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Acorn Archimedes A4000 Capacitor Guide

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Acorn Archimedes A4000 motherboard
Acorn A4000 main PCB, part 0194,600. The backup cell is the barrel at the top right.

Acorn's A3010/A3020/A4000 Technical Reference Manual includes a full parts list for the Acorn Archimedes A4000 main PCB (issue 2), giving the value, type and Acorn part number of every capacitor position. The power supply is a separate cased unit.[1]

Safety

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The A4000's power supply is a separate cased, primary switched-mode unit.[1] A supply of this kind rectifies the mains directly, so its input capacitor charges to the mains peak voltage and can stay charged after the plug is pulled. Disconnect the mains lead before opening the case. Before working inside the supply, discharge the input capacitor through a suitable resistor and confirm 0 V with a meter.

Acorn's service manual treats the A4000 supply as a service-replacement-only item and requires earth-continuity and DC insulation tests whenever a supply is fitted.[2] Repeat those tests after recapping the supply.

Main board capacitor list

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The parts list numbers resistors, capacitors and ICs in separate sequences. Most capacitor positions are surface-mount ceramics: 33 nF and 100 nF decouplers and small 12 pF to 47 nF parts. The polarised capacitors are:[1]

A4000 main PCB polarised capacitors (parts list issue 2, pp. 2-1 and 2-2)
Designators Value Type Acorn part
C1, C82, C84 10 µF 16 V Radial aluminium electrolytic 0635,100
C2, C14, C16, C73, C157 47 µF 16 V Radial aluminium electrolytic 0635,470
C74 4.7 µF 16 V Radial aluminium electrolytic 0635,047
C49, C110, C118, C161 220 µF 16 V Radial aluminium electrolytic 0635,230
C60, C68 100 µF 25 V Radial aluminium electrolytic 0642,103
C159, C164, C165 47 µF 10 V Axial aluminium electrolytic 0621,470
C62, C78, C80 47 µF 16 V Surface-mount aluminium electrolytic 0647,002
C88, C90, C93 10 µF 16 V Surface-mount aluminium electrolytic 0647,001
C83, C120, C131, C139 10 µF 10 V Tantalum, leaded 0610,010
C105 10 µF 16 V Surface-mount tantalum 0681,101
C106 47 µF Surface-mount tantalum 0681,100

That makes 18 leaded and 6 surface-mount aluminium electrolytics on the main board. Check around the six surface-mount parts for electrolyte residue and corroded pads. On Stardot, Ian Jeffray named only the early A5000 and the A4 as Acorn machines whose capacitors should be replaced as a matter of course, so replace a part on this board when it has failed or leaked.[3] An owner who replaced the surface-mount electrolytics in the audio circuit to cure speaker crackling on an A4000 traced the fault to broken pins on the plug-in audio hybrid (IM2).[4]

Power supply

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Neither Acorn manual hosted here gives a parts list or circuit diagram for the A4000 supply. Its outputs reach the main board on the four-way plug PL26: +5 V, 0 V, −12 V and +12 V.[1][2] Read the value, voltage and polarity of each capacitor off the part before removing it, and fit 105 °C parts of the same capacitance and the same or higher voltage. Replace any cracked mains suppression capacitor with an X- or Y-rated safety part of the same class and rating.

Replacement procedure

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  1. Disconnect the mains lead. Remove the two cover screws and slide the cover off.[5]
  2. Remove the drive tray (one push-in rivet between the floppy and hard drives), unplug the power cable at PL26, the LED cable at LK16 and the speaker lead at LK19, remove the screw holding the PSU to the case, slide the PSU forward slightly, lift the rear panel off its three locating tabs and unclip the front of the board.[2]
  3. Note the polarity marking of each capacitor before removing it.
  4. For leaded parts, add fresh solder, lift each lead in turn and clear the holes with solder wick or a desoldering tool. For the surface-mount electrolytics, heat both pads and lift the part without twisting it off the pads, then clean off any residue with isopropyl alcohol.
  5. Fit the new parts with the correct polarity.
  6. Reassemble, making sure the power connector to the main board is fitted correctly.[2]

Checks after recapping

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  1. Measure +5 V and +12 V at PL26 with the machine running.
  2. Run Acorn's functional tests from the ARM250 dealer test disc (Acorn part 0294,825), in particular the audio, video, memory and hard disc tests.[2]
  3. Repeat the earth continuity and DC insulation tests if the supply has been opened.[2]

If a function that worked before the recap fails afterwards, check the polarity of every replaced part first.

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References

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  1. ↑ 1.0 1.1 1.2 1.3 Acorn Computers Limited, Acorn A3010/A3020/A4000 Technical Reference Manual, issue 1, January 1993: power supply p. 1-18; plugs p. 1-20; A4000 main PCB assembly parts list (issue 2) pp. 2-1 and 2-2. Hosted on this wiki as Acorn A3010, A3020 and A4000 Technical Reference Manual; the pages of the hosted scan are out of order.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Acorn Computers Limited, Acorn A3010, A3020 and A4000 Module Level Service Manual, part number 0494,059, issue 1, September 1993: PSU failure p. 1-3; disassembly and refitting the A4000 PSU pp. 2-3 to 2-4; Appendices C and D, earth continuity and DC insulation tests; Appendix E, functional tests. Hosted on this wiki: Acorn A3010, A3020 and A4000 Service Manual.
  3. ↑ Stardot forums, "cap list for acorn a3010", July 2024 (reply by IanJeffray, 9 July 2024).
  4. ↑ Stardot forums, "A3010 - very crackly audio problem", replies by steve3000, April 2023.
  5. ↑ Whytehead, Chris. "Acorn A4000", Chris's Acorns, archived by The Centre for Computing History. Two-screw case, board layout and battery position.


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