Acorn Atom Troubleshooting Guide

This guide gives systematic, component-level diagnosis for the Acorn Atom (1980–1983, main PCB part 202,000). The Atom has no numeric POST code system — faults are diagnosed by symptom, by signal probing against the Atom circuit diagram (Acorn 102,000-C) reproduced on this page, and by substitution of socketed parts. The component designators below (IC numbers, crystals, links) are those used in the Acorn Atom Technical Manual parts list and the Acorn Atom General Maintenance page.

Power-on behaviour

A healthy Atom, on application of 8 V DC at SK3:

1. The on-board regulators IC53/IC54 (LM340T-5) produce +5 V for the two halves of the board.
2. The 4.00 MHz crystal (X2) is divided by the 74LS163/74LS393 counters (IC47/IC44) to the 1 MHz 6502 (IC22) clock; the 3.58 MHz crystal (X1) clocks the MC6847 VDG (IC31).
3. The 6502 fetches from the 8 KB mask ROM (IC20) and runs the operating system and Atom BASIC.
4. The INS8255 PPI (IC25) reads the keyboard matrix (driven by the 7445, IC26); the MC6847 generates the display.
5. The screen shows the BASIC sign-on and the > prompt with a flashing cursor.

If any stage fails, the machine is dark, shows a corrupt display, or hangs without a usable prompt. Work through the branches below.

Acorn Atom microcomputer circuit diagram (Acorn drawing 102,000-C). Use it to locate the test points named in this guide. (Source: 4corn Computers / The Centre for Computing History.)

Branch A — completely dead, no display

No display / no signs of life

Symptom	Probable cause	Action
Nothing at all, no screen activity	No / wrong supply	Verify 8 V at SK3; verify +5 V at IC53/IC54 outputs. A loose or cracked SK3 joint is common — reflow it.
+5 V present, still dead	Master clock dead	Scope X2 (4.00 MHz) and the 1 MHz 6502 clock at IC22. No clock → suspect X2, IC47 (74LS163) or IC44 (74LS393).
+5 V and clocks present, dead	CPU not running	Check 6502 (IC22) RESET and that the address lines are toggling. Reseat IC22.
+5 V good, regulator very hot then resets	Regulator thermal shutdown / over-current	Confirm heatsink fitted; check for a shorted tantalum/electrolytic or a shorted IC pulling the rail down.
Dead only when fully expanded	Supply current exceeded	Adaptor is marginal at ≥1.6 A; fit a higher-current 8 V supply, or convert to 5 V regulated via links LK6/LK7.

If +5 V is absent at the regulator outputs but 8 V is present at SK3, suspect IC53/IC54 themselves or a short on the +5 V rail. Lift one regulator output and look for the rail being clamped by a downstream short (a failed tantalum or IC).

Branch B — garbled or partial display

Display present but wrong

Symptom	Probable cause	Action
Regular pattern of corrupt characters/pixels	A failed RAM chip (2114)	A single bad 2114 corrupts one nibble across its address range. Reseat IC42/IC43/IC51/IC52 (and any expansion RAM); substitute known-good 2114s one at a time.
Sign-on text appears then hangs	ROM or higher RAM fault	Reseat the ROM (IC20). If still hanging, suspect RAM in the program area.
Wavy/rolling/wrong-shade picture	Video timing or MC6847	Verify 3.58 MHz at X1; reseat MC6847 (IC31). On a UK set with no colour card, a monochrome-only picture is normal.
No colour on a European TV	Expected behaviour	The MC6847 is a 60 Hz NTSC generator; UK/European colour needs the Acorn 50 Hz PAL colour card (102,006-C). Monochrome on a normal UK TV is correct for an un-modified Atom.
Snow / instability that worsens when warm	Decoupling or socket fault	Reseat ICs around the disturbed area; inspect the 47 nF decouplers (C14–C28).

The 2114 static RAMs are the most-reported display-corruption cause on the Atom. Because each 2114 is 1K×4, a single failed device affects one nibble of every byte in its block, which usually shows as a regular, repeating corruption rather than random noise.

Branch C — runs, but a subsystem fails

Keyboard dead or specific keys faulty

The keyboard matrix is read by the INS8255 PPI (IC25), with the 7445 (IC26) driving the row lines and decode handled by the 74LS138s (IC23/IC30).

1. Whole keyboard dead, display fine: suspect IC25 (8255). Reseat it; check that its data and control lines from the 6502 bus are present.
2. One row/column dead: trace from the dead keys back to IC25/IC26 for a broken track or a cold keyboard-lead joint. Reflow the through-plated key joints (do not flood them).
3. Single dead key: clean or replace that switch. Switches are soldered to the board.
4. Key chatters / double-types after cleaning: worn switch; the MOS debounce cannot mask a mechanically bad contact.

No sound

The loudspeaker is driven from an 8255 port bit. If BASIC sound commands produce nothing, reseat IC25 and check continuity from the speaker to the board. The speaker is glued to side 1 next to C5.

Cassette will not load or save

The cassette circuit is the LM358 op-amp (IC46) and transistors Q1/Q2 (BC107), connected through the 7-pin DIN (SK2), at 300 baud CUTS only.

1. Confirm the lead and the external recorder: a known-good recorder with correct level and clean heads loads reliably; most "won't load" faults are the tape, the recorder or the lead, not the Atom.
2. Clean SK2.
3. If save/load is dead with a known-good recorder, probe the read/write signals at IC46 (LM358) while playing a known tape; no activity points to IC46 or the 8255 cassette port.
4. Remember the rate: an Atom cannot load 1200 baud BBC tapes.

Expansion bus problems

• Machine fails only with an expansion card fitted: clean the 64-way edge fingers (PL8) and the card; look for bridged or oxidised contacts. A card drawing current can also push a marginal 8 V adaptor into collapse — retest on a higher-current supply.
• Printer/VIA port dead: the 6522 VIA and printer interface are optional fits; confirm the VIA is installed and seated before diagnosing the port.

No-display probing procedure

When the machine is dark with the adaptor connected:

1. Verify 8 V at SK3, then +5 V at the outputs of IC53 and IC54. Out of range → supply/regulator branch above.
2. Scope X2 (4.00 MHz). Absent → clock oscillator dead.
3. Scope the 1 MHz 6502 clock at IC22. Absent → IC47/IC44 divider chain or its supply.
4. Scope X1 (3.58 MHz) at the MC6847. Absent → video clock dead (display impossible).
5. With clocks present, check the 6502 address lines are toggling; if static, reseat or substitute IC22.
6. With the CPU running but no/garbled display, reseat the ROM (IC20) and the RAM (2114s), then the MC6847 (IC31).

Common fault catalogue

• Dead, +5 V present — clock chain (X2 / IC47 / IC44) or CPU; reseat IC22.
• Intermittent resets when warm — 7805 regulator (IC53/IC54) thermal shutdown; confirm heatsink, check for a rail short.
• Random crashes on an expanded machine — under-rated 8 V adaptor; fit a bigger supply or convert to 5 V (LK6/LK7).
• Repeating corrupt characters — a failed 2114; substitute to find it.
• Hang after sign-on — ROM socket dirty (reseat IC20) or program-area RAM bad.
• Monochrome only on a UK TV — normal; PAL colour needs the 102,006-C colour card.
• Keyboard dead, display fine — 8255 (IC25); reseat.
• Won't load tapes — recorder/tape/lead first; then IC46 (LM358); confirm 300 baud, not a 1200 baud BBC tape.
• Dies if jostled — cracked joint, usually SK3 or an edge connector; reflow.

Related pages

• Acorn Atom
• Acorn Atom General Maintenance
• Acorn Atom Capacitor Replacement Guide
• Capacitor Failure Symptoms
• Acorn Electron Troubleshooting Guide