BBC Micro Model A/B Capacitor Replacement Guide
The main board of the BBC Micro Model A/B has fifteen polarised capacitors: six aluminium electrolytics and nine tantalums. Every main-board value below is from the component location table and parts list in Acorn's BBC Microcomputer Service Manual, which covers main board issues 1 to 7.[1] Acorn published no component values for the power supply; the supply sections below say where each figure comes from.
Safety
[edit | edit source]The main board runs from +5 V and −5 V only; +12 V goes to the auxiliary power connector, not to the board.[2]
The switch-mode supply rectifies the mains directly. Its primary reservoir capacitors, C6 and C8 on Acorn's diagram, are connected in series across the rectified mains, about 325 V DC on 230 V mains, and they hold that charge after the machine is unplugged.[2] Before touching the primary side, unplug the machine, discharge C6 and C8 through a resistor of about 1 kΩ 5 W, and confirm 0 V with a meter. The early linear supply has its mains transformer first, so its smoothing capacitors are on the low-voltage secondary side.[3]
Acorn's instruction to dealers was to exchange a faulty supply, because any repair, including a new mains lead, had to be followed by an earth continuity test at not less than 10 A and a 1500 V AC dielectric withstand test between live and neutral and earth.[2]
Board issues
[edit | edit source]Issues 1, 2, 3, 4 and 7 were produced; issues 5 and 6 never went into production. The issue number is printed on the board.[4] Acorn records these capacitor changes:[4]
| Designator | Change | When |
|---|---|---|
| C34 | Cassette output coupling capacitor increased from 47 nF to 220 nF (listed as 200/220 nF) | Between issues 3 and 4 |
| C42 | At one stage a fixed capacitor with a trimmer in parallel to improve the 16 MHz waveform, so values vary between boards; 33 pF from issue 7 | From issue 3; final value at issue 7 |
| C51 | Select-on-test, 15–22 pF, to set the colour burst frequency to 4.4336 MHz ±100 Hz | From issue 3 |
| C58 | The 470 pF capacitor from the base of Q7 is connected to the emitter of Q9 through new link S39 | Issue 4 |
| C59 | 220 nF added in series with R90 to AC-couple the cassette log amplifier | Issue 4 |
Acorn marks C4, C19, C22 and C44 as not used. A part in one of those positions is a later modification.[1]
Main board polarised capacitors
[edit | edit source]| Value | Type | Designators | Acorn part no. |
|---|---|---|---|
| 4.7 µF 16 V | Radial | C2 | 635,047 |
| 10 µF 16 V | Radial | C5, C9 | 635,100 |
| 47 µF 10 V | Axial | C14, C16, C26 | 621,470 |
| Value | Designators | Acorn part no. |
|---|---|---|
| 1 µF 35 V | C27 | 613,100 |
| 4.7 µF 10 V | C28, C60 | 610,005 |
| 10 µF 10 V | C18, C30, C47, C57 | 610,010 |
| 47 µF 10 V | C36, C54 | 610,047 |
Acorn's parts list is for a Model B with the disc and Econet interfaces fitted. C18 is supplied with the Econet upgrade, so a machine without Econet has no part there.[5] A tantalum that fails usually fails short. A shorted tantalum across +5 V pulls the rail down for the whole board.
Main board non-polarised capacitors
[edit | edit source]The rest are ceramic or film types. They are listed so that they are not mistaken for electrolytics.[1]
| Value and type | Designators | Acorn part no. |
|---|---|---|
| 10 pF plate ceramic | C12 | 631,010 |
| 15/22 pF plate ceramic, select on test | C51 | 631M015/022 |
| 33 pF plate ceramic | C37, C42 | 631,033 |
| 39 pF plate ceramic | C56 | 631,039 |
| 47 pF plate ceramic | C50 | 631,047 |
| 47 pF plate ceramic | C43, C46 | 632,047 |
| 100 pF plate ceramic | C53, C55 | 631,100 |
| 150 pF plate ceramic | C49 | 631,150 |
| 220 pF plate ceramic | C41 | 631,220 |
| 270 pF plate ceramic | C48 | 631,270 |
| 390 pF plate ceramic | C52 | 630,039 |
| 470 pF plate ceramic | C58 | 633,047 |
| 820 pF plate ceramic | C31, C35 | 630,082 |
| 1 nF plate ceramic | C13 | 629,001 |
| 2.2 nF plate ceramic | C1, C3, C7, C11, C17, C29 | 629,002 |
| 2.2 nF plate ceramic | C38, C39 | 634,002 |
| 4.7 nF plate ceramic | C32, C33 | 634,004 |
| 10 nF plate ceramic | C10, C23, C40, C45 | 629,010 |
| 33 nF polyester | C25 | 650,333 |
| 47 nF disc ceramic | C20 | 628,470 |
| 100 nF disc ceramic | C6, C8, C15, C21, C24 | 628,101 |
| 200/220 nF | C34 | 651,204/224 |
| 220 nF | C59 | 651,224 |
| 2–22 pF trimmer | VC1 | 699,001 |
| 33/47 nF decoupler, 76 off | Unnumbered, marked "A" | 680,001 |
Power supply
[edit | edit source]Which supply
[edit | edit source]Most machines have a switch-mode supply, which Acorn's parts list gives as one item, "Power supply unit, ASTEC SMPS", part 103,003. A small number of early machines have a linear supply with a mains transformer and separate regulators, so +5 V can be present on two of the three pairs of leads and missing on the third.[2] The linear supply has a black painted case and no auxiliary power socket, and its −5 V lead is brown.[6] Inside it are a toroidal transformer, a rectifier, 4700 µF smoothing capacitors and three 7805 regulators, one for each +5 V pair.[3] Acorn and Derrick say the three +5 V points are joined by tracks on the main board, but Martin Jones found them unconnected on his issue 2 board.[2][6][3]
Some switch-mode supplies were built by Wong's Electronics. One stardot member's Wong's WA1020 had no RIFA suppression capacitors. Its start-up capacitor, 220 µF 10 V, is numbered C20 (C9 on the Astec board), and its C6 is a 2200 µF 16 V output capacitor, so the Astec designators below do not apply to that supply.[7] 1024MAK on stardot has noted that the printed Astec diagram differs in places from the production board, so read designators from the board itself.[8]
Mains suppression capacitors
[edit | edit source]Acorn's diagram shows the mains input through the switch and fuse F1, then R1 in the live line, C1 across the line, the common-mode choke T1, C2 and C3 across the line and C4 to earth, then the 230/115 V link and bridge rectifier DB1. The diagram prints no values.[2]
The two X2 suppression capacitors, C1 and C2 on the Astec board, are the parts that most often fail in this supply.[9][10] They are not electrolytics. The originals are usually RIFA parts, whose resin cases crack with age, and a cracked one can fail with a bang and a cloud of smoke.[11] Retro Kit and Tynemouth Software both give the pair as a 10 nF and a 100 nF X-class part; Retro Kit's original 100 nF part was rated 250 V AC, and modern replacements are rated 275 V AC.[11] Sources disagree on which designator holds which value: in a 2013 stardot thread one member lists C1 as 0.01 µF and C2 as 0.1 µF, while another gives the RS stock numbers the other way round.[9] Read the value printed on the old part.
The X2 capacitors are for suppression only, and the machine runs without them, though it then puts more interference back onto the mains.[11] Acorn's diagram also shows C3 and C4; no published source gives their values.
Supply electrolytics
[edit | edit source]The supply has its own designators: its C9 is not the main board's C9. The only published list of the Astec supply's electrolytics is a set of replacement parts posted to stardot in 2013, with RS stock numbers. It is one member's choice of replacements, not a reading of the original parts, so check the values and ratings on your own board before ordering.[9]
| Designators | Value listed | Position on Acorn's diagram |
|---|---|---|
| C6, C8 | 100 µF 250 V | Primary reservoir |
| C9 | 220 µF 25 V | Start-up, primary side |
| C16, C19, C20 | 1000 µF 16 V | Secondary output |
| C17 | 2200 µF 16 V | Secondary output |
| C18, C21 | 100 µF 25 V | Secondary output |
In the same thread RetroClinic advises replacing C9 every time and the others only when needed, and warns that the capacitor on the 5 V output after the choke must be a low-ESR type or the supply hunts audibly under load.[9] In a later thread one restorer reports that 28 of more than 30 BBC supplies he had repaired needed only C1, C2 and C9, and another describes a supply whose 5 V rail stayed at 3–4 V after the X2 capacitors were replaced until C9 was changed, although the old C9 had measured correct capacitance and 0.3 Ω ESR.[10] Tynemouth Software also replaces the start-up electrolytic with the X2 capacitors, because it can cause slow or intermittent start-up.[11]
Expected rails
[edit | edit source]| Rail | Rating | Notes |
|---|---|---|
| +5 V | 3.75 A | Fed to three points on the main board, connected together on the board |
| −5 V | 100 mA | Main board |
| +12 V | 1.25 A | Auxiliary connector only; the amount available depends on what is fitted inside |
Measure at the push-on connectors on the board, and check +5 V on each of the three pairs, particularly with the linear supply.[2] Acorn's service manual gives no tolerance for these rails.
If −5 V is missing, the processor, memory and video still work normally, but the cassette, sound, speech and RS423 interfaces do not. A machine that boots and displays but is silent and will not load from tape should have its −5 V lead checked first.[2]
Procedure
[edit | edit source]- Unplug the machine and remove the mains lead.
- Remove the two rear and two underside screws and lift the lid, keeping the two spire clips on the lid.[12]
- For the main board, unplug the keyboard (17-way), loudspeaker (2-way) and, if fitted, serial ROM (10-way) connectors, unplug the seven power leads, disconnect the two wires to the BNC socket SK2, remove the board's fixing screws (four, or five or seven on later issues) and slide the board forwards and lift it out at the rear.[12]
- For the switch-mode supply, unplug its leads, remove its three screws on the underside of the case, and discharge the primary reservoir capacitors (C6 and C8 on the Astec board) before working on the primary side.[12]
- Record the value, voltage and polarity of each part, desolder it and fit a part of the same value and at least the same voltage, with the same polarity.
- Refit, using the same type of screw for the supply, and check +5 V on each pair of leads and −5 V before reconnecting the main board.
See Capacitor Failure Symptoms, Recommended Tools and Battery Explosion, Capacitor or Corrosion Damage.
Related pages
[edit | edit source]- BBC Micro Model A/B · BBC Micro Model A/B General Maintenance · BBC Micro Model A/B Troubleshooting Guide
- BBC Microcomputer Service Manual October 1985
- BBC Micro Model B+ · BBC Micro B+ Capacitor Replacement Guide
References
[edit | edit source]- ↑ 1.0 1.1 1.2 Acorn Computers, BBC Microcomputer Service Manual, part no. 0433,001, issue 1, October 1985, Section 1: 8.4 "Capacitors", pp. 58–59, and 9.8 "Parts list, main circuit board model B + disc + Econet", pp. 82–83. Designator, value, type, circuit-diagram grid reference and Acorn part number for every capacitor on the main board.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Acorn Computers, BBC Microcomputer Service Manual, Section 1: 3.14 "Power supply", p. 24; 6.3 "Fault isolation", p. 39; 9.7 power supply circuit diagram. Output ratings, switch-mode and linear supplies, seven push-on connectors, exchange-only policy, earth continuity and dielectric withstand tests.
- ↑ 3.0 3.1 3.2 Martin Jones, "The BBC Micro linear power supply" (30 August 2013, retrieved 2026-10-01). An issue 2 machine's linear supply: toroidal transformer, rectifier, 4700 µF smoothing capacitors (10.5 V measured), three 7805 regulators and 1 µF 35 V tantalums; rated 2.25 A at 5 V; black case, later supplies gold-coloured. Secondary source.
- ↑ 4.0 4.1 Acorn Computers, BBC Microcomputer Service Manual, Section 1, 5.3 "Circuit modifications from issue 1 to issue 7", pp. 36–37.
- ↑ Acorn Computers, BBC Microcomputer Service Manual, Section 1, 4.5 "Modification E, add Econet interface to Model A", p. 29 (the notes also cover the Model B): parts include a 10 µF tantalum capacitor, C18.
- ↑ 6.0 6.1 A. D. Derrick, A Hardware Guide for the BBC Microcomputer, chapter 3.1, "The power supply unit", bbc.nvg.org (retrieved 2026-10-01). Linear supply on early machines, mainly issues 1 and 2, with a mains transformer, bridge rectifier and 7800-series regulators; black case and no auxiliary socket; brown −5 V lead. Secondary source.
- ↑ stardot.org.uk, "BBC Model B repair log. Well, actually the Power Supply..." (retrieved 2026-10-01). A Wong's Electronics WA1020 supply with no RIFA capacitors; start-up capacitor C20 (220 µF 10 V); C20, C6 (2200 µF 16 V) and C3 (220 µF 16 V) replaced. Secondary source.
- ↑ stardot.org.uk, "part list bbc psu ?" (retrieved 2026-10-01). 1024MAK: "there are some differences between the schematic circuit diagram and the actual circuit on the PCB". Secondary source.
- ↑ 9.0 9.1 9.2 9.3 stardot.org.uk, "Beeb PSU capacitors" (March–April 2013, retrieved 2026-10-01). Post by "IBM portable PC": RS numbers for an Astec supply, C1 0.01 µF 275 V AC X2, C2 0.1 µF X2, C6 and C8 100 µF 250 V, C9 220 µF 25 V, C16, C19, C20 1000 µF 16 V, C17 2200 µF 16 V, C18, C21 100 µF 25 V, chosen as replacements (Rubicon, some higher-temperature types). Post by flaxcottage giving RS 210-500 (0.1 µF) for C1 and 311-1024 (0.01 µF) for C2. Post by retroclinic: replace C9 every time, others only when needed; a low-ESR part is required on the 5 V post-choke output or the supply hunts audibly under load. Secondary source.
- ↑ 10.0 10.1 stardot.org.uk, "BBC Micro Power Supply advanced repair" (2020–2021, retrieved 2026-10-01). Posts by tricky (28 of over 30 supplies needed only C1, C2 and C9) and GoodToGo! (low 5 V rail cured by replacing C9; old part measured 0.3 Ω, new 0.1 Ω). Secondary source.
- ↑ 11.0 11.1 11.2 11.3 "All about the BBC Micro X2 capacitors", Retro Kit; and Dave Curran, "BBC Micro Power Supply Repair", Tynemouth Software, 15 May 2017 (both retrieved 2026-10-01). Two X-class capacitors, 0.01 µF and 0.1 µF; original 100 nF part rated 250 V AC, modern replacement 275 V AC; the capacitors are for noise suppression only and the machine runs without them. Secondary sources.
- ↑ 12.0 12.1 12.2 Acorn Computers, BBC Microcomputer Service Manual, Section 1, 1.4 "Mechanical assembly of case etc", p. 5.