RetroTechCollection

Acorn Electron Capacitor Replacement Guide

This guide lists the capacitors on the Acorn Electron main PCB (part 205,000) and covers the internal PSU PCB. The main-board values, designators and types are taken directly from the Acorn Electron Service Manual (Part no. 0405001, Issue 2, January 1987) parts list.[1]

Acorn Electron main PCB. The main-board electrolytics and tantalums are few; the larger smoothing electrolytics are on the separate PSU PCB (top right).

Why (and why not) recap an Electron

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Like the Acorn Atom, the Electron system unit has no internal mains and no CRT — the mains transformer is in the external adaptor, and the internal PSU PCB only ever sees the low-voltage ~19 V AC feed. There is therefore no lethal charge to discharge. The main board carries only a handful of electrolytics and tantalums; the larger smoothing electrolytics are on the PSU PCB, where aged caps cause ripple on the +5 V rail and the instability that follows. Recap the PSU board if the +5 V shows ripple or the machine resets randomly; recap the main board opportunistically while it is open or to chase a specific audio/video fault.

Visual inspection

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Inspect each electrolytic for a bulged/vented top, electrolyte residue on the board, a cracked sleeve, or a tired joint. On the Electron, weigh capacitor work against the two far more common faults — the unseated ULA and the cracked DC-jack joint — covered in the Acorn Electron Troubleshooting Guide.

Acorn Electron main PCB capacitor list

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These are the complete main-board values from the Service Manual parts list. The electrolytics and tantalums are the recap-relevant parts; the ceramics are stable and are normally left in place.

Acorn Electron main PCB — electrolytic and tantalum capacitors (recap targets)
Designator Value Voltage Type Acorn part #
C1 47 µF 16 V Electrolytic 622,470
C2 10 µF 16 V Electrolytic, axial 622,100
C7 10 µF 35 V Electrolytic, axial 624,100
C11 10 µF 35 V Electrolytic, axial 624,100
C25 33 µF 16 V Electrolytic, axial 622,330
C8 1 µF 35 V Tantalum 613,100
C9 1 µF 35 V Tantalum 613,100
C23 47 µF 16 V Tantalum 611,470
Acorn Electron main PCB — ceramic / film capacitors (normally leave in place)
Designator Value Type
C4, C6 33 pF Plate ceramic
C18 39 pF Plate ceramic
C21 47 pF Plate ceramic
C3, C5 100 pF Plate ceramic
C13 150 pF Plate ceramic
C17 470 pF Plate ceramic
C20, C22 820 pF Plate ceramic
C26 1.5 nF (1n5) Plate ceramic
C15 2.2 nF (2n2) Plate ceramic
C14, C16 4.7 nF (4n7) Plate ceramic
C12 33 nF Ceramic multilayer
C10, C19 220 nF Ceramic disc
Decouplers (18 off) 47 nF / 33 nF Per-IC supply decoupling
VC1 5.5–40 pF Variable trimmer (video/UHF)

Internal PSU PCB

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The PSU PCB (part 332,002) takes the ~19 V AC adaptor feed through a full-wave bridge rectifier, smooths it with electrolytic capacitor(s), and regulates +5 V with a 7805, while also generating the −5 V rail.[1][2] The Service Manual treats the PSU PCB as a replaceable module and does not enumerate its individual capacitor values; if you recap it, read the value and voltage from each fitted part and replace like-for-like with a 105 °C low-ESR equivalent (equal or higher voltage). The smoothing electrolytic and the −5 V-side electrolytic are the parts most worth replacing when chasing +5 V ripple.

A practical alternative to chasing a marginal PSU board is to substitute the supply entirely: because the bridge accepts either polarity, the Electron can be run from a DC supply (~18 V DC ideal, 9 V DC for a bare machine) via the 2.1 mm jack.[2]

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  • Electrolytics (C1, C2, C7, C11, C25): 105 °C low-ESR aluminium, equal capacitance, equal or higher voltage. C7 and C11 are 35 V parts; C1, C2 and C25 are 16 V.
  • Tantalums (C8, C9, C23): replace with tantalum or a good-quality electrolytic of equal value and equal/higher voltage. Observe polarity carefully — reverse-fitted tantalums fail short and can overheat.
  • Ceramics and the VC1 trimmer: leave in place unless physically damaged. Disturbing VC1 will require re-adjustment of the video timing.
Main-board recap summary
Value Voltage Type Quantity
1 µF 35 V Tantalum 2 (C8, C9)
10 µF 16 V Electrolytic 1 (C2)
10 µF 35 V Electrolytic 2 (C7, C11)
33 µF 16 V Electrolytic 1 (C25)
47 µF 16 V Electrolytic 1 (C1)
47 µF 16 V Tantalum 1 (C23)

Replacement procedure

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  1. Unplug the adaptor; separate the keyboard half and remove the board(s) from the base.
  2. Mark the polarity of each electrolytic/tantalum before removal (the silkscreen marks the positive end).
  3. Wet both leads with fresh solder/flux, lift each end in turn, and clear the holes with solder wick.
  4. Fit the new part with correct polarity, solder, and trim. Use a temperature-controlled iron and short dwell times to protect the pads.
  5. Inspect for clean joints and no bridges.

Post-recap verification

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  1. Reconnect the keyboard ribbon (the machine will not run without it).
  2. Power up; confirm the normal beep and the BASIC banner.
  3. Measure +5 V (4.75–5.25 V) and confirm ripple is within ~50 mV.[1]
  4. Test a cassette load (exercises the LM324 and the −5 V rail) and the keyboard.

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

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References

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  1. ↑ 1.0 1.1 1.2 Acorn Electron Service Manual (Part no. 0405001, Issue 2, January 1987), Acorn Computers — hosted on this wiki. Source for the complete main-PCB capacitor list (designators, values, types and Acorn part numbers), the PSU description (bridge rectifier, smoothing, 7805, −5 V generation), and the +5 V tolerance (4.75–5.25 V, ≤50 mV noise).
  2. ↑ 2.0 2.1 Wilson, Adam. "Acorn Electron Repair & Restoration", Adam's Vintage Computer Restorations (2021). Describes the Electron internal PSU board (bridge rectifier, smoothing, 7805) and DC-power substitution.


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