Acorn Atom & Acorn Electron Machine Hub
Cambridge 6502 engineering workbench reference — Acorn Atom (1980) & Acorn Electron (1983) architecture, PLCC-68 ULA socket restoration, 19 V AC PSU rails, RGB SCART wiring, and Plus 1 / Plus 3 expansion pinouts.
Electron 19V AC PSU / ULA Socket Alert & Atom Dual-7805 Thermal Warning
Over 75% of "dead" Acorn Electrons have nothing wrong with their silicon at all — the fault is tarnished tin contacts and lost spring tension inside the 68-pin 3M / Burndy leadless ceramic ULA socket (`IC1`)! Meanwhile, on the Acorn Atom, up to twenty-four `2114` static RAM chips draw heavy current through two onboard linear `7805` regulators that bake the PCB dark brown unless upgraded to modern switching buck modules.
1 Electron 19V AC Power & Internal Rails
The external Acorn Electron brick is a simple step-down transformer outputting 19 V AC (6.5 VA) on a coaxial jack. Inside the Electron, an L-shaped internal PSU board uses a self-oscillating step-down switching circuit to generate two DC rails:
| Internal Rail | Target Spec | Feeds |
|---|---|---|
| +5.0 V DC | 4.85–5.15 V DC (@ 1.0 A) | 6502A, Ferranti ULA, 4× 4164 DRAMs, OS/BASIC ROM & Expansion Bus |
| −5.0 V DC | −4.75 to −5.25 V DC | Cassette interface op-amp / comparator & expansion bus Pin 5/6 |
Can You Power an Electron from 19V DC (Laptop Brick)? Only for testing the logic board — feeding DC into the Electron's AC jack prevents the half-wave negative voltage doubler from generating the −5 V rail (which breaks cassette tape loading)!
2 Electron Ceramic ULA Socket Fix (`IC1`)
- The Electron's custom Ferranti 12C021 ULA (`IC1`) is a flat 68-pad ceramic Leadless Chip Carrier (LCC) held face-down in a brown plastic socket by a wire spring clip.
- Symptom: Pressing down on the metal ULA clip suddenly makes the machine boot with a `BEEP`!
- Proper Restoration: Unhook the wire bail, lift out the ceramic ULA (note the chamfered Pin 1 corner!), polish the 68 gold pads on the underside of the ceramic package with a soft white eraser and 99% IPA, and gently lift each of the 68 tiny spring contacts inside the socket upward ~0.3 mm with a sewing needle. Better still, replace the degraded original socket with a new through-hole PLCC-68 socket (using a ceramic LCC-to-PLCC adapter or modern CPLD ULA)!
3 Acorn Atom 8V DC Input & Dual 7805 Heat
- External Input: Unregulated +8 V to +10 V DC (at 1.5–2.5 A) via the rear 2.1 mm barrel jack (Centre-Positive on standard factory builds, though many kit-built Atoms in 1980 were wired by their owners — always trace the jack wires to the 7805 Input/GND pins with a multimeter before plugging in!).
- Dual 7805 Regulators (`IC1` & `IC47`): A fully populated 12 KB Atom (24× NMOS `2114` SRAMs) draws over 1.4 A at 5 V! Replace both linear TO-220 7805s with 1.5 A Recom R-78E5.0-1.5 / Traco TSR switching regulators to eliminate board scorching.
System Selector & Architecture Comparison
Toggle between the 1980 Acorn Atom (kit/assembled pioneer) and the 1983 Acorn Electron (single-ULA BBC Micro sibling).
Acorn Electron 1983–1985 • Issue 4 / Issue 6 • 32 KB RAM
Designed by Sophie Wilson and Steve Furber to bring the BBC Micro experience to the sub-£200 home market, the Acorn Electron condenses nearly all of the BBC Model B's video, sound, cassette, keyboard, and clock circuitry into a single 68-pin Ferranti Uncommitted Logic Array (12C021 ULA) — resulting in a remarkably compact motherboard with only 8 main integrated circuits!
Complete IC Map (Acorn Electron Motherboard)
- IC1 (Custom ULA): Ferranti
12C021/201699(LCC-68) — video Modes 0–6, DRAM refresh/nibble multiplexing, keyboard scanner, 1-bit sound generator, cassette FSK modem & RS-423/clock control - IC2 (CPU): Synertek / Rockwell / MOS 6502A @ 2.00 MHz (ROM access) / 1.00 MHz (RAM access)
- IC3 (OS + BASIC ROM): 32 KB Mask ROM (28-pin) — contains both 16 KB Electron MOS 1.00 (`$C000–$FFFF`) and 16 KB BBC BASIC II (`$8000–$BFFF`, paged by the ULA!)
- IC4, IC5, IC6, IC7 (32KB RAM): 4× 4164 (64K×1) DRAMs! How do four 1-bit chips make an 8-bit byte? The Electron ULA accesses all four 4164s twice per 2 MHz cycle (low nibble then high nibble), using 32 KB of the 4×64Kbit capacity!
- IC8, IC9, IC10 (74LS Glue): 74LS04 (`IC8` crystal oscillator/inverter), 74LS00 (`IC9`), and LM324 (`IC10` cassette input high-pass/low-pass filter op-amp)
Why the Electron Runs Slower in Modes 0, 1, 2 & 3 (Contended RAM)
| Memory / Screen Mode | 6502A Effective Clock | Architectural Reason |
|---|---|---|
| Executing from ROM (`$8000–$FFFF`) |
2.00 MHz | The 32 KB ROM has an 8-bit bus independent of the ULA video RAM fetches. |
| Modes 4, 5 & 6 (40-col / 2-col / 4-col) |
~1.00 MHz (RAM access) | Because the four 4164 DRAMs must be read twice (2×4 bits) to assemble one 8-bit CPU byte, all RAM accesses run at 1 MHz. |
| Modes 0, 1, 2 & 3 (80-col / 16-col high bandwidth) |
~0.57 MHz during active scanlines! | In 20 KB/16 KB screen modes, the ULA consumes 100% of DRAM bandwidth during each active TV scanline, stretching the 6502A clock high until Horizontal Flyback (HBLANK)! |
Turbo Board Fix: Modern plug-in 6502 socket upgrades (like the ElkSD64 / Turbo-Elk) place fast uncontended static RAM directly under the 6502A CPU, running all Electron software at a full, unthrottled 2 MHz (or 4 MHz)!
Acorn Atom 1980–1982 • Issue 2 – Issue 5 • Direct Ancestor of the BBC Micro
Sold both as a £120 self-assembly solder kit and a £170 built computer in custom-moulded wedge beige ABS by Industrial designer Allen Boothroyd, the Acorn Atom introduced Acorn's signature inline 6502 assembler inside Atom BASIC, Econet networking, and memory-mapped peripherals. In 1981, Acorn modified an Atom on a wire-wrapped board to build the working BBC Proton prototype in a single week!
Key ICs & Memory Map (Acorn Atom)
- CPU (`IC22`): MOS / Rockwell 6502 @ 1.00 MHz (derived from 4.00 MHz crystal)
- VDG (`IC27`): Motorola MC6847P / YP — 32×16 uppercase/inverse text (`CLEAR 0`) up to 256×192 monochrome (`CLEAR 4`)
- PPI (`IC25`): Intel 8255A-5 (`$B000–$B003`) — keyboard scanning, VDG mode select (`PA4–PA7`), 300-baud Kansas City cassette I/O & internal speaker
- VIA (`IC1` — Optional): MOS 6522 (`$B800–$B80F`) — Parallel Centronics Printer & User Timer interface
- Lower Text/Zero-Page RAM (`$0000–$03FF`): 2× 2114 (1K×4) SRAMs (`IC28/29`, 1 KB)
- Video RAM (`$8000–$97FF`): 2× to 12× 2114 SRAMs (`IC30–IC41`, 1 KB base expandable to 6 KB for high-res graphics)
- Upper User RAM (`$2800–$3BFF`): 10× 2114 SRAMs (`IC2–IC11`, 5 KB onboard expansion)
- ROM Sockets (`IC20, IC21, IC24`): 24-pin sockets for 8 KB Atom BASIC + OS (`$C000–$FFFF`), 4 KB Floating Point (`$D000`), and 4 KB Utility/DOS (`$A000`)
Acorn Atom vs Acorn Electron Hardware Comparison
| Feature | Acorn Atom (1980) | Acorn Electron (1983) |
|---|---|---|
| CPU & Clock | 6502 @ 1.0 MHz | 6502A @ 2.0 MHz (ROM) / 1.0 MHz (RAM) |
| Video Controller | Motorola MC6847 VDG | Ferranti 12C021 Custom ULA (Modes 0–6) |
| RAM Technology | Up to 24× 2114 SRAM (12 KB onboard) | 4× 4164 DRAM (32 KB onboard) |
| BASIC Dialect | Atom BASIC (quirky `@` variables, `$` strings, inline 6502 assembler!) | Full BBC BASIC II (`PROC`, `FN`, inline 6502 assembler) |
| Modern SD Upgrade | AtomMMC / AtoMMC2 (PIC + SD card at `$B400` replacing Econet/DOS) | Plus 1 / ElkSD64 / MGSCart (MMFS SD card filing system) |
Diagnostic Checklist & Common Faults
Step-by-step troubleshooting for Acorn Electron Black Screen / Nibble DRAM faults and Acorn Atom 2114 SRAM / keyboard issues.
Acorn Electron "Black Screen / No Boot" & 4164 DRAM Tree
Power LED (if mod fitted) or +5 V rail is live, but no startup beep and no `Acorn OS` prompt on RGB/Composite/RF.
- 1. Clean & Retension the 68-Pin ULA Socket (`IC1`): Before touching a soldering iron, clean the 68 gold pads under the ceramic Ferranti ULA and retension the socket pins! Poor contact on the ULA's clock or address pads stops the 6502A cold.
- 2. Check 16 MHz Master Oscillator (`IC8` 74LS04 + 16MHz Crystal): Probe ULA Pin 67 (`16MHz CLK IN`) and 6502A Pin 37 (`Φ0 IN`). If 16 MHz enters the ULA on Pin 67 but no 1/2 MHz clock leaves for the CPU, either the ULA socket pin isn't making contact or the ULA has failed.
- 3. High-Pitched Continuous Tone or Corrupt Screen (`IC4–IC7` 4164 DRAMs):
- Because each of the four 4164 DRAMs (`IC4, IC5, IC6, IC7`) supplies TWO data bits per byte (via time-multiplexed nibbles), a single dead 4164 corrupts a pair of bits across all 32 KB of RAM!
| Electron DRAM IC | Multiplexed CPU Data Bits | Symptom When Faulty |
|---|---|---|
| IC4 (4164) | Bit 0 (D0) & Bit 4 (D4) | Continuous boot tone or every character shifted by +1/+16 ASCII |
| IC5 (4164) | Bit 1 (D1) & Bit 5 (D5) | Uppercase/lowercase flipped (`Bit 5`) + vertical stripe pairs |
| IC6 (4164) | Bit 2 (D2) & Bit 6 (D6) | Boot crash (Zero-page/stack corruption) + vertical pixel pairs |
| IC7 (4164) | Bit 3 (D3) & Bit 7 (D7) | Solid vertical bars across screen; fails OS RAM size check |
Acorn Atom 2114 SRAM, MC6847 Video & Key Switch Fixes
Garbage characters on the `ACORN ATOM` startup screen, missing graphics memory, or dead mechanical key plungers.
- Isolating Bad `2114` (1K×4) SRAM Chips by Nibble:
- Each `1 KB` block of Atom memory uses a pair of 18-pin 2114 SRAMs: one chip holds the Low Nibble (`D0–D3`) and its partner holds the High Nibble (`D4–D7`)!
- Crashes immediately (no prompt at all): Check the Zero-Page/Stack pair at `IC28` (`D0–D3`) & `IC29` (`D4–D7`) (`$0000–$03FF`).
- Boots (`BEEP` works on CTRL+G), but screen shows wrong characters at `$8000`: Check the base Video RAM pair at `IC30` (Low Nibble) & `IC31` (High Nibble)! If a space (`$20`) turns into `$21` (`!`)–`$2F`, the Low-Nibble 2114 (`IC30`) is bad; if it turns into inverse/graphics blocks, the High-Nibble 2114 (`IC31`) is bad!
- Atom Video Snow & Monochrome-to-Composite / Colour Mods:
- A standard UK PAL Acorn Atom only feeds the MC6847's Luminance (`Y`, Pin 28) to the UHF modulator and rear phono jack (producing monochrome video even in colour modes!). Tapping MC6847 Pins 28 (`Y`), 9 (`φA`), 10 (`φB`), and 11 (`CHB`) into an LM1889/AD724 board or an RGBtoHDMI (6847 board) unlocks full colour!
- Atom Mechanical Key Switch Restoration: Early Atoms use individual open-frame gold-wire or reed key switches soldered directly to the main PCB. Flush oxidised wire contacts with DeoxIT D5 and check the `74LS42` 1-of-10 keyboard row decoder (`IC26`) if an entire row of keys fails.
RGB Video, Cassette & Electron Expansion Bus Pinouts
6-pin DIN RGB to SCART wiring (shared with BBC Micro!), 7-pin DIN Cassette, and the 50-way Electron rear Expansion Edge Connector.
Electron 6-Pin DIN RGB Video Out DIN 240° • BBC Micro Compatible!
The Acorn Electron's 6-pin DIN `RGB` socket has the exact same pinout and TTL signal levels as the BBC Micro — your existing BBC Micro RGB-to-SCART cable (with 330 Ω/470 Ω dividers) or RGBtoHDMI cable works directly on the Electron!
| DIN-6 Pin | Signal | SCART Target Pin & Attenuation |
|---|---|---|
| 1 | GREEN (TTL 5 V) | SCART Pin 11 via 330 Ω series + 470 Ω to GND |
| 2 | CSYNC (TTL Sync) | SCART Pin 20 via 470 Ω series resistor |
| 3 | +5 V DC Out | SCART Pin 16 (RGB fast-blanking) via 180 Ω resistor |
| 4 | GND (0 V) | SCART Pins 4, 5, 9, 13, 17, 18, 21 |
| 5 | RED (TTL 5 V) | SCART Pin 15 via 330 Ω series + 470 Ω to GND |
| 6 | BLUE (TTL 5 V) | SCART Pin 7 via 330 Ω series + 470 Ω to GND |
Electron & BBC 7-Pin DIN Cassette Port
Also identical to the BBC Micro (`Pin 1 = Data Out to MIC`, `Pin 2 = GND`, `Pin 3 = Data In from EAR`, `Pin 6 & 7 = Motor Control`). Note: Unlike the BBC Micro, the Electron has no internal reed relay for cassette motor control (Pins 6 & 7 are bridged/unswitched unless modified) and is famously picky about cassette volume levels — set smartphone/TZXDuino volume to ~70% when loading `.UEF` files!
50-Way Electron Expansion Bus Plus 1 / Plus 3 / ElkSD128
The rear 50-way (`2×25`, 0.1″ pitch) PCB edge fingers connect the Acorn Plus 1 (dual ROM cartridge slots + joystick + Centronics printer), Acorn Plus 3 (WD1770 3.5″ ADFS floppy unit), or modern MGSCart / ElkSD boards directly to the 6502A bus. Viewed from the rear of the Electron, Side A (Odd/Top) is on top and Side B (Even/Bottom) is underneath, numbered Left-to-Right (1–50):
| Top (A / Odd) | Signal | Bottom (B / Even) | Signal |
|---|---|---|---|
| 1, 3 | +5 V DC (Logic Out) | 2, 4 | 0 V GND |
| 5 | −5 V DC | 6 | −5 V DC |
| 7 | /IRQ (Interrupt Request) | 8 | /NMI (Non-Maskable Int) |
| 9 | /RST (System Reset) | 10 | R/W (Read / Write) |
| 11 | RDY (6502 Ready / Wait) | 12 | Φ0 (1/2 MHz CPU Clock) |
| 13 | 16 MHz Master Clock | 14 | SOUND OUT (Audio Bus) |
| 15 | 19 V AC (Raw PSU Tap) | 16 | 19 V AC Return |
| 17 | NC (Polarising Keyway) | 18 | NC (Polarising Keyway) |
| 19 – 33 (Odd) | D7 – D0 (Data Bus) | 20 – 34 (Even) | A0, A1, A12–A15 |
| 35 – 47 (Odd) | A11 – A2 (Address Bus) | 36 – 46 (Even) | A10 – A3 |
| 49 | 0 V GND | 48, 50 | +5 V / GND |
Plus 1 Sideways ROM Cartridge Slots: The Acorn Plus 1 adds two BBC-compatible 16 KB Sideways ROM slots (Banks 12 & 13 / 0 & 1) plus an ADC0804 analog joystick converter (`&FC70–&FC72`) and Centronics printer port (`&FC71`).
Chip & EPROM Reference
Interactive IC Pinout Finder deep-dives plus Acorn Atom & Electron ROM / EPROM upgrade notes.
MOS 6502 / 6502A
Atom `IC22` (1 MHz) & Electron `IC2` (2 MHz) — 8-bit microprocessor driving both systems.
View Pinout →Intel 8255A PPI
Atom `IC25` (`$B000`) — Programmable Peripheral Interface scanning the Atom keyboard, VDG modes, speaker, and tape I/O.
View Pinout →MOS 6522 VIA
Atom `IC1` (`$B800`) — Versatile Interface Adapter driving the Atom's parallel printer port and hardware timers.
View Pinout →2114 SRAM (1K×4)
Atom RAM array — Static RAM arranged in Low/High nibble pairs across Zero-Page, Video RAM, and Text RAM.
View Pinout →4164 DRAM (×4)
Electron `IC4–IC7` — Single-rail +5 V 64K×1 DRAMs accessed in double-rate 4-bit nibbles by the Ferranti ULA.
View Pinout →Electron 32KB OS + BASIC ROM (`IC3`) EPROM Replacement
On the Acorn Electron, `IC3` holds both the 16 KB OS (`$C000–$FFFF`) and 16 KB BASIC II (`$8000–$BFFF`) inside a single 28-pin mask ROM (`C30016` / `0102,000-01`). Because the ULA uses Pin 20 (`/CS`) and Pin 27 (`A14`) with standard JEDEC logic:
| EPROM Type | Size | Drop-in & Programming Notes |
|---|---|---|
| 27C256 (32 KB, 28-Pin) | 32 KB | 100% Direct Drop-In! Concatenate the 16 KB BASIC II ROM (`$0000–$3FFF` in EPROM, selected when `A14=0`) followed by the 16 KB Electron MOS 1.00 ROM (`$4000–$7FFF` in EPROM, selected when `A14=1`). |
Failed Electron Mask ROM Symptom: The Hitachi/NEC mask ROM at `IC3` is actually the second most common failure in the Acorn Electron (right after the ULA socket)! If cleaning the ULA socket doesn't cure a dead Electron, burn a 27C256 EPROM with BASIC II + MOS 1.00 and test it in `IC3`!
Acorn Atom 24-Pin ROM Sockets (`IC20, IC21, IC24`)
The Acorn Atom uses 24-pin 2332 (4 KB) / 2364 (8 KB) mask ROM sockets with active-high/low chip select links:
| Atom Socket | Address Range | Original Contents & EPROM Substitute |
|---|---|---|
| IC20 (24-Pin) | $C000–$CFFF & $F000–$FFFF |
8 KB Atom BASIC + OS Kernel (`2364` mask ROM). Replace via a 2364-to-2764 adapter or direct 24-pin `MCM68764`/`68766`. |
| IC21 (24-Pin) | $D000–$DFFF |
4 KB Floating Point & Colour extension ROM. Accepts a standard 2532 (24-pin 4 KB EPROM) directly! |
| IC24 (24-Pin) | $A000–$AFFF |
4 KB Utility / Acorn DOS / AtoMMC2 firmware ROM. Accepts a 2532 EPROM (or 2732 with Pin 18/21 adapter). |
All-in-One Atom RAM/ROM Piggyback Boards: Modern plug-in boards (like the gradec / Hoglet Atom 32K RAM + Flash ROM board) plug directly into the 6502 CPU socket (`IC22`), providing a full 32 KB of cool CMOS static RAM and switchable ROM banks while letting you remove all the hot 2114 chips!
Resources & Further Reading
Blog posts, interactive bench tools, and essential Acorn Atom & Electron communities.
Smiggy's Acorn Posts
Bench Tools & Machine Hubs
- IC Pinout Finder — 6502, 8255, 6522, 2114, 4164 & more
- Recap Planner — capacitor replacement guide
- Ohm's Law Calculator
- BBC Micro Hub — Model B, B+ & Master 128 reference
- C64 Machine Hub — Commodore 64 reference
- C128 Machine Hub — C128, C128D & DCR reference
- SX-64 Machine Hub — Executive Portable C64 reference
- Amiga Machine Hub — A500/A600/A1200 reference
- Atari ST Hub — 520ST, 1040STFM & STE reference
- Apple IIe Hub — Unenhanced, Enhanced & Platinum reference
- ZX Spectrum Hub — 16K/48K & +2/+3 reference
- Sinclair QL Hub — 68008, Microdrive & ULA reference
- Dragon 32/64 Hub — Dragon 32 & 64 reference
- Amstrad CPC 464 Hub — CPC 464 & Datacorder reference
- Osborne 1 & Kaypro II Hub — CP/M luggable reference
- Oric-1 & Atmos Hub — Oric 6502A & ULA reference
- Amstrad CPC 6128 Hub — 128K & 3″ disc reference
- MSX Machine Hub — MSX1, MSX2, MSX2+ & Turbo R reference
- Big-Box Amiga Hub — A1000, A2000, A3000 & A4000 reference
- Atari 8-Bit Hub — 400/800, 800XL & 130XE reference
- VIC-20 Machine Hub — Commodore VIC-20 reference
External Acorn Atom & Electron Links
- StarDot Forums — the premier Acorn Atom, Electron, and BBC Micro engineering community
- Acorn Electron World — software archive, Plus 1/Plus 3 hardware docs, and reviews
- AcornAtom.nl — Dutch & international Acorn Atom schematics, ROMs, and AtoMMC guides
- Chris's Acorns — complete museum archive of every Acorn board revision and expansion