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Acorn System 2 Capacitor Replacement Guide: Difference between revisions

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Replace generic 'regulator electrolytic on each card' text (System 2 cards have no regulators) with Acorn's System 2 parts lists for all four cards; note C8/C9 discrepancy in Acorn's manual
 
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[[File:Acorn System 2.jpg|thumb|360x360px|Acorn System 2 in its 19-inch card frame]]


This guide covers recapping the '''[[Acorn System 2]]''' (1980). The System 2 is a set of Eurocards, so the capacitors are per-card: each card carries ceramic decouplers plus a single electrolytic on its +5&nbsp;V regulator. The exact per-card values are in the card circuit diagrams and the [[Acorn System Catalogue]]; the CPU and cassette cards use the verified [[Acorn System 1]] values.<ref name="cat">[[Acorn System Catalogue]] and [[Acorn System 1 Technical Manual]], Acorn Computers &mdash; hosted on this wiki. Source for the Eurocard set and the System 1 CPU/cassette-card capacitor values (a 15&nbsp;µF/16&nbsp;V electrolytic plus ceramic decouplers per card).</ref>
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&nbsp;V supply on the backplane. All values below are from the parts lists in Acorn's System 2 Technical Manual.<ref name="s2tm">Acorn Computers, ''System 2 Technical Manual'', issue 1, January 1980. Text transcription by Ben Harris (1998), hosted on this wiki as [[:File:Acorn System 2 Technical Manual (text transcription).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.</ref>


== Approach ==
== Electrolytics ==


The System 2 has no mains and no CRT on its cards (only the optional Acorn PSU is mains-powered). Capacitor work is low-risk per-card bench work. On each Eurocard:
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''System 2 electrolytic capacitors'''
! Card !! Board !! Designator !! Value
|-
| 6502 CPU || 200,000 || C9 in the parts list; C8 in the assembly text || 15&nbsp;µF
|-
| Cassette interface || 200,001 || C5 || 15&nbsp;µF
|-
| VDU interface || 200,002 || C1 || 10 or 15&nbsp;µF
|-
| Memory || 200,003 || C8 || 15&nbsp;µF
|}


* The recap target is the '''regulator electrolytic''' (the CPU and cassette cards use 15&nbsp;µF/16&nbsp;V; the VDU and memory cards each have their own regulator electrolytic).
The manual gives no voltage rating for these parts. They sit on the 5&nbsp;V supply, apart from the cassette card's C5, which is on the LM358 bias divider.<ref name="s1cd">Acorn Computers, circuit diagrams 200,000/C and 200,001/C, 1979–1980. Hosted on this wiki as [[Acorn System 1 Circuit Diagrams]].</ref> The System 1 parts lists give the CPU and cassette card electrolytics as 15&nbsp;µF 16&nbsp;V.<ref name="s1tm">Acorn Computers, ''[[Acorn System 1 Technical Manual|Technical Manual, System 1 Micro-Computer]]'', 1979, parts lists pp. 25–30, and "Extending the system" p. 21. Hosted on this wiki.</ref>
* The ceramic decouplers are stable and are left in place.


== CPU and cassette cards ==
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.<ref name="s2tm" /> The System 1 manual also moves the electrolytic to C8 when the card is put in a rack.<ref name="s1tm" /> Check which position on the board has the + marking before fitting a replacement.


These are the [[Acorn System 1]] cards. Their full capacitor lists are in the [[Acorn System 1 Capacitor Replacement Guide]]; the only electrolytic on each is a '''15&nbsp;µF/16&nbsp;V''' part (C10 on the CPU card, C5 on the cassette card).
== Ceramic capacitors ==


== VDU and memory cards ==
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''System 2 ceramic capacitors (parts lists pp. 2-1 to 2-4)'''
! Card !! Designators !! Value
|-
| 6502 CPU || C1, C2, C5–C8 || 47 or 100&nbsp;nF
|-
| Cassette interface || C1 || 100&nbsp;pF
|-
| Cassette interface || C2, C4 || 1&nbsp;nF
|-
| Cassette interface || C3 || 10&nbsp;nF
|-
| Cassette interface || C6, C9, C10 || 47 or 100&nbsp;nF
|-
| Cassette interface || C8 || 22&nbsp;nF
|-
| VDU interface || C2–C7 || 47&nbsp;nF
|-
| VDU interface || C8 || 100&nbsp;pF
|-
| VDU interface || C9 || 150&nbsp;pF
|-
| Memory || C1–C7 || 47 or 100&nbsp;nF
|}


The 40×25 VDU card and the memory (RAM + BASIC ROM) card each regulate their own +5&nbsp;V and carry a regulator electrolytic plus ceramic decouplers. Read each value from the card itself and the card circuit diagram before replacing. Replace any electrolytic with a 105&nbsp;°C low-ESR part of equal capacitance and equal-or-higher voltage, observing polarity.
On the CPU card C3, C4 and C10 are not fitted in a System 2, and on the cassette card C7 is not fitted.<ref name="s2tm" /> Acorn notes that 47&nbsp;nF parts may be supplied in place of 100&nbsp;nF without affecting operation.<ref name="s2tm" />


== Procedure ==
== Replacing an electrolytic ==


# Power off; remove the card from the backplane.
# Switch off the supply and slide the card out of the frame.
# Record the value, voltage and polarity of each electrolytic.
# Note the + marking on the board; Acorn's instructions locate the positive end from the board marking.<ref name="s2tm" />
# Desolder, clean the holes, fit the new part with correct polarity, solder and trim.
# Desolder from side 1 only. Acorn warns against soldering through-holes that have no component lead, because the plating can break.<ref name="s2tm" />
# Verify +5&nbsp;V on the card and refit.
# Fit a part of the same value, rated 16&nbsp;V or more, observing polarity.
 
# Refit the card and check that the system signs on with "Acorn Cos".
== Optional mains PSU ==
 
If an Acorn mains power supply is fitted, it has its own bulk smoothing electrolytics (and mains-side parts). Recap it only if competent with mains supplies; discharge the bulk capacitor first.


== Related pages ==
== Related pages ==
Line 34: Line 65:
* [[Acorn System 2 Maintenance Guide]]
* [[Acorn System 2 Maintenance Guide]]
* [[Acorn System 2 Troubleshooting Guide]]
* [[Acorn System 2 Troubleshooting Guide]]
* [[Acorn System 1 Capacitor Replacement Guide]] · [[Capacitor Failure Symptoms]]
* [[Acorn System 1 Capacitor Replacement Guide]]


== References ==
== References ==

Latest revision as of 09:12, 1 October 2026

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.[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

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

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.[3] 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
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]

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 Acorn Computers, System 2 Technical Manual, issue 1, January 1980. Text transcription by Ben Harris (1998), hosted on this wiki as File:Acorn System 2 Technical Manual (text transcription).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. ↑ Acorn Computers, circuit diagrams 200,000/C and 200,001/C, 1979–1980. Hosted on this wiki as Acorn System 1 Circuit Diagrams.
  3. ↑ 3.0 3.1 Acorn Computers, Technical Manual, System 1 Micro-Computer, 1979, parts lists pp. 25–30, and "Extending the system" p. 21. Hosted on this wiki.


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