Acorn Archimedes A4000 Capacitor Guide - Revision history

Josh: Remove emoji from section headings; de-duplicate ref definitions that produced visible cite errors; merge the duplicated References section; targeted corrections noted in the drive report

2026-09-27T09:50:41Z

Remove emoji from section headings; de-duplicate ref definitions that produced visible cite errors; merge the duplicated References section; targeted corrections noted in the drive report

← Older revision		Revision as of 10:50, 27 September 2026
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	This guide covers the capacitors on the '''[[Acorn Archimedes A4000]]''' (ARM250 "Roadrunner" board, 0194,600) and its separate cased PSU. Unlike the [[Acorn Archimedes A3000]], Acorn documented the A4000 only at module level, so there is no machine-readable per-component parts list; the exact per-designator values are on the [[Acorn A3010, A3020 and A4000 Technical Reference Manual]] circuit diagrams, which are hosted on this wiki.		This guide covers the capacitors on the '''[[Acorn Archimedes A4000]]''' (ARM250 "Roadrunner" board, 0194,600) and its separate cased PSU. Unlike the [[Acorn Archimedes A3000]], Acorn documented the A4000 only at module level, so there is no machine-readable per-component parts list; the exact per-designator values are on the [[Acorn A3010, A3020 and A4000 Technical Reference Manual]] circuit diagrams, which are hosted on this wiki.

	== ⚠️ Safety: mains PSU — risk of fatal shock ==		== Safety: mains PSU — risk of fatal shock ==

	The A4000's power supply runs directly from the mains. Its primary side, the reservoir (bulk) capacitor and the mains-suppression capacitors '''can hold a lethal charge even after the machine is unplugged''', and a shock from the primary side '''can kill you'''. '''Always disconnect the mains lead before opening the case,''' and never work on the supply while it is plugged in.		The A4000's power supply runs directly from the mains. Its primary side, the reservoir (bulk) capacitor and the mains-suppression capacitors '''can hold a lethal charge even after the machine is unplugged''', and a shock from the primary side '''can kill you'''. '''Always disconnect the mains lead before opening the case,''' and never work on the supply while it is plugged in.

Josh: PSU: permit safe servicing/recap of the mains PSU for restoration; strengthen lethal-shock warnings and discharge procedure (replaces exchange-only/do-not-open wording)

2026-07-19T08:56:17Z

PSU: permit safe servicing/recap of the mains PSU for restoration; strengthen lethal-shock warnings and discharge procedure (replaces exchange-only/do-not-open wording)

← Older revision		Revision as of 09:56, 19 July 2026
Line 4:		Line 4:
	This guide covers the capacitors on the '''[[Acorn Archimedes A4000]]''' (ARM250 "Roadrunner" board, 0194,600) and its separate cased PSU. Unlike the [[Acorn Archimedes A3000]], Acorn documented the A4000 only at module level, so there is no machine-readable per-component parts list; the exact per-designator values are on the [[Acorn A3010, A3020 and A4000 Technical Reference Manual]] circuit diagrams, which are hosted on this wiki.		This guide covers the capacitors on the '''[[Acorn Archimedes A4000]]''' (ARM250 "Roadrunner" board, 0194,600) and its separate cased PSU. Unlike the [[Acorn Archimedes A3000]], Acorn documented the A4000 only at module level, so there is no machine-readable per-component parts list; the exact per-designator values are on the [[Acorn A3010, A3020 and A4000 Technical Reference Manual]] circuit diagrams, which are hosted on this wiki.

	== ⚠️ Safety ==		== ⚠️ Safety: mains PSU — risk of fatal shock ==

	The A4000 ~~has a~~ mains-~~powered switch-mode PSU~~. '''~~Disconnect~~ the mains lead before opening the case.''' ~~The Service Manual prohibits opening or repairing~~ the PSU module and ~~requires~~ the earth-continuity and DC-insulation (class 1) safety tests to ~~be re-applied after any internal work~~.<ref name="sm">[[Acorn A3010, A3020 and A4000 Service Manual]], Acorn Computers — hosted on this wiki. Module-level service manual: the PSU ~~is exchanged~~ as ~~a sealed~~ unit ~~(do not open/repair)~~, the safety tests ~~must be re-applied~~, and the ~~supply provides~~ +5 V/+12 V/−5 V.</ref>		The A4000's power supply runs directly from the mains. Its primary side, the reservoir (bulk) capacitor and the mains-suppression capacitors '''can hold a lethal charge even after the machine is unplugged''', and a shock from the primary side '''can kill you'''. '''Always disconnect the mains lead before opening the case,''' and never work on the supply while it is plugged in.

			Recapping the PSU is a legitimate part of restoring these machines. Acorn's original service manual told dealers to exchange the whole PSU module rather than repair it, but that advice assumed replacement modules were on the shelf; none have been made for decades, so for restoration the supply is '''recapped, not discarded'''. Only attempt PSU work if you are competent with mains switch-mode supplies. Before touching the primary side, '''discharge the reservoir capacitor through a suitable bleeder resistor and confirm 0 V with a meter''', and treat the X/Y mains-suppression capacitors as charged until proven otherwise. After any internal work, re-apply the earth-continuity and DC-insulation (class 1) safety tests before returning the machine to service.<ref name="sm">[[Acorn A3010, A3020 and A4000 Service Manual]], Acorn Computers — hosted on this wiki. Module-level service manual: it treats the PSU as an exchange unit, specifies the earth-continuity and DC-insulation safety tests, and gives the +5 V/+12 V/−5 V rails. The exchange-only guidance dates from when spare PSU modules were stocked.</ref>

	== Is the A4000 a recap candidate? ==		== Is the A4000 a recap candidate? ==
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	== PSU module ==		== PSU module ==

	The A4000 uses a '''separate cased switch-mode PSU''' supplying +5 V, +12 V and −5 V, with an internal fuse FS1 and thermal-shutdown protection.<ref name="sm" /> Acorn ~~ships it as~~ a ~~sealed~~, ~~non-serviceable unit. If~~ the supply ~~is unstable~~ (random resets, ripple, thermal cut-outs):		The A4000 uses a '''separate cased switch-mode PSU''' supplying +5 V, +12 V and −5 V, with an internal fuse FS1 and thermal-shutdown protection.<ref name="sm" /> Acorn originally handled a faulty supply by exchanging the whole module, but with no replacement modules made for decades the practical fix for an unstable supply (random resets, ripple, thermal cut-outs) is to recap it. Observe the mains-shock warning at the top of this page: the supply is dangerous until discharged.

	* The supported repair is '''exchange''' of the ~~PSU module~~.		# Unplug the mains lead.
	* If you are competent with mains switch-mode supplies and ~~choose to recap it: unplug from the mains,~~ '''discharge the ~~primary~~ bulk capacitor''' before touching the primary side~~, then replace~~ the smoothing electrolytics and any cracked mains-suppression (RIFA-style) capacitor with ~~correctly~~-rated ~~parts, reading each value off~~ the ~~fitted part~~. Re-apply the earth-continuity and insulation safety tests ~~afterward~~.		# Open the PSU case and '''discharge the reservoir (bulk) capacitor''' through a bleeder resistor; confirm 0 V with a meter before touching the primary side.
			# Replace the smoothing electrolytics, reading the value, voltage and polarity off each fitted part; use 105 °C low-ESR parts of equal capacitance and equal-or-higher voltage.
			# Replace any cracked or leaking mains-suppression (RIFA-style) capacitor with a '''safety-rated X/Y-class''' part of the same rating — never an ordinary capacitor.
			# Reassemble and re-apply the earth-continuity and DC-insulation (class 1) safety tests before returning the machine to service.

	== Replacement procedure (board electrolytics) ==		== Replacement procedure (board electrolytics) ==

Josh: Create: capacitor guide (leaded vs SMD distinction, PSU recap; values via on-wiki TRM)

2026-06-17T20:07:12Z

Create: capacitor guide (leaded vs SMD distinction, PSU recap; values via on-wiki TRM)

New page

<templatestyles src="Template:StyledTable/styles.css" />
[[File:Acorn Archimedes A4000 motherboard.jpg|thumb|360x360px|Acorn A4000 "Roadrunner" main PCB. The logic is surface-mount, but the electrolytics are conventional leaded parts; the A4000 is not affected by the SMD electrolyte leakage of the A5000/A4/Risc PC.]]

This guide covers the capacitors on the '''[[Acorn Archimedes A4000]]''' (ARM250 "Roadrunner" board, 0194,600) and its separate cased PSU. Unlike the [[Acorn Archimedes A3000]], Acorn documented the A4000 only at module level, so there is no machine-readable per-component parts list; the exact per-designator values are on the [[Acorn A3010, A3020 and A4000 Technical Reference Manual]] circuit diagrams, which are hosted on this wiki.

== ⚠️ Safety ==

The A4000 has a mains-powered switch-mode PSU. '''Disconnect the mains lead before opening the case.''' The Service Manual prohibits opening or repairing the PSU module and requires the earth-continuity and DC-insulation (class 1) safety tests to be re-applied after any internal work.<ref name="sm">[[Acorn A3010, A3020 and A4000 Service Manual]], Acorn Computers — hosted on this wiki. Module-level service manual: the PSU is exchanged as a sealed unit (do not open/repair), the safety tests must be re-applied, and the supply provides +5 V/+12 V/−5 V.</ref>

== Is the A4000 a recap candidate? ==

The A4000 board is almost entirely surface-mount logic, but its '''electrolytics are conventional leaded parts plus surface-mount ceramic decouplers''' — it is '''not''' one of the Acorn machines that suffer from leaking surface-mount electrolytic capacitors. Those are the [[Acorn Archimedes A5000]], the A4 laptop and the Risc PC; the through-hole-capacitor machines such as the A3020/A4000 are generally unaffected.<ref name="stardot">[https://stardot.org.uk/forums/ Stardot forums] (Acorn restoration community). The electrolyte-leakage problem is specific to the Acorn machines fitted with SMD electrolytic capacitors (A5000, A4, Risc PC); the through-hole-capacitor machines such as the A3020/A4000 are generally fine.</ref> On the A4000 the recap priorities are therefore the '''PSU module''' and any individually aged or leaking board electrolytic, not a blanket board recap.

== Board capacitors ==

The A4000 main board ("Roadrunner", 0194,600) carries:

* '''Surface-mount ceramic decouplers''' of '''33 nF''' and '''100 nF''' across the supply pins of the ICs — these are stable and are not replaced.<ref name="trm">[[Acorn A3010, A3020 and A4000 Technical Reference Manual]], Acorn Computers — hosted on this wiki. Main-PCB ("Roadrunner") circuit diagrams; the decoupling values (100 nF/33 nF) and the per-designator electrolytic values are shown on the circuit-diagram sheets.</ref>
* A small number of '''leaded aluminium electrolytics''' on the supply-filtering, audio and video sections. These are the recap targets.

Because Acorn published the A4000 only at module level, there is no official tabular parts list giving each electrolytic's designator and value. The exact values are legible on the [[Acorn A3010, A3020 and A4000 Technical Reference Manual]] circuit-diagram sheets (on the wiki), and can be read directly off each fitted capacitor. When recapping, '''record the value, voltage and polarity of each electrolytic before removing it''' and replace like-for-like with a 105 °C low-ESR part of equal capacitance and equal-or-higher voltage. This guide does not reproduce a per-designator table rather than risk publishing unverified values.

== PSU module ==

The A4000 uses a '''separate cased switch-mode PSU''' supplying +5 V, +12 V and −5 V, with an internal fuse FS1 and thermal-shutdown protection.<ref name="sm" /> Acorn ships it as a sealed, non-serviceable unit. If the supply is unstable (random resets, ripple, thermal cut-outs):

* The supported repair is '''exchange''' of the PSU module.
* If you are competent with mains switch-mode supplies and choose to recap it: unplug from the mains, '''discharge the primary bulk capacitor''' before touching the primary side, then replace the smoothing electrolytics and any cracked mains-suppression (RIFA-style) capacitor with correctly-rated parts, reading each value off the fitted part. Re-apply the earth-continuity and insulation safety tests afterward.

== Replacement procedure (board electrolytics) ==

# Disconnect the mains lead; remove the two cover screws and lift out the board.
# Record each electrolytic's value, voltage and polarity (the silkscreen marks the positive pad).
# Wet both leads with fresh solder/flux, lift each end in turn, and clear the holes with solder wick.
# Fit the new part with correct polarity; solder and trim. The A4000 board is multi-layer, so use a temperature-controlled iron and short dwell times to avoid lifting pads.
# Inspect for clean joints and no bridges.

== Post-recap verification ==

# Reassemble; reconnect the mains lead.
# Power on and confirm RISC OS 3.11 reaches the desktop.
# Check the +5 V, +12 V and −5 V rails.
# Run the audio and video functional tests; confirm the hard disc and floppy.
# Re-apply the earth-continuity and DC-insulation safety tests before returning the machine to service.

If a previously-working function fails after the recap, re-check the polarity of every replaced electrolytic first.

== Related pages ==

* [[Acorn Archimedes A4000]]
* [[Acorn Archimedes A4000 Maintenance Guide]]
* [[Acorn Archimedes A4000 Troubleshooting Guide]]
* [[Acorn Archimedes A3000 Capacitor Guide]] — the discrete-chip predecessor (full cap list)
* [[Capacitor Failure Symptoms]]

== References ==

<references />

{{Navbox-AcornComputers|state=collapsed}}

[[Category:Acorn Computers]]
[[Category:Capacitor Replacement Guides]]