Acorn Archimedes A3010 Capacitor Guide

A3010 main board with the ARM250. The supply is at the top left; the radial electrolytics are through-hole parts.

The Acorn Archimedes A3010 main board uses through-hole radial and axial aluminium electrolytics and tantalum capacitors, with surface-mount ceramic decouplers. It has no surface-mount electrolytics. Acorn's Technical Reference Manual gives a parts list for each A3010 board, with every capacitor's value, voltage, type and Acorn part number, and the main board circuit diagram shows where each one sits.

On Stardot, Ian Jeffray pointed out that the A3010 is not known for capacitor problems and has none of the surface-mount electrolytics that leak on other Acorn machines, and advised against recapping one as a matter of course. Replace a capacitor when it has failed or leaked. Deal with the backup cell, BT2, first: Retro Kit lists the A3010 among the Archimedes models whose rechargeable cell needs servicing to avoid leakage damage to the board. See the Acorn Archimedes A3010 Maintenance Guide.

Safety

Warning: the supply is part of the main board, and mains voltage reaches the board through the inlet PL1, switch SW2 and fuse FS1 to the transformer primary. Acorn warns of hazardous voltages on the board once the metal safety case is removed, and on the underside of the board if it is run outside the metalwork. Unplug the machine before working on it.

None of the capacitors fitted to the A3010 board is on the mains side. The circuit diagram shows positions for an X capacitor (C4) and two Y capacitors (C5, C7) across the mains input, but the parts lists mark all three not fitted.

After any repair, Acorn applies an earth continuity test and a DC insulation test to the reassembled machine (Service Manual Appendices C and D).

Capacitor list

ARM250 main board

A3010 1 MB (ARM250) main PCB electrolytic and tantalum capacitors (Technical Reference Manual pp. 2-13 to 2-15)

Designator	Value	Voltage	Type	Acorn part no.
C10	1000 µF	35 V	Radial electrolytic	0635,106
C18	470 µF	35 V	Radial electrolytic	0635,476
C19	2.2 µF	50 V	Radial electrolytic	0635,024
C30, C60, C68	100 µF	25 V	Radial electrolytic	0642,103
C2, C6, C20, C21, C39, C49, C110, C118, C161, C228	220 µF	16 V	Radial electrolytic	0635,230
C22	220 µF	16 V	Radial electrolytic	0635,226
C14, C16, C28, C62, C73, C78, C80, C86, C157	47 µF	16 V	Radial electrolytic	0635,470
C1, C9, C33, C82, C84, C88, C90, C93, C220	10 µF	16 V	Radial electrolytic	0635,100
C74	4.7 µF	16 V	Radial electrolytic	0635,047
C159, C164, C165	47 µF	10 V	Axial electrolytic	0621,470
C59, C83, C105, C120, C131, C139	10 µF	10 V	Tantalum	0610,010
C106	47 µF	10 V	Tantalum	0610,047

That is 39 electrolytics and seven tantalums. The rest of the board's capacitors are surface-mount ceramics (33 nF and 100 nF decouplers and small ceramics), metallised polyester types (C12, C15, C148, C150) and a 5-65 pF trimmer (C25).

Early board with the CPU daughter card

Acorn's list for the early 1 MB board (issue 3, pp. 2-5 to 2-8) has the same electrolytics and tantalums in the same positions, with these differences:

• C19 is part 0635,022, also 2.2 µF 50 V radial.
• C22 is part 0635,230, 220 µF 16 V.
• C27 is fitted, 100 µF 25 V (0642,103). It is not fitted on the ARM250 board.
• There is no C228; the list ends at C223.

The CPU daughter card (0194,002) carries four 10 µF 16 V surface-mount tantalums, C1, C2, C11 and C12 (part 0681,101), and no electrolytics.

Acorn's separate lists for the 2 MB boards begin on p. 2-9 (early board) and p. 2-17 (ARM250 board).

Where the capacitors are

The main board circuit diagram, which Acorn says is common to both boards, places these capacitors as follows.

A3010 supply and audio capacitors (circuit diagram 0194,000 sheet 2)

Designator	Value	Position in the circuit
C10, C18	1000 µF 35 V, 470 µF 35 V	Smoothing after the bridge rectifier D5-D8 on the main transformer winding, the about 23 V supply at TP1
C19	2.2 µF 50 V	Soft-start capacitor of the L4960 regulator, Q2
C20, C21	220 µF 16 V	+5 V output of the L4960, after the 150 µH inductor L4
C9	10 µF 16 V	Output of the 78L12 regulator, Q1 (regulated +12 V)
C2	220 µF 16 V	Unregulated +12 V rail (TP2)
C6	220 µF 16 V	Unregulated -12 V rail (TP3)
C59, C83, C105, C120, C131, C139	10 µF 10 V tantalum	Bulk decoupling on +5 V
C60, C68	100 µF 25 V	Left and right audio outputs from the audio hybrid
C62	47 µF 16 V	Supply decoupling for the LM386 speaker amplifier, IC10
C78	47 µF 16 V	LM386 bypass pin (pin 7)
C118	220 µF 16 V	Couples the LM386 output to the internal speaker

The TRM describes the supply: the main secondary is rectified and filtered to about 23 V at TP1, the L4960 switches at about 100 kHz to make 5 V, the 78L12 makes the regulated 12 V for the SCART interface and the audio hybrid, and the centre-tapped second winding gives about ±12 V at 50 mA for the RS232 circuits.

Audio hybrid

Most of the audio circuit is on a plug-in hybrid module (IM2, part 0194,007) carrying LM324 op-amps. Acorn's parts lists give no component list for it. On Stardot, members who took one apart found ceramic capacitors on it, not electrolytics, and traced crackling sound to broken hybrid pins and to the switched contacts in the headphone socket; Ian Jeffray suggested C60, C68, C78 and C118 on the main board as the electrolytics to check.

Parts to buy

A3010 ARM250 main board, electrolytics and tantalums by value

Value	Voltage	Type	Quantity
1000 µF	35 V	Radial electrolytic	1
470 µF	35 V	Radial electrolytic	1
220 µF	16 V	Radial electrolytic	11
100 µF	25 V	Radial electrolytic	3
47 µF	16 V	Radial electrolytic	9
10 µF	16 V	Radial electrolytic	9
4.7 µF	16 V	Radial electrolytic	1
2.2 µF	50 V	Radial electrolytic	1
47 µF	10 V	Axial electrolytic	3
10 µF	10 V	Tantalum	6
47 µF	10 V	Tantalum	1

Use the same or a higher voltage rating and a part that fits the footprint. The early board needs one more 100 µF 25 V (C27) and one fewer 220 µF 16 V (no C228).

Replacement procedure

1. Unplug the machine. Remove the top cover, the keyboard (two flexi strips) and the metal safety case, then separate the two halves of the case, disconnecting the floppy drive cables and the speaker (Service Manual Part 2).
2. Disconnect the mains cable at the top left of the lower envelope, remove the screw locks on the rear connectors and the ring lock on SK4, remove the earth lead if fitted, and lift the board out.
3. Note the polarity of every electrolytic and tantalum before removing it.
4. Desolder each part with a desoldering station or with fresh solder and wick, and fit the replacement with the correct polarity.
5. Check every joint for bridges before refitting the board.

Checks after recapping

1. Measure about 23 V at TP1, +12 V at TP2 and -12 V at TP3 against 0 V (Acorn's nominal figures at 240 V input), and 5 V on the logic.
2. Check the machine completes its power-on self-test: the screen changes colour as the test runs and ends black with a memory size message, within 2 to 12 seconds.
3. Test sound through the speaker and headphones, the floppy drive and, if used, the TV output.
4. Carry out the earth continuity and insulation tests before putting the machine back into use.

Related pages

• Acorn Archimedes A3010
• Acorn Archimedes A3010 Maintenance Guide
• Acorn Archimedes A3010 Troubleshooting Guide
• Acorn Archimedes A3020 Capacitor Guide
• Acorn Archimedes A3000 Capacitor Guide
• Capacitor Failure Symptoms