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Acorn Cambridge Workstation Capacitor Guide

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Acorn Cambridge Workstation 32016 processor board
The 4 MB 32016 processor board from an ACW (photo: Chris Whytehead, CC BY-SA 3.0)

Acorn's ACW 443 Service Manual includes the parts lists for the Acorn Cambridge Workstation's I/O processor board, its 32016 processor board, the Winchester host adapter and the power distribution board, so the polarised capacitors on all four are known from Acorn's own data.[1] The 85 W power supply, the Microvitec monitor, the Adaptec controller and the drives are bought-in units with no component list in the manual. Acorn's description prefixes: ALEC is an aluminium electrolytic, TANT a tantalum, RAD radial, AX axial.

I/O processor board

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The ABM control board (assembly 0133,400 on bare board 0233,000) is the BBC Model B+ board as fitted to the ACW.[1] Its parts list (drawing A4/0133,400, issue 3, 28 July 1986, PDF page 186) gives these polarised capacitors:[1]

I/O processor board: aluminium electrolytics
Designators Value Type Acorn part
C30 4.7 µF 16 V Radial 0635,047
C27, C51, C52 10 µF 16 V Radial 0635,100
C15 4.7 µF 10 V Axial 0621,047
C25, C46, C50, C54, C56, C60 47 µF 10 V Axial 0621,470
I/O processor board: tantalums
Designators Value Tolerance Acorn part
C33 1.0 µF 35 V 20 % 0613,100
C57 10 µF 10 V 20 % 0610,010
C37 47 µF 10 V 20 % 0610,047

That is 11 aluminium electrolytics and three tantalums. The rest of the board is ceramic and polyester capacitors and 67 axial 100 nF decouplers marked "A".[1]

The list matches the polarised capacitors in Acorn's BBC Model B+ parts list on BBC Micro B+ Capacitor Replacement Guide position for position, with two differences: the ACW list has no C14 (10 µF 6.3 V on the B+), and it gives C33's voltage as 35 V, which the B+ list leaves out.[1][2] Check C14's position on your board; the ACW omits some B+ parts, among them the UHF modulator.[3]

C33, C37 and C57 are tantalums. A shorted capacitor across a supply rail pulls that rail down for the whole board, and Acorn's I/O board current check finds it: see measuring the I/O board current.[4]

32016 processor board

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The "Large" 32016 board (assembly 0118,100 on bare board 0218,100) is listed without designators. Its parts list (issue 1, 28 July 1986, PDF page 190) gives:[1]

32016 board: polarised capacitors
Quantity Value Type Acorn part
4 47 µF 10 V 20 % Tantalum 0610,047
1 1.0 µF 35 V 20 % Tantalum 0613,100

The other capacitors on this board are a 10 nF ceramic disc, a 1 nF 50 V capacitor and 182 decouplers of 33 or 47 nF for the DRAM array. Find the tantalums from the board layout drawing on PDF page 164 or on the board itself.[1]

Winchester host adapter and power distribution board

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The host adapter (assembly 0127,000) has one electrolytic, C1, 22 µF 16 V (part 0622,220), and sixteen 47 nF decouplers.[1] The power distribution board (0120,005) has no capacitors; its only component apart from the plugs is R1, 10 ohms 1 W.[1]

Power supply and monitor

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The 85 W supply (Acorn part 0831,104) is a field replacement unit and the manual gives no circuit or component list for it.[5] It is mains-powered: discharge and measure its primary capacitors before touching the board.

The monitor is a Microvitec Cub chassis. Acorn's manual reproduces Microvitec's circuit diagram and PCB layouts (PDF pages 172 to 175), but the scan is too coarse to read component values reliably, and Acorn refers servicing to Microvitec's service manual, GP0019AA0.[6] A later issue of that Microvitec manual, GP0019AA3 for the Series 3 Cub, is on archive.org; its uploader describes it as covering most of the Cub monitors used with the BBC Micro. Acorn's manual identifies the ACW's monitor only as a medium-resolution Cub.[7] Discharge the CRT before working on the monitor: see CRT Discharge Procedure.

Procedure

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  1. Park the Winchester heads, switch off and unplug the mains.
  2. Open the case and discharge the CRT as described in opening the case.
  3. For the 32016 board, lower the right-hand tray, disconnect the four Faston DC leads, the IDC ribbon and the keyboard wire on PL9, and remove the six nylon screws. Do not overtighten them when refitting.
  4. For the I/O processor board, pivot the cradle beyond its intermediate position, remove the seven Faston DC plugs, the five IDC plugs on the far edge, the keyboard plug, the loudspeaker and volume plugs and the monitor plug, then the four Phillips screws, and draw the board out of the base.
  5. For the host adapter, lower the left-hand tray, disconnect its two Faston DC leads and two IDC cables, and lift it off its four nylon pillars. The host adapter and the Adaptec board are static-sensitive.
  6. Note the polarity of each part, then fit a replacement of the same value and at least the same voltage rating.
  7. Refit the boards and put every Faston back on its correct post: red +5 V, black 0 V, purple −5 V, orange +12 V. The posts are identical, so a reversed lead is easy to fit.
  8. Before closing the case, measure +5 V (4.9 to 5.2 V) and +12 V (11.4 to 12.6 V), and at least 5.00 V at both Faston pairs of the 32016 board.[8]
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Acorn Computers, ACW 443 Service Manual, reference 0420,001, issue 1, 7 January 1987, parts lists at the end of the scan (image pages). Hosted on this wiki: File:Acorn ACW 443 Service Manual.pdf; described on Acorn Cambridge Workstation Documentation.
  2. ↑ Acorn Computers, BBC Microcomputer Service Manual, October 1985, Section 2 (Model B+), parts list, items 66 to 101, as transcribed on BBC Micro B+ Capacitor Replacement Guide.
  3. ↑ ACW 443 Service Manual, chapter 4, p. 33: the I/O processor board is the BBC Model B+ board with a small number of omissions, chiefly the UHF modulator.
  4. ↑ ACW 443 Service Manual, section 4.7.3, p. 62: I/O board power supply current check.
  5. ↑ ACW 443 Service Manual, section 1.3.2, p. 6, and final assembly parts list, PDF page 182.
  6. ↑ ACW 443 Service Manual, chapter 3, p. 25.
  7. ↑ Microvitec plc, Series 3 service manual GP0019AA3 (Microvitec Cub), archive.org item microvitec-series-3-sm-gp-0019-aa-3 (retrieved 2026-10-02).
  8. ↑ ACW 443 Service Manual, chapter 2, pp. 16-22 (reassembly, Faston colours and polarity, removing the 32016 board, the host adapter and the I/O processor board), and pp. 6, 100 and 117 (supply limits).


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