Jump to content

Acorn System 2 Capacitor Replacement Guide

From RetroTechCollection
Acorn System 2
Acorn System 2 in its 19-inch card frame

The Acorn System 2 has four Eurocards with one electrolytic capacitor each and ceramic capacitors for everything else. None of the cards has a regulator; the system runs from one 5 V supply on the backplane. All values below are from the parts lists in Acorn's System 2 Technical Manual, which is hosted as a 1998 OCR text transcription with the pictures removed.[1]

Electrolytics

[edit | edit source]
System 2 electrolytic capacitors
Card Board Designator Value
6502 CPU 200,000 C9 in the parts list; C8 in the assembly text 15 µF
Cassette interface 200,001 C5 15 µF
VDU interface 200,002 C1 10 or 15 µF
Memory 200,003 C8 15 µF (4.7 µF on the issue 1 drawing)

The manual gives no voltage rating for these parts. They sit across the 5 V supply, apart from the cassette card's C5, which is on the LM358 bias divider.[2][3][4] The System 1 parts lists give the CPU and cassette card electrolytics as 15 µF 16 V.[5]

On the CPU card, the parts list names C9 as the electrolytic, while the assembly instructions say it is fitted at C8 and that C10 (the regulator capacitor on a System 1) must not be used.[1] The System 1 manual also moves the electrolytic to C8 when the card is put in a rack.[5] The hosted manual is a transcription, so the conflict may be in Acorn's original or may have come from the transcription; the manual's assembly drawings are not in it. Check which position on the board has the + marking before fitting a replacement.

Ceramic capacitors

[edit | edit source]
System 2 ceramic capacitors (parts lists pp. 2-1 to 2-4)
Card Designators Value
6502 CPU C1, C2, C5–C8 47 or 100 nF
Cassette interface C1 100 pF
Cassette interface C2, C4 1 nF
Cassette interface C3 10 nF
Cassette interface C6, C9, C10 47 or 100 nF
Cassette interface C8 22 nF
VDU interface C2–C7 47 nF
VDU interface C8 100 pF (270 pF on drawing 200,002/C)
VDU interface C9 150 pF
Memory C1–C7 47 or 100 nF

On the CPU card C3, C4 and C10 are not fitted in a System 2, and on the cassette card C7 is not fitted.[1] Acorn notes that 47 nF parts may be supplied in place of 100 nF without affecting operation.[1] Acorn's circuit diagrams for the VDU and memory cards disagree with the transcribed parts lists in two places: the VDU card's C8 is 270 pF on the drawing and 100 pF in the list, and the memory card's C8 is 4.7 µF on the issue 1 drawing and 15 µF in the list for the issue 2 board. Read the value of the fitted part.[2][3][1]

Replacing an electrolytic

[edit | edit source]
  1. Switch off the supply and slide the card out of the frame.
  2. Note the + marking on the board; Acorn's instructions locate the positive end from the board marking.[1]
  3. Desolder from side 1 only. Acorn warns against soldering through-holes that have no component lead, because the plating can break.[1]
  4. Fit a part of the same value, rated 16 V or more, observing polarity.
  5. Refit the card and check that the system signs on with "Acorn Cos".
[edit | edit source]

References

[edit | edit source]
  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Acorn Computers, System 2 Technical Manual, issue 1, January 1980. Text transcription by Ben Harris (1998), hosted on this wiki as Acorn System 2 Technical Manual (PDF). Parts lists: CPU card p. 2-1, cassette interface p. 2-2, VDU interface p. 2-3, memory board p. 2-4; capacitor substitution and polarity pp. 3-1 and 3-5; CPU card set-up p. 3-6; power supply p. 5-1.
  2. ↑ 2.0 2.1 Acorn Computers, V.D.U. Interface P.C.B. (Teletext), drawing 200,002/C, issue 1, 29 August 1979, hosted as File:Acorn teletext VDU interface Eurocard circuit diagram (200,002).png: C1 (electrolytic) and C2 to C7 across +5 V; C8 270 pF and C9 150 pF on the IC3 oscillator.
  3. ↑ 3.0 3.1 Acorn Computers, 8K RAM + 8K PROM, drawing 200,003/C, issue 1, hosted as File:Acorn 8K RAM and 8K PROM Eurocard circuit diagram (200,003).png: C1 to C7 47 nF and C8 4.7 µF electrolytic across +5 V.
  4. ↑ Acorn Computers, circuit diagrams 200,000/C and 200,001/C, 1979–1980. Hosted on this wiki as Acorn System 1 Circuit Diagrams.
  5. ↑ 5.0 5.1 Acorn Computers, Technical Manual, System 1 Micro-Computer, 1979, parts lists pp. 25–30, and "Extending the system" p. 21. Hosted on this wiki.


Retrieved from "https://retrotechwiki.com/index.php?title=Acorn_System_2_Capacitor_Replacement_Guide&oldid=25358"