Acorn Atom Capacitor Replacement Guide
The Acorn Atom main PCB (part 202,000) has eight small aluminium electrolytics and twenty ceramic capacitors. The values below are from the parts list in Acorn's Acorn Atom Technical Manual, and the circuit positions are from Acorn's circuit diagram.[1][2] The case holds no mains wiring and no CRT; the machine runs on 8 V DC from an external adaptor, and the largest capacitor on the board is 22 µF.
Acorn allows any value from 10 to 47 µF, rated 10 V or more, in each electrolytic position, and notes that substitute parts were sometimes supplied in kits, for example 100 nF in place of 47 nF.[1] A board that differs slightly from the list may be original.
Visual inspection
[edit | edit source]Look at each electrolytic for a bulged or vented top, electrolyte residue on the board, a split sleeve or a cracked joint. See Capacitor Failure Symptoms for what failure looks like.
Acorn Atom capacitor list
[edit | edit source]| Designator | Value | Minimum rating | Position on the circuit diagram |
|---|---|---|---|
| C2 | 22 µF | 10 V | Video coupling into the UHF modulator |
| C3 | 22 µF | 10 V | Modulator supply, after choke L1 |
| C4 | 22 µF | 10 V | MC6847 Y output to the video amplifier Q1 |
| C5 | 22 µF | 10 V | Loudspeaker coupling (the speaker leads go to the holes beside C5) |
| C8 | 22 µF | 10 V | Tape input amplifier IC46 |
| C11 | 10 µF | 10 V | BREAK key reset circuit |
| C12 | 22 µF | 10 V | 8 V DC input at SK3 |
| C13 | 22 µF | 10 V | +5 V supply, IC53 side |
| Designator | Value | Position on the circuit diagram |
|---|---|---|
| C1 | 10 nF | X1 (3.58 MHz) oscillator |
| C6 | 10 nF | Tape output |
| C7 | 47 or 100 nF | Tape input amplifier |
| C9 | 22 or 47 nF | Tape input coupling |
| C10 | 10 nF | X2 (4.00 MHz) oscillator |
| C14–C22 (9) | 47 nF | +5 V decoupling, IC53 side |
| C23–C28 (6) | 47 nF | +5 V decoupling, IC54 side |
Values: Technical Manual parts list.[1] Positions: circuit diagram.[2] The ceramics rarely fail with age; replace one only if it is cracked or measures short.
Replacement parts
[edit | edit source]| Value | Quantity | Positions |
|---|---|---|
| 22 µF | 7 | C2, C3, C4, C5, C8, C12, C13 |
| 10 µF | 1 | C11 |
Acorn's minimum is 10 V. A 16 V or 25 V part of the same value fits the same holes and gives more margin, which matters most at C12, which sits across the unregulated adaptor input.[1][2] Use radial 105 °C parts.
Replacement procedure
[edit | edit source]- Unplug the adaptor and open the case (Acorn Atom General Maintenance#Opening the case). The capacitors are on side 2 and soldered on side 1, so the board has to come out of the top moulding.[1]
- Note the polarity of each electrolytic. The board marks the positive end with a +; the capacitor marks its negative end with a band or arrow, which must face away from the +.[1]
- Desolder each capacitor from side 1. The board is double-sided and through-plated, so keep heating short and clear each hole with wick or a desoldering pump.
- Fit the new capacitor with its positive lead to the +, solder on side 1 and trim the leads.[1]
- Check for bridges and good fillets before reassembly.
After recapping
[edit | edit source]- Power up on a known-good supply (8 V DC, or 5 V if LK6 and LK7 are fitted).
- Press BREAK and check the sign-on screen appears.[1]
- Measure +5 V at both regulator outputs.
- Check the picture, the loudspeaker, the keyboard and a tape load. Each electrolytic except C12 and C13 sits in one of these circuits, so a fault that appears after the work points at the capacitor in that circuit; check its polarity first.
Related pages
[edit | edit source]- Acorn Atom
- Acorn Atom General Maintenance
- Acorn Atom Troubleshooting Guide
- Acorn Atom Technical Manual
- Capacitor Failure Symptoms
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Acorn Computers, Atom Technical Manual, issue 2, October 1980, hosted on this wiki. Parts list pp. 6–7: capacitors C1–C28 and the note "The electrolytic capacitors (marked *) may be replaced by any value in the range 10 to 47uF. They should be rated at 10 Volts or greater"; capacitor marking and substitution notes p. 4; fitting electrolytics p. 10; loudspeaker beside C5 p. 11; power supply p. 13.
- ↑ 2.0 2.1 2.2 Acorn Computers, Circuit Diagram for Atom Microcomputer, drawn 28 February 1980, revised to issue 3 on 14 January 1981. Hosted on this wiki as File:Acorn Atom circuit diagram.png.
