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Acorn System 5 Capacitor Replacement Guide: Difference between revisions

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[[File:Acorn System 5.jpg|thumb|360x360px|Acorn System 5 Eurocard rack with two floppy drives.]]


This guide covers recapping the '''[[Acorn System 5]]''' (late 1982). As a Eurocard rack it has per-card capacitors (a regulator electrolytic plus ceramic decouplers on each card), and as a fully-cased machine it also has bulk smoothing electrolytics in its integral mains power supply. The [[Acorn System 5 Handbook]] documents the boards.<ref name="hb">[[Acorn System 5 Handbook]] (1 March 1983), Acorn Computers &mdash; hosted on this wiki. Per-board reference for the System 5 Eurocards. The [[Acorn System Catalogue]] and [[Acorn System 1 Technical Manual]] give the wider Eurocard capacitor detail.</ref>
The Eurocards of the [[Acorn System 5]] are all powered from the rack's central switch-mode supply and carry no regulators; each has one or two small electrolytic or tantalum capacitors across its supply and a set of ceramic decouplers. The values below are from the parts lists in Acorn's [[Acorn System 5 Handbook]].<ref name="hb">Acorn Computers, ''[[Acorn System 5 Handbook]]'', 400,056/1, 1 March 1983, hosted on this wiki. Parts lists: Rack, PSU and Backplane p. 4; 6502A CPU Board (400,005) p. 21; 32K DRAM Board (400,010) p. 8; 80 × 25 VDU Board (400,019) p. 15; Floppy Disc Controller Board (400,004) p. 12; Acorn Keyboard (400,013) p. 6; Econet Board p. 4. Circuit diagrams: FDC p. 13, keyboard p. 7.</ref>


== Per-card capacitors ==
== Polarised capacitors ==


On each Eurocard the recap target is the +5&nbsp;V regulator electrolytic; the ceramic decouplers are stable and left in place. Read each value from the card and its circuit diagram, and fit a 105&nbsp;°C low-ESR part of equal capacitance and equal-or-higher voltage, observing polarity.
These are the only electrolytic and tantalum parts on the standard boards. Where the parts list gives no voltage, read the rating from the fitted part and fit at least that rating.<ref name="hb" />


* '''CPU card''' (6502 or 6502A) &mdash; regulator electrolytic plus decouplers (the 1&nbsp;MHz 6502 card follows the [[Acorn System 1]] values).
{| class="wikitable styled-table" style="width:100%; text-align:center;"
* '''VDU card''' (80×25 or 40×25) &mdash; regulator electrolytic plus decouplers.
|+'''System 5 polarised capacitors (Acorn parts lists)'''
* '''32&nbsp;KB DRAM cards''' &mdash; regulator electrolytic plus decouplers; the DRAM cards draw the most current, so their regulator electrolytics are worth checking first.
! Board !! Ref !! Value !! Type as listed !! Notes
* '''Floppy-controller and Econet cards''' &mdash; regulator electrolytic plus decouplers.
|-
| 6502A CPU (400,005) || C1 || 22 µF 16 V || Electrolytic ||
|-
| 6502A CPU (400,005) || C2 || 10 µF 16 V || Tantalum ||
|-
| 32K DRAM (400,010) || C2 || 22 µF 16 V || Electrolytic ||
|-
| 80 × 25 VDU (400,019) || C10 || 10 µF 25 V || Electrolytic ||
|-
| Floppy disc controller (400,004) || C1 || 15 µF || Not stated; drawn polarised || The circuit diagram labels it 22 µF, across +5 V and 0 V
|-
| Keyboard (400,013) || C3 || 470 nF || Tantalum || Drawn as 0µ47
|-
| Keyboard (400,013) || C7 || 22 µF || Electrolytic || The circuit diagram labels it 15µ
|-
| Econet || C3, C7 || 10 µF 16 V || Electrolytic || Acorn part 635,100
|}


== ⚠️ Mains power supply ==
== Other capacitors ==


The System 5's integral PSU is mains-powered and contains the bulk smoothing electrolytics and mains-side suppression parts. '''Disconnect the mains lead and discharge the bulk capacitor before any PSU work.''' Recap the PSU only if competent with mains supplies; otherwise replace it. Read each value from the fitted part.
The rest are ceramic or unspecified non-polarised types.<ref name="hb" />


== Procedure (per card) ==
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''System 5 non-polarised capacitors (Acorn parts lists)'''
! Board !! Capacitors
|-
| Backplane || One 47 nF ceramic disc
|-
| 6502A CPU || C3 100 pF ceramic disc; C4–C9 47 nF ceramic
|-
| 32K DRAM || C1 47 pF ceramic disc; C3–C14 47 nF; C15 100 pF ceramic disc
|-
| 80 × 25 VDU || C1 330 pF; C2 47 pF; C3 150 pF; C4, C5 470 pF; C6 330 pF; C7–C9 47 nF; C11 150 pF
|-
| Floppy disc controller || C2–C4 and C7 47 nF; C5 180 pF and C6 100 pF are listed as not used in the System 5
|-
| Keyboard || C1 1 nF and C2 100 nF ceramic disc; C4–C6 47 nF ceramic disc; C8–C10 100 nF for the power-on reset option
|-
| Econet || C1 10 nF (629,010); C2 2.2 nF (629,002); C4–C6 and C8–C10 47 nF (680,001)
|}


# Power off; remove the card from the backplane.
The 32K DRAM board has a local oscillator, trimmed by a select-on-test resistor R13 of about 1&nbsp;kΩ, for use with CPUs that do not supply the bus clocks. In the System 5 the board is supplied linked to run from the Acorn Bus clocks at 2&nbsp;MHz, so the oscillator is not used.<ref name="hbdram">Acorn Computers, ''[[Acorn System 5 Handbook]]'', 32K Dynamic RAM Board (400,010) pp. 5–6, paras 2.3 to 2.3.3: clock circuit, Acorn Bus clock operation, local oscillator and its setting-up procedure; test connection +5 V to side A pin 1 and 0 V to side A pin 32.</ref>
# Record each electrolytic's value, voltage and polarity.
 
# Desolder, clean the holes, fit the new part with correct polarity, solder and trim.
== Power supply ==
# Verify +5&nbsp;V on the card and refit.
 
The supply is a mains-connected AC9231 switch-mode unit mounted behind the card frame (+5&nbsp;V 6&nbsp;A, −5&nbsp;V 0.5&nbsp;A, +12&nbsp;V 2.5&nbsp;A, −12&nbsp;V 0.5&nbsp;A). Acorn's parts list carries it as a single item and the handbook gives no component data for it, so any recap of the supply has to be planned from the fitted parts.<ref name="hb1">Acorn Computers, ''[[Acorn System 5 Handbook]]'', Part 2 Section 1, ''Rack, PSU and Backplane'', pp. 2 and 4.</ref> A switch-mode supply rectifies the mains directly, so its primary reservoir capacitor can hold a dangerous charge after the plug is pulled. Unplug the machine and confirm that the primary capacitor has discharged before touching the supply board.
 
== Procedure ==
 
# Switch off and unplug the System 5.
# Pull the card from the frame and note the orientation of each polarised capacitor.
# Desolder the old part, clean the holes and fit the new part with the same polarity and at least the listed voltage.
# Refit the card and check +5&nbsp;V at the card's edge connector (side A pin 1, 0&nbsp;V on side A pin 32).<ref name="hbdram" />


== Related pages ==
== Related pages ==
Line 28: Line 70:
* [[Acorn System 5 Maintenance Guide]]
* [[Acorn System 5 Maintenance Guide]]
* [[Acorn System 5 Troubleshooting Guide]]
* [[Acorn System 5 Troubleshooting Guide]]
* [[Acorn System 1 Capacitor Replacement Guide]] · [[Capacitor Failure Symptoms]]
* [[Capacitor Failure Symptoms]]


== References ==
== References ==
Retrieved from "https://wiki.retrotechcollection.com/Acorn_System_5_Capacitor_Replacement_Guide"