8 KB 2364 mask ROMs at IC17 & IC18, or single 16 KB ROM)
- Colour Encoder: LM1889N PAL chroma modulator on upper analogue board
- Sound DAC: 6-bit resistor ladder DAC driven by PIA1 Port A (PA2–PA7)
Dragon 32 Motherboard Revisions & RAM Configs
Dragon Data used three different RAM populations across production depending on chip pricing in 1982–83:
| Board Type | RAM Population | Power Rails Used | 64K Upgrade Path |
|---|---|---|---|
| Early Issue 1 / 2 (16-socket board) |
16× 4116 (16K×1) or 16× 4132 "half-good" DRAMs in two banks of 8 | +5 V, +12 V, −5 V | Replace lower 8 sockets with 4164s (+5 V mod) and route SAM pin Z2 (pin 12) for 64K addressing. |
| Mid Production (Half-good 4164s) |
16× Motorola MCM4532 / TI TMS4532 (32K×1 half-valid 64K chips) | +5 V only | Fit 8× healthy 4164s in one bank, remove the second bank, and adjust the upper/lower address jumper link. |
| Late Issue 3 / Eurohard (8-chip board) |
8× 4132 or 8× 4164 (with upper 32K disabled) | +5 V only | Often already has 4164s fitted from factory or needs 8× 4164s + single wire from SAM pin 12 to RAM pin 9 (A7). |
Identifying Blue vs Green/Tan Boards: Many Welsh-built Dragon 32s feature distinctive dark-blue solder mask PCBs. Always inspect the underside of the blue boards for factory track cuts and jumper wires around the RAM bank before desoldering!
Dragon 64 & Tano Dragon 1983–1985 • Dark Charcoal Case • 64 KB RAM
Upgraded with a full 64 KB of single-rail DRAM, two switchable 16 KB BASIC ROMs (32K mode at $8000 and 64K mode copied to RAM at $C000 via EXEC 48000), and a dedicated hardware RS-232C serial port driven by a MOS/Rockwell 6551 ACIA. The US Tano Dragon is an NTSC Dragon 64 built in New Orleans with a heavy switching power supply.
Key ICs & Upgrades over Dragon 32
- CPU: MC6809E / HD68B09E @ 0.895 MHz (1.79 MHz bursts via SAM poke)
- SAM: MC6883 / SN74LS783 configured in 64K dynamic mode
- VDG: MC6847P (or MC6847T1 on rare prototypes)
- RAM: 8× 4164 (64K×1, +5 V single-rail) at IC1–IC8
- Dual OS ROMs: Two 16 KB mask ROMs (23128): IC17 (32K Mode BASIC) & IC18 (64K Mode BASIC + Bootloader + Serial routines)
- Serial ACIA: MOS / Synertek / Rockwell 6551P ACIA (mapped at
$FF04–$FF07) with dedicated 1.8432 MHz baud rate crystal - RS-232 Line Drivers: MC1488 (+12 V/−12 V TX) & MC1489 RX level shifters
Dragon 32 vs Dragon 64 Hardware Comparison
| Feature | Dragon 32 | Dragon 64 / Tano |
|---|---|---|
| Case Colour | Light Cream / Beige | Dark Charcoal Grey |
| RAM Architecture | 32 KB (16× 4116/4532 or 8× 4132) | 64 KB (8× 4164 single-rail +5 V) |
| ROM Socket Type | 2× 24-pin (8 KB 2364 pinout) | 2× 28-pin (16 KB 23128 / 27128 pinout) |
| 64K Mode Entry | N/A (unless upgraded) | Type EXEC (or EXEC 48000) to copy ROM to RAM and bank-switch |
| RS-232 Serial Port | None | 5-pin DIN on rear driven by 6551 ACIA (up to 19,200 baud) |
| Keyboard Connector | Single 16-pin inline Molex | Split 2× 8-pin Molex headers (watch orientation!) |
Dragon 64 Keyboard Header Trap: When reconnecting the Dragon 64's two 8-pin keyboard ribbon connectors, ensure Pin 1 matches carefully — plugging them in swapped or offset by one pin causes phantom keypresses on boot.
Diagnostic Checklist & Common Faults
Isolating MC6883 SAM failures, MC6847 VDG video glitches, and 4116/4164 DRAM faults on the bench.
SAM (MC6883 / SN74LS783) Failure Symptoms
The 40-pin MC6883 SAM is the heartbeat of the Dragon: it divides the 14.31818 MHz crystal down to the CPU E/Q quadrature clocks, generates the VDG clock, handles transparent interleaved DRAM refresh, and decodes the entire memory map (S0–S2).
Diagnostic Steps
- Total Black Screen / Dead CPU: Probe SAM Pin 35 (E Clock) and Pin 36 (Q Clock) with an oscilloscope. Both must show a clean 0.895 MHz square wave separated by 90° phase. If missing, check the 14.318 MHz crystal (XTAL1) on SAM pins 1 & 2, or suspect a dead SAM.
- Thermal Check: Because the SN74LS783 is a dense bipolar Schottky chip, it naturally runs very warm (~55–60 °C). Fitting a 40-pin stick-on aluminium heatsink is mandatory preventative maintenance. If it burns your finger immediately or stays stone cold, it has failed.
- Corrupted Screen / RAM Not Refreshing: Check SAM Pin 37 (/RAS) and Pin 38 (/CAS), plus multiplexed address outputs Z0–Z7 (Pins 5–12). Stuck Z-lines cause repeating screen blocks and instant crashes.
- Device Select Decoding (S0, S1, S2 on Pins 21, 20, 19): These feed a 74LS138 demultiplexer to generate chip selects for ROM, PIA0, PIA1, and the cartridge port. If the CPU runs but never reads the BASIC ROM, check S0–S2.
- Modern Replacement: Original MC6883 / SN74LS783 chips are scarce, but modern CPLD drop-in replacements (such as the robcfg / Stewart Orchard EPM7128 SAM replacement) work flawlessly and run completely cool.
VDG (MC6847) Video Faults, Snow & Artefacts
The Motorola MC6847 Video Display Generator reads display RAM during the half-cycle when the 6809E is off the bus, outputting luminance (Y) and colour difference signals (φA, φB) to the LM1889 encoder.
Diagnostic Steps
- Green screen with black characters, or garbled characters with working BASIC: Blind-type
SOUND 100,5and press ENTER. If you hear the beep through the TV/monitor, the 6809E, SAM, ROM, and PIA are working — the fault is isolated to the MC6847 VDG, the 74LS244/74LS245 video bus buffers, or bad lower RAM bit lines. - Black & white picture only (no green background): Check the 4.433619 MHz PAL colourburst crystal on the upper video board, the +12 V supply to the LM1889 (Pin 14), and the chroma phase trimpot.
- "Snow", jailbars, or shimmering ghosting: Usually caused by dried-out decoupling capacitors on the upper video/PSU board or oxidised pins on the MC6847 socket. Clean the 40-pin VDG socket with DeoxIT and reseat the IC.
- Wrong graphics mode / stuck inverse characters: PIA1 Port B (PB3–PB7) drives the VDG mode control pins (
/A/G,GM0–GM2,CSS). If PIA1 fails, the VDG gets stuck in semigraphics or wrong colour set.
RAM Fault Detection (4116 on Dragon 32 vs 4164 on Dragon 64)
On boot, Extended BASIC clears text memory at $0400–$05FF with $60 (96 decimal, dark-green space character) and tests RAM upward until a read-back fails. A single stuck data bit changes $60 into a repeating grid of identical characters across the whole screen!
Screen Character Fault Decoder (Base = $60 / Space)
| Data Bit | Stuck LOW (0) | Stuck HIGH (1) | Dragon 64 IC |
|---|---|---|---|
| D0 (Bit 0) | Normal Space ($60) — crashes or fails RAM count | ! (Exclamation, $61) | IC1 |
| D1 (Bit 1) | Normal Space ($60) | " (Quote, $62) | IC2 |
| D2 (Bit 2) | Normal Space ($60) | $ (Dollar, $64) | IC3 |
| D3 (Bit 3) | Normal Space ($60) | ( (Left Paren, $68) | IC4 |
| D4 (Bit 4) | Normal Space ($60) | 0 (Digit Zero, $70) | IC5 |
| D5 (Bit 5) | @ (Inverse At, $40) | Normal Space ($60) | IC6 |
| D6 (Bit 6) | Black/Green block ($20) | Normal Space ($60) | IC7 |
| D7 (Bit 7) | Normal Space ($60) | Green/Orange graphic block ($E0) | IC8 |
Step-by-Step RAM Troubleshooting
- Measure DRAM rails first: On 16-chip Dragon 32 boards (4116), measure Pin 1 (−5 V), Pin 8 (+12 V), and Pin 9 (+5 V). On 4164/4532 boards and Dragon 64, measure Pin 8 (+5 V) and Pin 16 (GND).
- Check
PRINT MEMin BASIC:- A healthy Dragon 32 prints
24871bytes free. - If it boots to BASIC but prints a much lower number (e.g.
8487), the upper 16K bank of 4116s (or the CAS1 line from the SAM) has a faulty chip.
- A healthy Dragon 32 prints
- Piggyback Test (for 4164s / non-shorted 4116s): If one chip has an open data output (without a dead short on power), carefully press a known-good DRAM chip on top of each suspect RAM IC one by one while powering on to see if the screen clears or
PRINT MEMreturns to 24871. - Use a Diagnostic Cartridge: Plugging a Dragon Dead-Test / DiagROM cartridge into the 40-pin side expansion slot runs straight from the
$C000cartridge space without needing working system RAM, testing all 32K/64K bit-by-bit and flashing the screen red/green with the bad bit number.
Power, A/V & Cable Pinouts
External AC transformer & internal PSU headers, 5-pin DIN Composite A/V, Cassette DIN, and 5-pin DIN analogue Joystick wiring.
External AC Power Socket & Internal Header DIN / Inter-Board
The external brick supplies raw AC to the upper regulator PCB, which then feeds regulated DC to the lower logic board.
External Rear Power Socket (Viewed from Rear)
| Pin | Dragon 32 / 64 UK PSU | Function |
|---|---|---|
| 1 | 0 V AC (Center Tap) | Transformer secondary common return |
| 2 | 8.5 V AC (Unregulated) | Feeds 5 V bridge rectifier & 7805 (+5 V @ 1.5 A) |
| 3 | NC (Key / Blank) | Not connected |
| 4 | 14 V AC (Winding A) | Feeds +12 V / −5 V rectifier circuit |
| 5 | 14 V AC (Winding B) | 28 V AC end-to-end with Pin 4 (14-0-14 V AC) |
Internal DC Header Check: On the Molex header linking the top PSU board to the main PCB, verify clean +5.0 V (heavy tracks), +12.0 V, −5.0 V, and 0 V GND before suspecting logic chips.
Monitor / Composite A/V Port 5-Pin DIN 180°
Standard 180° 5-pin DIN (labelled MONITOR). Provides composite video and mono audio for SCART or RCA TV inputs — vastly sharper than the RF modulator socket!
| DIN Pin | Signal | RCA / SCART Connection |
|---|---|---|
| 1 | Audio Out | To RCA Audio (White/Red) or SCART Pins 2 & 6 (bridge for dual-mono) |
| 2 | GND (0 V) | Video & Audio Ground shield (SCART Pins 4, 17, 18) |
| 3 | Composite Video Out | To Yellow RCA Video In or SCART Pin 20 (1 V p-p 75 Ω) |
| 4 | NC (or Chroma on mods) | Unconnected on stock boards |
| 5 | NC (or +5V mod) | Unconnected on stock boards |
Pro Tip: The Dragon's 5-pin Monitor DIN uses the exact same Pin 1 (Audio), Pin 2 (GND), and Pin 3 (Video) assignment as the 5-pin Commodore VIC-20 / early C64 A/V cable — your existing C64 composite cable works straight out of the box!
Cassette Recorder Port 5-Pin DIN 180°
Labelled TAPE. Includes an internal reed relay for automatic cassette motor control (`MOTOR ON` / `MOTOR OFF` or `CLOAD`).
| DIN Pin | Signal | 3.5 mm Jack Plug Target |
|---|---|---|
| 1 | Motor Control A | 2.5 mm Sub-mini REM plug (Tip) |
| 2 | GND (0 V) | 3.5 mm EAR & MIC Sleeve (Common Ground) |
| 3 | Motor Control B | 2.5 mm Sub-mini REM plug (Sleeve) — relay contact |
| 4 | Audio IN (from Tape EAR) | 3.5 mm plug to Cassette EAR / Phones output |
| 5 | Audio OUT (to Tape MIC) | 3.5 mm plug to Cassette MIC / Aux input |
Relay Faults: If you don't hear a faint mechanical click from the board when typing CLOAD, the reed relay or its driver transistor (driven by PIA1 CA2) is stuck. Also ensure smartphone/laptop playback volume is at ~75–85% with all EQ/enhancements off.
Left & Right Analogue Joystick Ports 5-Pin DIN 180°
Dragon joysticks are analogue potentiometric (0–100 kΩ) between +5 V and GND, read via an internal 6-bit DAC and LM339 comparator! Standard Atari digital 9-pin joysticks will not work without an active resistor-divider adapter.
| DIN Pin | Signal | Function |
|---|---|---|
| 1 | X-Axis Wiper | 0–5 V analogue wiper from 100 kΩ horizontal pot (Left = 0 V, Center = 2.5 V, Right = 5 V) |
| 2 | Y-Axis Wiper | 0–5 V analogue wiper from 100 kΩ vertical pot (Up = 0 V, Center = 2.5 V, Down = 5 V) |
| 3 | GND (0 V) | Ground reference for pots & Fire button return |
| 4 | /FIRE Button | Active-low switch to Pin 3 (GND) — read on PIA0 PA0 / PA1 |
| 5 | +5 V Reference | +5 V supply for top end of both 100 kΩ joystick pots |
Tandy CoCo Compatibility: The Dragon's 5-pin DIN joystick electrically matches the Tandy TRS-80 Color Computer (CoCo 1/2), which uses a 6-pin DIN with the exact same signals on pins 1–5.
Chip & EPROM Reference
Interactive IC pinout links plus BASIC / OS ROM socket compatibility and modern EPROM substitution rules.
Motorola MC6809E
Advanced 8/16-bit microprocessor with dual accumulators (A/B → D), two 16-bit index registers, hardware multiply, and external E/Q clock inputs.
View Pinout →MC6821 PIA (×2)
Peripheral Interface Adapter. Two 6821s manage the keyboard matrix, 6-bit DAC audio, joystick multiplexer, cassette I/O, and VDG mode lines.
View Pinout →4116 DRAM (16K×1)
Triple-rail (−5 V, +12 V, +5 V) dynamic RAM fitted to early 16-socket Dragon 32 motherboards.
View Pinout →4164 / 4532 DRAM
Single-rail (+5 V) DRAM used in Dragon 64 (8× 4164) and mid/late Dragon 32 boards (4532 / 4132).
View Pinout →Core Silicon Substitution Reference
| Chip | Original Part | Direct Drop-In Substitutes |
|---|---|---|
| CPU | MC6809EP (1 MHz) | MC68A09EP (1.5 MHz), MC68B09EP (2 MHz), or Hitachi HD63C09EP (CMOS upgrade — runs cool and adds extra registers/instructions!). Note: Must be the "E" external-clock suffix, NOT the internal-clock MC6809P! |
| SAM | MC6883P / SN74LS783N | SN74LS785N (enhanced version) or modern CPLD SAM replacement module. |
| VDG | MC6847P (Non-interlaced) | MC6847YP (interlaced capable, pin-compatible in Dragon socket) or S68047P. |
| PIA | MC6821P / MC68B21P | EF6821P (Thomson), HD6821P (Hitachi), W65C21N (WDC CMOS — check totem-pole vs open-drain PA lines), or MOS 6520. |
| ACIA (D64) | MPS 6551 / R6551P | SY6551A, UM6551A, or W65C51N (note: modern WDC W65C51S has a known TX interrupt hardware bug; use vintage NMOS/CMOS Rockwell or Synertek parts). |
BASIC & OS ROM / EPROM Substitution Guide
The Dragon 32 uses 24-pin 2364 mask ROM sockets (8 KB each), whereas the Dragon 64 uses 28-pin 23128 mask ROM sockets (16 KB each).
| Machine | Original ROM | EPROM Substitute | Adapter / Jumper Notes |
|---|---|---|---|
| Dragon 32 (2× 8K sockets) |
2× 2364 (24-pin, 8 KB) | MCM68764 / 68766 (24-pin 8K) | Direct 24-pin drop-in! No adapter required, though 68766 chips require a classic programmer capable of 25 V VPP. |
| Dragon 32 (Single ROM or 2764/27128) |
2× 2364 (IC17 & IC18) | 27C64 (8K) or 27C128 (16K) (28-pin) | Use a standard 2364-to-2764/27128 adapter board, or fit a single 27C128 into one socket with a wire from the 74LS138 combined 16K ROM select line. |
| Dragon 64 (IC17 & IC18, 28-pin) |
2× 23128 (28-pin, 16 KB) | 27C128 (16K) or 27C256 (32K) | 27C128 is a direct drop-in into the Dragon 64's 28-pin sockets! For a 27C256, program the 16K image into the upper half (or tie Pin 27 / A14 appropriately). |
Cartridge Port Auto-Start Note: Cartridge ROMs map at $C000–$FEFF and trigger auto-boot by tying the Cartridge Sense line (/CART, tied to PIA1 CB1) to the Q clock so the PIA fires an interrupt once the system initialises!
Resources & Further Reading
Blog posts, interactive bench tools, and Dragon 32 / 64 community archives.
Smiggy's Dragon & 6809 Posts
Bench Tools & Machine Hubs
- IC Pinout Finder — 6809, 6821, 4116, 4164 & more
- Recap Planner — capacitor replacement guide
- Ohm's Law Calculator
- C64 Machine Hub — Commodore 64 reference
- Amiga Machine Hub — A500/A600/A1200 reference
- BBC Micro Hub — Model B, B+ & Master 128 reference
- ZX Spectrum Hub — 16K/48K & +2/+3 reference
External Dragon Resources
- World of Dragon Archive — schematics, service manuals, ROMs, and wiki
- World of Dragon Forums — active Dragon 32/64 hardware & repair community
- Ciaran Anscomb's Dragon Pages — XRoar emulator & technical deep-dives
- Dragon Diagnostic ROMs — dead-test & memory test ROM images