Acorn Archimedes A3000 Capacitor Guide

A3000 main board 0180,000 Issue 3. The radial electrolytics are the blue cans; B1, the backup cell, is at the bottom left.

The Acorn Archimedes A3000 main board uses leaded radial and axial aluminium electrolytics and leaded tantalums, with surface-mount ceramic decouplers. Every capacitor on the board, and on Acorn's two internal upgrades, is listed with its value, voltage, type and Acorn part number in the parts lists of the Acorn A3000 Service Manual. The tables below are taken from those lists. The leaking backup cell, B1, does more damage to these boards than the capacitors do; deal with it first (see the Acorn Archimedes A3000 Maintenance Guide).

Safety

Warning: the power supply inside the case runs from the mains. Acorn's service manual warns that dangerous voltages are exposed when the cover is removed and that the computer must be disconnected from the mains first. Capacitors on the primary side of the supply can hold a charge after the plug is pulled. Discharge them and confirm with a meter before touching that side.

The main board is a four-layer PCB, and Acorn says components should be removed from it only with equipment designed for the job. A through-hole pad pulled from an inner-layer connection is hard to repair, so use a desoldering station or plenty of fresh solder and wick, and do not pull a lead until the joint is fully molten.

Failure signs

• An electrolytic with a domed top, a vented seal, residue around the base or a cracked sleeve.
• A tantalum that has gone short circuit. Acorn's video fault procedure checks C100 for a short when L18 is open, and its audio procedure checks C108 when L19 is open.
• Printer faults. Acorn notes that C23-C33 on the printer port occasionally go open circuit.

See Capacitor Failure Symptoms for general signs.

A3000 capacitor list

Electrolytic and tantalum capacitors, main PCB

A3000 main PCB electrolytic and tantalum capacitors (Service Manual pp. 52-53)

Designator	Value	Voltage	Type	Acorn part no.
C4, C13, C22, C34, C55, C57, C78, C89, C94, C100, C108, C111	10 µF	10 V	Tantalum	0610,010
C6, C7, C8, C9, C10, C19, C93, C97	10 µF	16 V	Aluminium electrolytic, radial	0635,100
C12, C14, C61, C66, C69, C112	220 µF	16 V	Aluminium electrolytic, radial	0635,230
C15, C60, C63, C95	47 µF	16 V	Aluminium electrolytic, radial	0635,470
C39, C83, C109, C110	47 µF	10 V	Aluminium electrolytic, axial	0621,470
C65, C70	100 µF	25 V	Aluminium electrolytic, radial	0642,103
C67	4.7 µF	16 V	Aluminium electrolytic, radial	0635,047

That is 25 aluminium electrolytics and 12 tantalums. Acorn numbers capacitors in a single sequence across all types, so the electrolytics are scattered through C1-C116 among the ceramics.

Upgrade boards

Electrolytics on Acorn's internal upgrades (Service Manual p. 54)

Board	Designator	Value	Voltage	Acorn part no.
2 MB RAM upgrade	C9, C10	47 µF	16 V	0635,470
User Port/MIDI upgrade	C1	220 µF	16 V	0635,230

The RAM upgrade also carries eight 100 nF axial decouplers (C1-C8), and the User Port/MIDI board seven 33/47 nF decouplers (C2-C8).

Ceramic and film capacitors

The remaining positions are ceramic or film capacitors. Of these, only C23-C33 and C107 appear in Acorn's fault procedures.

• 33 nF surface-mount decouplers (1210): C1, C5, C11, C17, C18, C20, C21, C35-C37, C40-C47, C50-C54, C59, C64, C76, C87, C98, C99, C105, C106, C113, C500-C506.
• 100 nF surface-mount decouplers (1210): C48, C49, C56, C58, C71-C73, C84.
• 47 nF ceramic: C3, C62, C68, C74, C96, C107.
• Plate ceramics: 10 pF C114; 12 pF C16; 33 pF C2, C115; 100 pF C23, C32, C33; 1 nF C79, C85; 2.2 nF C24-C31, C75, C81, C82, C86, C90, C91; 2.7 nF C116.
• Metallised polyester, 50 V: 22 nF C77, C92; 100 nF C80, C88.
• Not fitted: C38, C101-C104.

Capacitors in Acorn's fault procedures

Capacitors named in the A3000 Service Manual fault chapter (pp. 49-50)

Fault	Acorn's check
No video	+5 V should be present on both ends of L18. If L18 is open, check C100 for a short and replace L18 and C100 as needed. IC41 (VIDC) pin 43 should be about 3.5 V; if not, check R109, D16 and C93.
No audio at all	+5 V should be present on both ends of L19. If L19 is open, check C108 before replacing it. VIDC pin 12 should be about 3 V; if not, check R500, C107 and C108.
Clock and CMOS RAM lose data	With the power off, IC6 pin 8 should read about 1.1 V. Below 1 V, change B1 and check D2 and C15.
Printer problems	C23-C33 on the printer port occasionally go open circuit.

Parts to buy

A3000 main PCB electrolytics and tantalums by value

Value	Voltage	Type	Quantity
4.7 µF	16 V	Radial electrolytic	1
10 µF	16 V	Radial electrolytic	8
47 µF	16 V	Radial electrolytic	4
100 µF	25 V	Radial electrolytic	2
220 µF	16 V	Radial electrolytic	6
47 µF	10 V	Axial electrolytic	4
10 µF	10 V	Tantalum	12

Add two 47 µF 16 V for the RAM upgrade and one 220 µF 16 V for the User Port/MIDI board if fitted. A replacement can have the same or a higher voltage rating, provided it fits the footprint. For the axial positions, an axial part keeps the original layout.

Replacement procedure

1. Disconnect the mains lead. Remove the top cover, keyboard, floppy drive and any RAM upgrade, then the main PCB (Service Manual pp. 31-32).
2. Photograph the board and note the polarity mark of every electrolytic and tantalum before removing anything.
3. Remove B1 at the same time if it is still fitted, and clean any leakage before fitting new parts.
4. Desolder each part with a desoldering station, or add fresh solder and clear the holes with wick. Do not lever parts out of the four-layer board.
5. Fit the new part with the correct polarity, solder and trim. A reversed tantalum fails short.
6. Check every joint for bridges before refitting the board.

Power supply

The supply is part 0180,010, "22W 240V PSU", in the final assembly parts list. Acorn's service manual covers only removing and refitting it as a unit and gives no component list or circuit for it, so read each value and voltage off the part being replaced. When refitting, replace the plastic insulating film beneath it and locate its cover correctly.

Checks after recapping

1. Measure the +5 V rail. Acorn's specification is 4.9-5.1 V, with ripple and noise up to 50 mV peak to peak.
2. Check that the machine reaches the desktop, then test sound through the speakers and headphones, a floppy read, and the printer port if it is used.
3. Run Acorn's functional tests from the service manual if a test disc is available (pp. 36-43).
4. If a function that worked before the recap now fails, check the polarity of the parts in that area first.

After refitting or replacing an assembly, Acorn requires an earth continuity test on the assembled computer, between the mains plug earth pin and the expansion-card fixing screws and printer-port fixing bolts (pp. 33 and 45; Appendix B, p. 57).

Related pages

• Acorn Archimedes A3000
• Acorn Archimedes A3000 Maintenance Guide
• Acorn Archimedes A3000 Troubleshooting Guide
• Acorn Archimedes A3010 Capacitor Guide
• Capacitor Failure Symptoms