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Acorn Archimedes 400/1 Series Maintenance Guide

From RetroTechCollection
An A420/1 with the top cover off: power supply with the fan and the CMOS batteries in front of it (photo: Chris Whytehead, CC BY-SA 3.0)

The Archimedes 400/1 series, the A410/1, A420/1 and A440/1 of 1989, share one main board, the Acorn Universal Board (part 0283,000), one case, one 55 W power supply and one service manual, so their care is the same.[1][2] The procedures and values here come from Acorn's 400/1 & R140 Service Manual (0483,075, July 1989) unless stated otherwise. The 1987 A440 has a different board and its own manual; see Archimedes 440 Service Manual.

Identify the machine

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Archimedes 400/1 range
Model Product code RAM Hard disc fitted LK14 LK15
A410/1 AKB40 1 MB none B B
A420/1 AKB42 2 MB 20 MB (0177,001) A B
A440/1 AKB44 4 MB 50 MB (0912,012) A A

Product codes from Chris's Acorns; memory links and drive part numbers from Acorn's manual and Technical Service Bulletin 2/89.[1][2][3] Chris Whytehead lists the A440/1 with a 20 MB disc; Acorn's parts list gives 50 MB.[4][2]

Badges and serial numbers do not always agree: Chris Whytehead has an A440/1 with an A410/1 serial code and an A420/1 with an A440/1 Learning Curve code. Go by the memory and drive fitted.[1] Acorn's test disc reports the machine as "Annn/1" when RISC OS and the MEMC1a are fitted, so an upgraded A310 reports as an A410/1 and an A410/1 with one RAM upgrade as an A420/1.[5]

Safety

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  • Switch off and pull the mains plug before removing the top cover. The cover gives access to the power supply; it is built to BS5850 class 1, but keep screws and other metal out of its ventilation holes.[6]
  • The supply has a double-pole switch and fuses in both the live and the neutral.[6]
  • The IEC outlet on the rear is unswitched: it is live whenever the inlet is powered, and is rated 3 A continuous.[7]
  • After refitting or replacing any assembly, Acorn requires an earth continuity check, with a tester set to 25 A, between the earth pin of the mains plug and the base metalwork, the rear panels including blanking panels, and the top cover.[6]
  • Use anti-static matting and a wrist strap at every level of servicing.[8]

Backup batteries

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The clock and the 240 bytes of configuration RAM run from two LR06 (AA) 1.5 V manganese alkaline cells, listed as "alkaline 1V5 2AH AA", in battery holder assembly 0176,009 connected to PL10 on the main board. The holder sits at the front left of the case beside the fan. Acorn's specification says the batteries "require replacement once a year".[7][6][9]

Alkaline cells left in place leak potassium hydroxide over the holder, its leads and the board. Retro-Kit found minor leak damage on the holder of an A410/1 it bought in 2013, cleaned it and re-soldered the wires to the contacts.[10] A stardot member, paulv, describes replacing the holders in the A300, A400 and A400/1 with two Bulgin BX0035 holders: drill out the pop rivets, rivet the new holders in place and move the wiring across.[11]

  1. Remove the cells as soon as the machine is opened.
  2. Check the holder, its wires, PL10 and the board area around them for green or white deposits. Clean and neutralise as described in Battery Explosion, Capacitor or Corrosion Damage.
  3. With fresh cells fitted and the power off, IC29 pin 8 should read about 2.8 V. If it does not, check PL10, diode D16 and the cells, each of which should be above 1.4 V.[12]
  4. After new cells, configuration defaults can be restored with a DELETE power-on: hold DELETE while switching on until a red border shows.[5]

See Battery Refurbishment for holder replacement options.

Opening the case

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Top cover

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  1. Switch off at the rear, unplug the mains lead and disconnect every peripheral, including the keyboard.
  2. Put the unit on a soft, clean surface with the rear panel towards you.
  3. Remove the two screws in the sides of the top cover just behind the front moulding, then the three screws along the top of the rear panel.
  4. Slide the cover off towards the rear.[6]

Front moulding

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  1. Remove the top cover and the two screws at each side of the front moulding.
  2. Stand the unit on one side and remove the three screws under the front moulding.
  3. Stand it back on its feet, unplug the LED and speaker connector PL8 from the main board, and pull the front moulding straight off.[6]

Acorn's second floppy drive instructions, written for both the 300 and 400/1 series, call this connector PL9; the 400/1 manual and its plug table give PL8.[13][14]

Main board

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  1. Unplug PL8 (speaker and LED), PL10 (battery), PL12 (fan) and the four power tags: PL11 yellow +12 V, PL5 black 0 V, PL6 red +5 V, PL7 mauve −5 V. With a hard disc fitted, the fan connector can only be reached after the board has been partly withdrawn.
  2. Unplug the floppy cable from SK9.
  3. Remove any expansion cards, take out the two backplane screws and rest the backplane on the power supply without unplugging its leads.
  4. Unplug the hard disc control (SK11, 34-way) and data (SK10, 20-way) cables.
  5. Stand the unit on its left side and remove the two screws and star washers under the rear bus bars, then the three screws holding the rear moulding.
  6. Stand it upright and draw the rear moulding out with the board attached, undoing the nut on the earth strap to the drive support bracket when it can be reached, and supporting the front edge of the board as it comes out.[6]

Put the four power tags back on the posts they came from.

Power supply and fan

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The 55 W 240 V supply, part 0183,002, is a service replacement item; the manual has no circuit or component data for it.[6][9] To remove it, remove the top cover and the fan, free all its low-voltage leads, take out the four M3 x 6 mm screws under the base metalwork, slide the supply forward clear of the rear moulding and lift it out. Run the earth continuity check after refitting.[6]

The fan is mounted at the front left of the case beside the battery holder. Undo the two nuts holding the filter clamp and remove the clamp and filter, unplug PL12, then undo the four nuts holding the fan. When refitting, put the two filter nuts on diagonally opposite corners.[6] The filter is a separate spare part, 0276,034.[9]

Keyboard

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Turn the keyboard over on a soft surface and remove the eight Pozidriv screws holding the two halves. The PCB is fixed to the top moulding by four No. 6 x 0.25 inch screws, two at each end of its metal support tray, and on some units three more screws marked A, B and C; take those out first, then the two screws at the cable end, and loosen the other two to lift the board out. Move the reset switch cap across to any replacement board. On reassembly, check that every key clears its cut-out before tightening the screws.[6]

Drives

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  • Floppy drive: a 3.5-inch 1 MB drive, part 0183,004; Chris Whytehead's machines have Citizen OSDC-65A and OSDC-65C drives.[9][1] If a drive misbehaves, check the configuration items STEP and FLOPPIES and that the drive's ID switch is at 0 (1 for a second drive).[15] On an A420/1 with a Citizen OSDC-65C, a stardot member traced Disc Error 04 and erratic head movement to the drive's stepper motor power lead, which sits next to the drive's power connector and had been knocked loose when the power connector was unplugged.[16] See Floppy Drive Repair.
  • Hard disc: an ST506 drive on the on-board HD63463 interface. Park the heads before moving the machine; Acorn's test disc has a park option.[17] Acorn's hard disc tests and formatter destroy the data on the disc. See Hard Drive Maintenance and Repair.

Memory and ROMs

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  • RAM upgrades use Acorn's AKA52 1 MB kit. Fitting from 1 MB to 2 MB, the new chips interleave with the soldered ones; from 2 MB to 4 MB they go in the upper row of empty sockets, bevelled edge towards the back of the machine. Set LK14 and LK15 as in the table above. There is no 3 MB option and 4 MB is the maximum. Acorn made this an approved service centre job and asks for static precautions to BS 5783.[3]
  • The four RISC OS ROMs are in 32-pin sockets IC16 to IC19. Links LK7a, LK7b and LK8 set the ROM size and type.[14] Acorn warns that levering ROMs out with a screwdriver can break tracks or socket pins.[18]

Expansion cards

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The four-slot backplane (0283,040) has three 64-way slots for any card that is not a co-processor and one 96-way slot that also takes a co-processor. With an Econet module fitted, only a half-width card, or one shaped to clear the module, fits in the lower position.[7] A backplane fault shows as every card failing, cards failing in one slot only, or a known good card failing that passes in another machine.[19]

Capacitors

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The main board's polarised capacitors are listed on the Acorn Archimedes 400/1 Series Capacitor Guide, with the community report on the +12 V position C53. See also Capacitor Failure Symptoms.

After any work

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  1. Check that the four power tags are on PL11 (+12 V), PL5 (0 V), PL6 (+5 V) and PL7 (−5 V).
  2. Check that PL8, PL10 and PL12 are plugged in and no cable is trapped.
  3. Run the earth continuity check if the supply, base, rear panels or top cover were disturbed.
  4. Measure about 2.8 V at IC29 pin 8 with the power off.[6][12]
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References

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  1. ↑ 1.0 1.1 1.2 1.3 Whytehead, Chris. "Acorn Archimedes A400/1 Series", Chris's Acorns, The Centre for Computing History (retrieved 2026-10-02). Acorn Universal Board 0283,000 issues 2 and 3 in all models; documented machines with serial numbers, drives and photographs.
  2. ↑ 2.0 2.1 2.2 Acorn Computers, 400/1 & R140 Service Manual, part number 0483,075, issue 1, July 1989. Hosted on this wiki: File:Acorn A400-1 and R140 Service Manual.pdf; described on Acorn A400-1 and R140 Service Manual. Printed page numbers are cited.
  3. ↑ 3.0 3.1 Acorn Computers, Technical Service Bulletin 2/89, 400/1 RAM Upgrade, part number 0483,052, issue 1, June 1989, bound into the 400/1 & R140 Service Manual, pp. 91-92 of the PDF.
  4. ↑ Whytehead, Chris. "Archimedes", Chris's Acorns (retrieved 2026-10-02): product codes, A440/1 listed with a 20 MB hard disc.
  5. ↑ 5.0 5.1 400/1 & R140 Service Manual, main PCB functional test, p. 26.
  6. ↑ 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 6.11 400/1 & R140 Service Manual, disassembly and assembly, pp. 17-20.
  7. ↑ 7.0 7.1 7.2 400/1 & R140 Service Manual, technical specification, pp. 7-11.
  8. ↑ 400/1 & R140 Service Manual, fault-finding information, p. 21.
  9. ↑ 9.0 9.1 9.2 9.3 400/1 & R140 Service Manual, parts list, pp. 51-64: lower case assemblies (battery holder 0176,009, fan 0176,010, fan filter 0276,034, 55 W 240 V PSU 0183,002, floppy drive 0183,004, hard discs 0177,001 and 0912,012) and final assembly items (two alkaline AA cells).
  10. ↑ "Acorn Archimedes A410/1 with 8MB RAM", Retro-Kit (retrieved 2026-10-02): minor battery leak damage to the holder cleaned, wires trimmed and re-soldered; new batteries fitted; the real time clock was found paused and had to be reset to keep time.
  11. ↑ paulv, "Replacing battery holders in the A300/A400 and A400/1 series machines", stardot forums, 13 May 2016, post 137842 in the thread "32 bit Fault finding index".
  12. ↑ 12.0 12.1 400/1 & R140 Service Manual, repairs following functional testing, pp. 47-48.
  13. ↑ Acorn Computers, Second floppy disc drive installation instructions, part number 0483,062, issue 1, January 1989, bound into the 400/1 & R140 Service Manual (PDF pp. 79-86).
  14. ↑ 14.0 14.1 400/1 & R140 Service Manual, main PCB links, test points and plugs, p. 67 (PDF p. 87).
  15. ↑ 400/1 & R140 Service Manual, other tests, floppy disc drive, p. 32.
  16. ↑ "Machine: A420/1", stardot forums, 30 January 2025, post 447026 in the thread "32 bit Fault finding index".
  17. ↑ 400/1 & R140 Service Manual, fault-finding information, pp. 24-30: general test procedure, hard disc formatter, verifier, soak test and park.
  18. ↑ 400/1 & R140 Service Manual, main PCB fault diagnosis and repair, pp. 43-48.
  19. ↑ 400/1 & R140 Service Manual, backplane tests, p. 41.


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