Editing Acorn Atom Capacitor Replacement Guide

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[[File:Acorn Atom motherboard.jpg|thumb|360x360px|Acorn Atom main PCB (202,000 Issue 4). The electrolytics are the small radial/axial cans among the logic ICs; there is no large reservoir capacitor on the board.]]

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.<ref name="atomtm">[[Acorn Atom Technical Manual]] (Issue 2, October 1980), Acorn Computers &mdash; hosted on this wiki. Capacitor designators C1&ndash;C28 and the note: "The electrolytic capacitors (marked *) may be replaced by any value in the range 10 to 47&nbsp;µF. They should be rated at 10&nbsp;Volts or greater." Also the regulator (IC53/IC54 LM340T-5) and supply detail.</ref>

== 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''' &mdash; it runs on 8&nbsp;V DC (or 5&nbsp;V regulated) from an external adaptor &mdash; 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&ndash;47&nbsp;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 &mdash; 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&ndash;47&nbsp;µF rated 10&nbsp;V or greater''', so exact-value matching is not critical for those positions.

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{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Acorn Atom main PCB &mdash; electrolytic capacitors (recap targets)'''
! Designator !! Value !! Min. voltage !! Notes
|-
| C2 || 22&nbsp;µF || 10&nbsp;V || Electrolytic (*) &mdash; supply/decoupling
|-
| C3 || 22&nbsp;µF || 10&nbsp;V || Electrolytic (*)
|-
| C4 || 22&nbsp;µF || 10&nbsp;V || Electrolytic (*)
|-
| C5 || 22&nbsp;µF || 10&nbsp;V || Electrolytic (*) &mdash; near the loudspeaker connection
|-
| C8 || 22&nbsp;µF || 10&nbsp;V || Electrolytic (*)
|-
| C11 || 10&nbsp;µF || 10&nbsp;V || Electrolytic (*)
|-
| C12 || 22&nbsp;µF || 10&nbsp;V || Electrolytic (*)
|-
| C13 || 22&nbsp;µF || 10&nbsp;V || Electrolytic (*)
|}

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{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Acorn Atom main PCB &mdash; non-electrolytic capacitors (normally leave in place)'''
! Designator !! Value !! Type
|-
| C1 || 10&nbsp;nF || Ceramic
|-
| C6 || 10&nbsp;nF || Ceramic
|-
| C7 || 47 or 100&nbsp;nF || Ceramic
|-
| C9 || 22 or 47&nbsp;nF || Ceramic
|-
| C10 || 10&nbsp;nF || Ceramic
|-
| C14&ndash;C28 (15 off) || 47&nbsp;nF || Ceramic supply decoupling, one per IC group
|}

The fifteen 47&nbsp;nF decouplers (C14&ndash;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&nbsp;°C low-ESR aluminium electrolytics'''. Equal capacitance is ideal, but per the manual any value '''10&ndash;47&nbsp;µF''' at '''&ge;10&nbsp;V''' is acceptable in these positions; a 16&nbsp;V or 25&nbsp;V part adds margin at no cost.

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{| class="wikitable styled-table" style="width:80%; text-align:center;"
|+'''Replacement summary'''
! Value !! Suggested rating !! Quantity
|-
| 22&nbsp;µF || 16&ndash;25&nbsp;V, 105&nbsp;°C radial || 7 (C2, C3, C4, C5, C8, C12, C13)
|-
| 10&nbsp;µF || 16&ndash;25&nbsp;V, 105&nbsp;°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.<ref name="atomtm" />
# 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 &mdash; 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&nbsp;°C and short dwell times avoid lifting pads.

== Post-recap verification ==

# Power up on a known-good 8&nbsp;V DC (or 5&nbsp;V regulated) supply.
# Confirm the BASIC sign-on and prompt appear.
# Confirm +5&nbsp;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 &mdash; reversed polarity is the most common recap error.

== Note on the power adaptor ==

The Atom's bulk smoothing is inside the external 8&nbsp;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&nbsp;V DC feed, or on 5&nbsp;V regulated with links LK6/LK7 fitted (see [[Acorn Atom General Maintenance]]).

== Related pages ==

* [[Acorn Atom]]
* [[Acorn Atom General Maintenance]]
* [[Acorn Atom Troubleshooting Guide]]
* [[Acorn Electron Capacitor Replacement Guide]]
* [[Capacitor Failure Symptoms]]

== References ==

<references />

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