Acorn A4 Capacitor Guide
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The Acorn A4 main board has 23 surface-mount aluminium electrolytic capacitors and one surface-mount tantalum. The same positions and values appear in Acorn's parts lists for both the 2 MB and the 4 MB main boards.[1] These parts leak. Ian Jeffray calls the A4 "the only machine where a full blind re-cap is a recommended course of action", and owners have traced freezes, failure to start and a dead battery management circuit to electrolyte on the board.[2][3][4]
Main board
[edit | edit source]| Value | Voltage | Type | Acorn part | Designators | Qty |
|---|---|---|---|---|---|
| 47 µF | 16 V | Aluminium, surface-mount | 0647,002 | C13, C38, C55, C57, C62, C66, C78, C89, C138 | 9 |
| 10 µF | 16 V | Aluminium, surface-mount | 0647,001 | C19, C65, C67, C93, C108, C133, C135, C137 | 8 |
| 4.7 µF | 25 V | Aluminium, surface-mount | 0647,000 | C107, C134, C136 | 3 |
| 100 µF | 6.3 V | Aluminium, surface-mount | 0647,003 | C61, C97 | 2 |
| 220 µF | 4 V | Aluminium, surface-mount | 0647,004 | C102 | 1 |
| 1 µF | 16 V | Tantalum, surface-mount, 10 % | 0681,102 | C125 | 1 |
Every other main board capacitor is a multilayer ceramic: 33 nF, 47 nF and 100 nF decouplers and small values from 15 pF to 22 nF. C75 is listed as a 150 nF ceramic in a 1210 case, although its part number (0681,050) is in the series Acorn uses for tantalums elsewhere in the list.[1] On the circuit diagram C135 is on the BMU supply, and Philip Pemberton uses it as a test point for the BMU's 5 V.[5][4]
Other lists
[edit | edit source]Adam Wilson's published recap list for the A4 has 18 parts (eight 10 µF, seven 47 µF, two 100 µF and one 220 µF) and leaves out the three 4.7 µF 25 V positions and two of the 47 µF ones, yet his write-up describes losing the pads of C134, which Acorn lists as a 4.7 µF part.[6] Use Acorn's list above.
Econet module
[edit | edit source]The optional A4 Econet module (ALA66) has three surface-mount aluminium electrolytics, C4, C15 and C16, all 47 µF 16 V (0647,002), and two surface-mount tantalums, C5 47 µF (0681,100, voltage not given) and C6 10 µF 16 V (0681,101).[1]
Boards without a parts list
[edit | edit source]Acorn's manual lists the DC/DC converter board (0190,083) and the backlight control board (0190,128) only as assemblies, with no components.[1] The external power adaptor is not documented either. Philip Pemberton repaired adaptors by replacing a 1 µF 50 V electrolytic near the mains inlet with a 105 °C low-ESR part, and suggests replacing all of the adaptor's electrolytics while it is open.[4]
Procedure
[edit | edit source]- Remove the main board. Every other part of the base has to come out first; follow Acorn A4 Maintenance Guide#Opening the A4.
- Photograph the board and record the value and polarity of each part before removing it.
- Remove the old capacitors with hot air, two irons or hot tweezers. Do not twist them off: corroded pads on this board lift easily.[6][3]
- Clean off the electrolyte and corrosion, neutralise, rinse with isopropyl alcohol and dry. Look under nearby parts as well: Philip Pemberton found leakage under the crystal X6 that stopped the BMU, and had to remove X6 to clean beneath it.[4]
- Check the tracks and vias around each position with a meter and repair any break.
- Fit new parts of the same capacitance and the same or higher voltage, with the polarity matching the board marking. Where pads have gone, Adam Wilson fitted the part to the vias on the other side of the board and secured it with epoxy.[6]
- After reassembly, check that the battery gauge works with the adaptor connected and that the machine starts.
Adam Wilson replaced the electrolytics with solid tantalum parts, which do not leak.[6] Aluminium replacements of the original values are also used. Whichever you choose, keep at least Acorn's voltage rating.
See Capacitor Failure Symptoms and Battery Explosion, Capacitor or Corrosion Damage.
Related pages
[edit | edit source]- Acorn A4
- Acorn A4 Maintenance Guide
- Acorn A4 Troubleshooting Guide
- Acorn A4 Technical Reference Manual
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 Acorn Computers, Acorn A4 Technical Reference Manual, part no. 0490,058, issue 1, February 1993. Hosted on this wiki: Acorn A4 Technical Reference Manual. 2 MB main PCB parts list pp. 2-1 to 2-5 and 4 MB main PCB parts list pp. 2-6 to 2-10 (polarised capacitors on pp. 2-1, 2-2, 2-6, 2-7); Econet PCB parts list p. 2-11. Acorn part number series: 0647 surface-mount aluminium electrolytic ("ALEC SMD"), 0681 surface-mount tantalum.
- ↑ "Several A4 repairs", stardot.org.uk forum thread, November 2022. IanJeffray on recapping never-serviced A4s.
- ↑ 3.0 3.1 "A4 Laptop No Display", stardot.org.uk forum thread, December 2016 to April 2017. vectorlight found most capacitors leaked and some pads lifted during replacement; RobC cured persistent freezes by recapping and cleaning the board.
- ↑ 4.0 4.1 4.2 4.3 Markettos, Theo. "Acorn A4 laptop faults", Acorn A4 pages, 2019 (retrieved 2026-10-01). Philip Pemberton: a dead A4 caused by capacitor leakage under the BMU crystal X6, cured by removing all the tin-can electrolytics, cleaning, repairing tracks and fitting new capacitors; the power adaptor's 1 µF 50 V electrolytic.
- ↑ Acorn Computers, "Perth Main PCB" circuit diagram 0190,000/C, sheet 2 of 2: battery management (IC33, X6 4.1943 MHz, C135 10 µF on the BMU supply from Q2 LP2950), PL5 to PL7. Hosted on this wiki: File:Acorn A4 Technical Reference Manual drawings.pdf, page 2.
- ↑ 6.0 6.1 6.2 6.3 Wilson, Adam. "1992 Acorn A4 Laptop Repair, Restoration, & Modifications", Adam's Vintage Computer Restorations, September 2024 (retrieved 2026-10-01). "Eighteen SMD electrolytic capacitors"; replacement with solid tantalum parts; C134 and both its pads came off, so the replacement was fitted to the vias on the underside and secured with epoxy.