Acorn Atom Capacitor Replacement Guide

This guide lists the capacitors on the Acorn Atom main PCB (part 202,000) and gives the replacement procedure. Values, designators and the note on permissible substitutions are taken directly from the Acorn Atom Technical Manual (Issue 2, October 1980) parts list.^[1]

Why (and why not) recap an Atom

The Atom is one of the lower-risk vintage machines to recap. The system unit has no mains wiring and no CRT — it runs on 8 V DC (or 5 V regulated) from an external adaptor — so there is no lethal charge to discharge and no large mains-side reservoir capacitor to fail dangerously. The board carries only eight small aluminium electrolytics (10–47 µF) plus ceramic decouplers. These are now 40-plus years old and are reasonable pre-emptive replacements while a board is open, but the Atom does not have a notorious "must recap or it dies" reputation: do it when a board is already apart, or to chase a specific fault, not reflexively.

Visual inspection

Before replacing anything, inspect each electrolytic for: a bulged or vented top, brown electrolyte crust on the board around the can, a cracked or discoloured sleeve, or a lifted/cracked solder joint. Note that on the Atom the more common age faults are dirty sockets and tired 7805 regulators rather than failed capacitors — see Acorn Atom Troubleshooting Guide.

Acorn Atom capacitor list

The figures below are the base-kit values from the Technical Manual parts list. The eight electrolytics are marked *; the manual states they may be any value in the range 10–47 µF rated 10 V or greater, so exact-value matching is not critical for those positions.

**Acorn Atom main PCB — electrolytic capacitors (recap targets)**
Designator	Value	Min. voltage	Notes
C2	22 µF	10 V	Electrolytic (*) — supply/decoupling
C3	22 µF	10 V	Electrolytic (*)
C4	22 µF	10 V	Electrolytic (*)
C5	22 µF	10 V	Electrolytic (*) — near the loudspeaker connection
C8	22 µF	10 V	Electrolytic (*)
C11	10 µF	10 V	Electrolytic (*)
C12	22 µF	10 V	Electrolytic (*)
C13	22 µF	10 V	Electrolytic (*)

**Acorn Atom main PCB — non-electrolytic capacitors (normally leave in place)**
Designator	Value	Type
C1	10 nF	Ceramic
C6	10 nF	Ceramic
C7	47 or 100 nF	Ceramic
C9	22 or 47 nF	Ceramic
C10	10 nF	Ceramic
C14–C28 (15 off)	47 nF	Ceramic supply decoupling, one per IC group

The fifteen 47 nF decouplers (C14–C28) and the small ceramics (C1, C6, C7, C9, C10) are stable and do not normally fail with age. Replace a ceramic only if it is physically cracked; replacing all of them en masse risks lifting pads on the 40-plus-year-old board for no benefit.

Recommended replacement parts

For the eight electrolytics, use modern 105 °C low-ESR aluminium electrolytics. Equal capacitance is ideal, but per the manual any value 10–47 µF at ≥10 V is acceptable in these positions; a 16 V or 25 V part adds margin at no cost.

**Replacement summary**
Value	Suggested rating	Quantity
22 µF	16–25 V, 105 °C radial	7 (C2, C3, C4, C5, C8, C12, C13)
10 µF	16–25 V, 105 °C radial	1 (C11)

Replacement procedure

Unplug the adaptor; open the case and separate the board/keyboard assembly.
Photograph or mark the polarity of each electrolytic. The Atom silkscreen marks the positive end with a +; the can marks the negative end with a stripe (or arrow). The positive of the new cap goes to the + on the board.^[1]
Add a little fresh flux/solder to both leads from side 1, then heat one pad and lift that side of the can; heat the other and remove it.
Clear both holes with solder wick. The board is double-sided with plated-through holes — do not overheat; limit each heat cycle to a few seconds.
Fit the new cap, matching polarity, solder both leads on side 1 and trim flush.
Inspect for clean fillets and no bridges before reassembly.

The Atom board's plated-through holes are tolerant but not indestructible; a temperature-controlled iron at around 350 °C and short dwell times avoid lifting pads.

Post-recap verification

Power up on a known-good 8 V DC (or 5 V regulated) supply.
Confirm the BASIC sign-on and prompt appear.
Confirm +5 V at the regulator outputs.
Test sound (a BASIC tone), the keyboard, and a known-good cassette load.

If a previously-working function fails after a recap, re-check the polarity of every replaced electrolytic first — reversed polarity is the most common recap error.

Note on the power adaptor

The Atom's bulk smoothing is inside the external 8 V adaptor, not on the board. If you have mains-voltage concerns about a 40-plus-year-old moulded adaptor, replace the whole adaptor with a modern regulated supply rather than opening it; the board is happy on a clean 8 V DC feed, or on 5 V regulated with links LK6/LK7 fitted (see Acorn Atom General Maintenance).

Related pages

References

↑ ^1.0 ^1.1 Acorn Atom Technical Manual (Issue 2, October 1980), Acorn Computers — hosted on this wiki. Capacitor designators C1–C28 and the note: "The electrolytic capacitors (marked *) may be replaced by any value in the range 10 to 47 µF. They should be rated at 10 Volts or greater." Also the regulator (IC53/IC54 LM340T-5) and supply detail.

[atomtm-1] 1.0 ^1.1 Acorn Atom Technical Manual (Issue 2, October 1980), Acorn Computers — hosted on this wiki. Capacitor designators C1–C28 and the note: "The electrolytic capacitors (marked *) may be replaced by any value in the range 10 to 47 µF. They should be rated at 10 Volts or greater." Also the regulator (IC53/IC54 LM340T-5) and supply detail.

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