Acorn Archimedes A3020 Capacitor Guide

A3020 main board 0194,500. The supply is at the top left; the backup cell BT2 is at the top right.

The Acorn Archimedes A3020 main board (0194,500) mixes through-hole radial and axial electrolytics with seven surface-mount electrolytics and two surface-mount tantalums. Acorn's Technical Reference Manual lists every capacitor with its value, voltage, type and Acorn part number, and the hosted circuit diagram shows where each one sits. Deal with the backup cell, BT2, before the capacitors: Retro Kit lists the A3020 among the Archimedes models whose rechargeable cell needs servicing to avoid leakage damage to the board. See the Acorn Archimedes A3020 Maintenance Guide.

Safety

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

None of the fitted capacitors is on the mains side. The circuit diagram has positions for an X capacitor (C4) and two Y capacitors (C5, C7) across the mains input, marked not fitted. After any repair, Acorn applies earth continuity and DC insulation tests to the reassembled machine (Service Manual Appendices C and D).

Capacitor list

A3020 2 MB main PCB electrolytic and tantalum capacitors (Technical Reference Manual pp. 2-23 and 2-24)

Designator	Value	Voltage	Type	Acorn part no.
C200, C201	1000 µF	35 V	Radial electrolytic	0635,106
C203, C204, C207, C208	2.2 µF	50 V	Radial electrolytic	0635,022
C60, C68, C211, C212, C213	100 µF	25 V	Radial electrolytic	0642,103
C2, C6, C39, C49, C110, C118, C161	220 µF	16 V	Radial electrolytic	0635,230
C14, C16, C28, C73, C86, C157	47 µF	16 V	Radial electrolytic	0635,470
C1, C9, C82, C84	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
C62, C78, C80, C218	47 µF	16 V	Surface-mount electrolytic	0647,002
C88, C90, C93	10 µF	16 V	Surface-mount electrolytic	0647,001
C59, C83, C120, C131, C139	10 µF	10 V	Tantalum, leaded	0610,010
C105	10 µF	16 V	Tantalum, surface-mount	0681,101
C106	47 µF	Not stated	Tantalum, surface-mount	0681,100

That is 32 through-hole electrolytics, seven surface-mount electrolytics and seven tantalums. The other capacitors are surface-mount ceramics and five metallised polyester parts (C15, C148, C150, C206, C209).

Several positions differ from the A3010, whose list uses C10 and C18 for the supply reservoir, C20 and C21 for the 5 V output and leaded parts throughout; the A3020's supply capacitors are numbered from C200, and C62, C78, C80, C88, C90 and C93 are surface-mount parts where the A3010 has leaded ones. Do not use one machine's list for the other.

Where the capacitors are

A3020 supply and audio capacitors (circuit diagram 0194,500 sheet 2)

Designator	Value	Position in the circuit
C200, C201	1000 µF 35 V	Smoothing after the bridge rectifier D5-D8 on the main transformer winding (TP1)
C203, C204, C207, C208	2.2 µF 50 V	Around the L4974 switching regulator, IC38
C211, C212, C213	100 µF 25 V	+5 V output of the L4974, after the 150 µH inductor L4
C9	10 µF 16 V	Output of the 78L12 regulator, Q1 (regulated +12 V)
C2, C6	220 µF 16 V	Unregulated +12 V and -12 V rails
C59, C83, C105, C120, C131, C139	10 µF tantalum	Bulk decoupling on +5 V
C60, C68	100 µF 25 V	Left and right audio outputs from the audio hybrid to the headphone socket
C62, C78, C118	47 µF, 47 µF, 220 µF	Supply, bypass and output of the LM386 speaker amplifier

The supply works like the A3010's, with an L4974 in place of the L4960: about 23 V at TP1 from the main winding, 5 V from the switching regulator, a regulated 12 V for SCART and the audio hybrid, and ±12 V at 50 mA from a centre-tapped second winding.

Surface-mount electrolytics

The seven surface-mount electrolytics are the parts to inspect closely. Look at the board around each one for residue or darkened solder mask, and check the nearby tracks. Several of them are in the audio stage (C62 and C78 at the speaker amplifier) and the rest are decoupling. Replace a leaking part with a surface-mount electrolytic of the same value and at least the same voltage, after cleaning the board under it.

On Stardot, Ian Jeffray named the early A5000 and the A4 laptop as the Acorn machines whose surface-mount electrolytics should be replaced as a matter of course.

Parts to buy

A3020 main board, electrolytics and tantalums by value

Value	Voltage	Type	Quantity
1000 µF	35 V	Radial electrolytic	2
220 µF	16 V	Radial electrolytic	7
100 µF	25 V	Radial electrolytic	5
47 µF	16 V	Radial electrolytic	6
10 µF	16 V	Radial electrolytic	4
4.7 µF	16 V	Radial electrolytic	1
2.2 µF	50 V	Radial electrolytic	4
47 µF	10 V	Axial electrolytic	3
47 µF	16 V	Surface-mount electrolytic	4
10 µF	16 V	Surface-mount electrolytic	3
10 µF	10 V	Tantalum, leaded	5
10 µF	16 V	Tantalum, surface-mount	1
47 µF	Read from the part	Tantalum, surface-mount	1

Replacement procedure

1. Unplug the machine. Remove the top cover (Appendix E of the A3020 Welcome Guide), the keyboard flexi strips and keyboard, and the metal safety case, then separate its two halves after unplugging the floppy cables and the speaker. Remove the hard disc if fitted (four screws in the lower envelope).
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. For through-hole parts, desolder with a desoldering station or fresh solder and wick. For surface-mount electrolytics, heat both pads together with hot air or two irons; twisting the part off can lift the pads.
5. Fit the replacement with the correct polarity and check every joint for bridges.

Checks after recapping

1. Measure about 23 V at TP1, +12 V at TP2 and -12 V at TP3, and 5 V on the logic (Acorn's nominal figures, 240 V input).
2. Check the power-on self-test completes, ending on a black screen with a memory size message.
3. Test sound, the floppy drive and the hard disc if fitted.
4. Carry out the earth continuity and insulation tests before putting the machine back into use.

Related pages

• Acorn Archimedes A3020
• Acorn Archimedes A3020 Maintenance Guide
• Acorn Archimedes A3020 Troubleshooting Guide
• Acorn Archimedes A3010 Capacitor Guide
• Acorn Archimedes A4000 Capacitor Guide
• Capacitor Failure Symptoms