RetroTechCollection

Acorn Electron Troubleshooting Guide

This guide gives systematic, component-level diagnosis for the Acorn Electron (main PCB part 205,000). The Electron has no numeric POST display; faults are diagnosed by symptom, by probing against the Electron main-PCB circuit diagram (Acorn 105,000-C) reproduced on this page and the Acorn Electron Service Manual, and by substituting socketed parts. IC designators below are those of the Service Manual parts list and the Acorn Electron Maintenance Guide.

Architecture relevant to faults

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  • IC1 — Ferranti ULA (12C021), 68-pin, in a holder on Issue 1–4 boards. Coordinates clocks (divides the 16 MHz input), all RAM accesses (RAS/CAS/WE), video, sound, the cassette serialiser/deserialiser, keyboard scan (KBD0–3), and reset (RST is asserted at power-up and when BREAK is pressed).[1]
  • IC2 — single 32 KB ROM holding MOS 1.0 and BBC BASIC II (28-pin: pin 28 +5 V, pin 14 ground, pin 22 output-enable).
  • IC3 — 6502A CPU (Synertek SY6502A or Rockwell R6502AP). Runs at 2 MHz when accessing ROM, 1 MHz when accessing RAM, and is periodically halted by the ULA during the active display.[1][2]
  • IC4–IC7 — four 4164 DRAMs (64K×1) forming 32 KB. The bus is 4 bits wide, so each byte needs two accesses; IC4 carries bits 0 and 1, and so on.[1][2]
  • IC8 — 74S04, the 16 MHz crystal oscillator.
  • IC13 — LM324, cassette/audio amplifier (needs +5 V and −5 V).
  • IC14, IC15 — 74LS86 XOR gates, used in RGB/CSYNC generation.
 
Acorn Electron main PCB circuit diagram (Acorn drawing 105,000-C). Use it to locate the test points named in this guide. (Source: 4corn Computers / The Centre for Computing History.)

Power-on behaviour

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A healthy Electron, on power-up: the PSU PCB brings up +5 V and −5 V; the 16 MHz oscillator (IC8) runs and the ULA divides it for the 6502; the ULA releases RST; the 6502 runs MOS 1.0 from the ROM; the machine produces a short beep and shows the BASIC banner and > prompt. A continuous tone instead of a short beep is the classic ULA-not-seated symptom (below).[2]

Branch A — dead, or hangs / continuous tone

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No life, freeze, or continuous tone
Symptom Probable cause Action
Completely dead, no beep, no LED No power reaching the board Reflow / replace the DC jack (a very common cracked-joint fault). Verify ~19 V AC at the grey leads and +5 V at the red lead.
Power LED on, but dead/no beep CPU/ROM/clock not running, or ULA unseated Press the ULA's metal cover to reseat (Issue 1–4). Verify 16 MHz at IC8. Reseat ROM (IC2) and CPU (IC3).
Continuous tone at power-on, freeze/hang ULA risen out of its holder (thermal) Press down the ULA cover; if it recurs, remove cover, clean ULA pins and holder, refit.
+5 V low or absent, AC present PSU PCB fault Fault is on the PSU board (bridge, smoothing cap, 7805). If +5 V > 5.25 V or dead, replace/repair the PSU PCB.

The ULA reseating fault is the Electron's signature failure on socketed (Issue 1–4) boards; always try reseating the ULA before deeper diagnosis.[2] Confirm the supply first, though: the cracked DC-jack joint is equally common and trivially fixed.[3]

Branch B — no or wrong video

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Display faults
Symptom Probable cause Action
RGB monitor blank, but composite/UHF works RGB sync not reaching the connector The RGB sync is generated through the 74LS86 gates (IC14/IC15). Probe the sync gate output; a stuck-low output means a failed LS86 — substitute it. Check LK3 is set for CSYNC.
Composite is greyscale only Normal, or LK4 open The composite output is monochrome by design; closing the chrominance link LK4 adds colour at some quality cost.
Garbage / corrupt display, may still beep RAM fault Substitute the 4164 DRAMs (IC4–IC7). A single bad device corrupts two bits of every byte.
No video at all, machine seems alive Video clock/ULA Verify 16 MHz at IC8; the ULA generates all video, so a dead/under-seated ULA gives no picture.

The 74LS86 RGB-sync failure is a documented, real-world Electron fault: a machine that beeps and works on composite but shows nothing over RGB, traced to a stuck-low sync output on a 74LS86 (IC14/IC15) that is cured by replacing the gate.[3]

Branch C — RAM, ROM and CPU

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RAM (4164)

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The four 4164 DRAMs (IC4–IC7) hold all 32 KB. Because the memory is 4 bits wide, each chip provides two bits of every byte, so a single failed device produces a regular, repeating corruption (and frequently crashes) rather than random noise.[1][2]

  1. If the machine boots but is unstable or shows corrupt text, substitute the 4164s one at a time with known-good devices.
  2. If it does not boot at all but the ULA and clocks are good, a RAM fault can prevent the OS from initialising — substitute the DRAMs.
  3. The Electron has no parity, so RAM errors are not reported; they appear as corruption or crashes.

ROM (IC2)

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A single 32 KB ROM holds MOS 1.0 and BBC BASIC II. A boot that hangs after the beep, or a corrupt banner, points to the ROM or its socket — reseat IC2 first (verify +5 V on pin 28, ground on pin 14, and that output-enable on pin 22 is active).[1]

CPU (IC3)

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The 6502A must tolerate having its clock stopped by the ULA during the active display; this is why Acorn specified the Synertek SY6502A (the Rockwell R6502AP is also found). A substitute CPU must be a type that allows the clock to be halted.[2] If the ULA, clock, ROM and RAM all check out and the machine is still dead, reseat/substitute IC3.

Branch D — cassette, sound and keyboard

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Cassette (1200 baud)

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The cassette read/write circuit is built around the LM324 (IC13), which needs both +5 V (pin 4) and −5 V (pin 11).[1]

  1. Press BREAK and measure the quiescent voltage of the three amplifier outputs (IC13 pins 7, 8, 14): each should be no more than ~100 mV. A pin sitting high indicates a failed LM324 — replace it.[1]
  2. If those are good, load a known-good tape and confirm activity; the ULA CAS IN/CAS OUT lines carry the 1.8 V p-p cassette tone.
  3. Verify the −5 V rail — a dead −5 V supply takes cassette I/O down with it.

Sound

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Sound is a single ULA channel (with software noise emulation) to the internal speaker. No sound with a working display points to the speaker connection or the ULA sound output; reseat the ULA and check the speaker lead.

Keyboard

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The 56-key keyboard is scanned by the ULA over KBD0–3 and the address bus.[1]

  1. Machine won't boot at all: confirm the keyboard ribbon is connected — the Electron will not run without it.[1]
  2. Whole keyboard dead, display OK: reseat the ribbon both ends; check the KBD lines into the ULA.
  3. Individual unreliable keys: contact cleaner into the switch, worked up and down; a dead switch is replaced from a donor.[3]

Expansion port

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The rear edge connector exposes the full address and data bus plus control signals, and also carries the ~18 V AC feed to expansions.[1] If a machine misbehaves only with the Plus 1, Plus 3 or a third-party interface fitted, clean the edge fingers, check for bridged contacts, and confirm the expansion is getting its AC/DC feed.

No-display probing procedure

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  1. Confirm ~19 V AC at the grey leads and +5 V (4.75–5.25 V) at the red lead. Out of range → PSU branch.
  2. Reflow the DC jack if power is intermittent.
  3. On Issue 1–4, press the ULA cover to reseat; retry.
  4. Scope 16 MHz at IC8. Absent → oscillator dead, no clocks, no picture.
  5. Reseat ROM (IC2) and CPU (IC3); substitute the 4164s (IC4–IC7).
  6. If composite works but RGB does not, probe the 74LS86 sync gates (IC14/IC15).

Common fault catalogue

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  • Continuous tone / hang at power-on — ULA risen out of holder (Issue 1–4); press the cover, then clean/reseat.
  • Dead / intermittent power — cracked DC-jack joints; reflow or replace the jack.
  • +5 V high or dead — PSU PCB; repair or replace it.
  • RGB blank, composite fine — 74LS86 sync gate (IC14/IC15); substitute.
  • Corrupt display / crashes — a 4164 DRAM (IC4–IC7); substitute to find it.
  • Hang after the beep — ROM socket (reseat IC2) or RAM.
  • No cassette I/O — LM324 (IC13) or the −5 V rail; check pins 7/8/14 quiescent and pin 11.
  • Won't boot, keyboard suspected — reconnect the keyboard ribbon; the machine refuses to run without it.
  • Misbehaves only with expansion fitted — dirty edge connector or missing AC feed.
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References

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  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 Acorn Electron Service Manual (Part no. 0405001, Issue 2, January 1987), Acorn Computers — hosted on this wiki. Source for the IC complement (IC1 ULA 12C021, IC2 32 KB OS/BASIC ROM, IC3 6502A, IC4–7 4164 DRAM, IC8 74S04 16 MHz oscillator, IC13 LM324 cassette amp, IC14/IC15 74LS86), the ULA pin functions (CAS OUT 1.8 V p-p, RST on power-up/BREAK), the supply rails and tolerances, and the diagnostic procedures used below.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 "Acorn Electron", Wikipedia. Source for the SY6502A clock-stopping requirement, the 2 MHz/1 MHz/halted clocking, the 4-bit-wide RAM (two accesses per byte), the socketed-ULA thermal fault and continuous-tone symptom, and the design-team ROM credit text.
  3. ↑ 3.0 3.1 3.2 Wilson, Adam. "Acorn Electron Repair & Restoration", Adam's Vintage Computer Restorations (2021). Documents the cracked DC-jack fault, the internal bridge/7805 PSU, DC-power substitution, and a worked RGB-sync fault traced to a 74LS86 gate (RGB blank while composite worked).


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