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Acorn System 1 Capacitor Replacement Guide

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Acorn System 1 CPU board
Acorn System 1 CPU card, 200,000 (photo: Chris's Acorns)

The Acorn System 1 has two electrolytic capacitors, one on each card. Everything else is ceramic. Values are from Acorn's parts lists (issue 2, July 1979) and positions from Acorn's circuit diagrams.[1][2] The machine runs on 7 to 35 V DC and has no mains wiring.[1]

CPU card, 200,000

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System 1 CPU card capacitors
Designator Value Type Position
C10 15 µF 16 V Electrolytic Regulator IC13 input, from the DC supply on pin 31
C1, C2, C5, C6, C7, C8, C9 100 nF Ceramic +5 V decoupling
C3 10 nF Ceramic NMI switch network (with R3)
C4 10 nF Ceramic IRQ switch network (with R4)

The CPU card parts list prints "NOT SUPPLIED" between C3 and C4; the single board controller list puts that note against C4.[1] Only one push switch came with the kit.[1]

Acorn's kits could contain 47 nF capacitors in place of the 100 nF parts, which Acorn says does not affect operation.[1] When the CPU card is converted to run from a rack supply, IC13 and C10 are removed and C8 is replaced by an electrolytic; Acorn suggests reusing the one taken out of C10.[1][2]

Keyboard card, 200,001

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System 1 keyboard card capacitors
Designator Value Type Position
C5 15 µF 16 V Electrolytic LM358 bias divider (R6, R7)
C1 100 pF Ceramic Cassette tone oscillator
C2, C4 1 nF Ceramic Cassette circuit coupling
C3 10 nF Ceramic Cassette circuit
C6 100 nF Ceramic Cassette input filter
C8 22 nF Ceramic Cassette input coupling
C9, C10 100 nF Ceramic +5 V decoupling
C7 – – Not fitted

The keyboard card has no regulator of its own; it takes +5 V from the CPU card through the connecting strip.[2]

Replacing the electrolytics

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Both electrolytics are polarised, and Acorn's construction notes show the + end's position on the board.[1] Replacement parts are 15 µF (or the nearest standard value, 22 µF) at 16 V or more. C10 sees the full DC input, which can be up to 35 V, so a 16 V part is only correct if the supply is 16 V or less; use a 50 V part if the machine is run from a higher-voltage supply.[1]

  1. Disconnect the supply. Undo the four screws and spacers that hold the keyboard above the CPU card, taking care not to flex the 20-way strip.[1]
  2. Note the polarity of the old capacitor.
  3. Desolder it from side 1 of the board. Acorn notes that through-holes without component leads should not be soldered, because the plating can break.[1]
  4. Fit the new part with the correct polarity, solder and trim.
  5. Power up, press RST and check for the eight dots on the display.[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 Acorn Computers, Technical Manual, System 1 Micro-Computer, 1979, hosted on this wiki. Capacitor substitution and polarity notes in the construction procedure; extending the system p. 21; parts lists pp. 25–30 (CPU card 200,000/P, keyboard 200,001/P, single board controller 200,010/P, issue 2, July 1979).
  2. ↑ 2.0 2.1 2.2 Acorn Computers, circuit diagrams 200,000/C and 200,001/C, 1979–1980. Hosted on this wiki as Acorn System 1 Circuit Diagrams.


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