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Acorn Electron Capacitor Replacement Guide

From RetroTechCollection
Acorn Electron opened, with the issue 4 main PCB in the case base and the separate power supply board at the top right

The Acorn Electron has two boards in the case base: the main PCB (205,000) and a small power supply board (332,002).[1] Acorn's parts list gives every capacitor on the main PCB. The Service Manual has a circuit diagram of the power supply board but no parts list for it, so its values come from owners who have recapped one. The adaptor holds the mains transformer; inside the case the highest voltage is the rectified 19 V AC feed on the power supply board.[1]

Main PCB electrolytic and tantalum capacitors

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Values, types and part numbers are from Acorn's main PCB parts list (Service Manual pp. 43–44). The circuit positions are read from Acorn's circuit diagram 105,000/C.[1][2]

Acorn Electron main PCB polarised capacitors
Designator Value Voltage Type Acorn part no. Position on the circuit diagram
C1 47 µF 16 V Electrolytic 622,470 +5 V to 0 V at the supply connector PL3
C2 10 µF 16 V Electrolytic, axial 622,100 −5 V to 0 V at PL3
C7 10 µF 35 V Electrolytic, axial 624,100 Cassette input coupling, ULA pin 59 to the LM324
C8 1 µF 35 V Tantalum 613,100 Cassette high-tone timing on ULA CAS RC (pin 47)
C9 1 µF 35 V Tantalum 613,100 Power-on reset timing on ULA POR (pin 54)
C11 10 µF 35 V Electrolytic, axial 624,100 Loudspeaker coupling
C23 47 µF 16 V Tantalum 611,470 UHF modulator supply, after R65
C25 33 µF 16 V Electrolytic, axial 622,330 Sound amplifier supply, after R66

Other main PCB capacitors

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Non-polarised capacitors (parts list p. 43)
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 Plate ceramic
C15 2.2 nF Plate ceramic
C14, C16 4.7 nF Plate ceramic
C12 33 nF Ceramic multilayer
C10, C19 220 nF Ceramic disc
A (18 off) 47 nF or 33 nF Decouplers
VC1 5.5–40 pF Trimmer on the 17.7345 MHz colour oscillator

VC1 sets the colour oscillator to 17.7345 MHz ±200 Hz, measured at IC8 pin 6 with a frequency counter.[1] Leave it alone unless the colour on the UHF output has failed.

Power supply board

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The power supply board takes the adaptor's AC through a filter and a bridge rectifier (D1–D4) to the reservoir capacitor C4, then a transistor switching regulator and a small transformer, T1, produce +5 V and −5 V.[1] Acorn's diagram marks six polarised capacitors on this board: C4, C5, C9, C11, C12 and C14. It gives no values, and the Service Manual has no parts list for the board.[1]

On stardot in 2017, Mark (1024MAK) listed the electrolytics on his boards, and another owner read the value of the sixth part:[3]

Electron power supply board electrolytics (stardot, 2017)
Designator Value Voltage Form Position on Acorn's diagram
C4 3300 µF 35 V Axial Reservoir after the bridge rectifier
C5 10 µF 16 V Radial Q1/Q2 control circuit
C9 1000 µF 10 V Radial +5 V, before choke L2
C11 470 µF 10 V Axial −5 V, before choke L3
C12 47 µF 10 V Radial +5 V output, after L2
C14 10 µF 35 V or 50 V Axial −5 V output, after L3

The 10 µF part on the −5 V output is printed C13 on some boards, which then carry two C13s (the other is a ceramic).[3] Read each fitted part before ordering, and use a voltage rating equal to or higher than the original.[3]

Replacement procedure

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  1. Unplug the adaptor. Undo the four screws underneath: two recessed in the front corners and two in the cut-away on the back edge.[1]
  2. Lift the top half (with the keyboard) and unplug the keyboard ribbon from the main PCB.[1]
  3. Unplug the three-wire supply connector (red +5 V, blue −5 V, black 0 V), the two 18V AC faston tabs and the loudspeaker lead.[1]
  4. Remove the main PCB (four screws) and the power supply board (three screws).[1]
  5. Note the polarity of each electrolytic and tantalum before removing it. Desolder, clear the holes and fit the new part.
  6. Refit the boards and reconnect everything. The Electron will not run without the keyboard connected.[1]

Checks after recapping

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Acorn's own supply test, from the Service Manual:[1]

  1. With the supply connector unplugged from the main PCB, put a 10 Ω 2.5 W resistor across the red and black leads (about 500 mA). The +5 V must be between 4.75 and 5.25 V with no more than 50 mV of noise on an oscilloscope.
  2. Keeping that load, put 47 Ω across the blue and black leads and check −5 V.
  3. Reconnect the main PCB. It should draw 700–900 mA from +5 V and about 25 mA from −5 V.
  4. If the supply cuts out on a short, unplug the adaptor for at least 15 seconds to reset it.

Then check that the machine starts, and load and save a tape to exercise the −5 V rail and C7. Acorn notes that a −5 V fault shows up as a cassette fault.[1]

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References

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  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 Acorn Computers, Electron Service Manual, part no. 0405001, issue 2, January 1987, hosted on this wiki. Adaptor p. 2; power supply specification p. 4; disassembly p. 6; power supply tests p. 10; power supply board circuit diagram p. 37; final assembly and main PCB parts lists pp. 41–44.
  2. ↑ Acorn Computers, Circuit Diagram for Electron Microcomputer Main PCB, drawing no. 105,000/C, revision table to June 1983. Hosted on this wiki as File:Acorn Electron circuit diagram.png.
  3. ↑ 3.0 3.1 3.2 "New PSU for an Electron", stardot.org.uk forum thread, 4–5 June 2017. Post by 1024MAK listing C4, C5, C9, C11, C12 and the 10 µF 35 V electrolytic marked C13 on his board; post by RetroMike reading C14 as 10 µF 50 V 85 °C.


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