BBC Master 128 Capacitor Replacement Guide
The BBC Master 128 main board has four aluminium electrolytic capacitors and eight tantalums. Every main board value below comes from the parts list in Acorn's Master series service manual and has been checked against Acorn's circuit diagram 0143,000/C.[1][2] The power supply is a separate unit with no published Acorn data; its capacitors are the ones that fail, and the figures for it below come from repairers' reports.
Visual inspection
[edit | edit source]- On the supply, look at the two X2 mains suppression capacitors at the mains input end of the board. The originals are often RIFA paper capacitors, the older ones in a clear case; these crack with age, and a cracked one can fail with a bang and a cloud of smoke.[3][4][5][6]
- Look for brown residue under the secondary-side electrolytics. One owner found a lot of electrolyte under the orange capacitors on an Acorn-branded Master supply.[7]
- On the main board the electrolytics are not under any significant heat or ripple, and repairers on stardot see no reason to replace them routinely. One names the audio circuit as the only place where they matter much; 1024MAK notes that even the speaker coupling capacitor C18, the one that carries AC, does so too rarely for it to shorten its life; and danielj has not heard of a tantalum failing in a Master.[8]
- Battery leakage damages parts near PL8 and corrodes the plug and its wires; see Battery Explosion, Capacitor or Corrosion Damage and the maintenance guide.
Main board capacitors
[edit | edit source]Aluminium electrolytics
[edit | edit source]| Designator | Value | Position on drawing 0143,000/C |
|---|---|---|
| C18 | 47 µF 16 V | Output of the LM386 audio amplifier IC13 to the speaker plug PL9 and the audio output socket SK13 |
| C71 | 47 µF 16 V | +5VA input filter, after L4 and L5 from PL17 and PL18 |
| C72 | 47 µF 16 V | +5VB input filter, after L6 and L7 from PL19 and PL20 |
| C73 | 47 µF 16 V | +5VC input filter, after L8 and L9 from PL21 and PL22 |
The drawing shows a fifth position, C70, on a +12 V input (PL15 and PL16); it is marked not fitted, and the parts list has no part for it.[2][1] One stardot member counted three electrolytics on his board, so check the board in front of you.[8]
Tantalums
[edit | edit source]| Designator | Value | Acorn part | Position on drawing 0143,000/C |
|---|---|---|---|
| C11 | 10 µF 35 V | 0613,101 | Supply pin of the 146818 clock chip IC11 |
| C14 | 4.7 µF 16 V | 0611,047 | Sound circuit around IC9 (LM324), to −5 V |
| C15 | 10 µF 16 V | 0611,100 | −5 V feed to IC9 through R37 |
| C17 | 10 µF 16 V | 0611,100 | +5VB feed to IC9 through R27 |
| C21 | 47 µF 16 V | 0611,470 | Power-on reset timing (R52, IC19, IC39) |
| C32 | 1 µF 35 V | 0613,100 | Cassette input |
| C49 | 1 µF 35 V | 0613,100 | Across the reference of the analogue-to-digital converter IC49 (VREF, pin 8, with diodes D17 to D19). Acorn's fault-finding text calls C49 the integration capacitor, but on the drawing the capacitor across the converter's CI pins is C36, 33 nF polyester[2][9] |
| C60 | 10 µF 16 V | 0611,100 | Supply to the UHF modulator |
C39, a 47 µF tantalum beside the serial processor IC48, is shown on the drawing as not fitted.[2] A tantalum that fails usually fails short and pulls down the rail it decouples.
Acorn changed C11 in the field. Its repair notes say that a machine that loses its configuration should have C11 replaced by a 100 µF part (0610,100), D8 replaced by a 1N4001 and a 1.8 kΩ resistor fitted between the base and emitter of Q3; the service manual applies the same change to some early computers.[10][11]
Non-polarised capacitors
[edit | edit source]These are ceramic or film parts and seldom need attention. They are listed so that they are not mistaken for the polarised parts, and because Acorn's field fixes change some of them.[1]
| Value and type | Designators | Acorn part |
|---|---|---|
| 10 pF plate ceramic | C19 | 0631,010 |
| 22 pF plate ceramic | C26 | 0631,022 |
| 27 pF plate ceramic | C80 | 0631,027 |
| 33 pF plate ceramic | C6, C7, C83, C84 | 0631,033 |
| 39 pF plate ceramic | C81 | 0631,039 |
| 47 pF plate ceramic | C23, C44, C45, C88, C92 | 0631,047 |
| 100 pF plate ceramic | C3, C85, C86 | 0631,100 |
| 150 pF plate ceramic | C87 | 0631,150 |
| 220 pF plate ceramic | C24, C27 | 0631,220 |
| 270 pF plate ceramic | C62, C63, C64, C69 | 0631,270 |
| 820 pF plate ceramic | C55, C56 | 0630,082 |
| 2.2 nF plate ceramic | C9, C12, C35 | 0629,002 |
| 4.7 nF plate ceramic | C40, C43 | 0629,004 |
| 10 nF plate ceramic | C47 | 0629,010 |
| 10 nF plate ceramic | C91 | 0634,010 |
| 33 nF polyester | C36 | 0650,333 |
| 47 nF | C16 | 0637,473 |
| 100 nF | C2, C10, C22 | 0637,104 |
| 220 nF ceramic | C34, C46 | 0651,224 |
| 330 nF polyester | C4, C79 | 0650,334 |
| 33/47 nF decouplers | C1 and 45 unnumbered positions marked "A" | 0680,005 and 0680,002 |
| 2–22 pF trimmer | VC1 (clock crystal) | 0699,001 |
| 5.5–40 pF trimmer | VC2 (17.7345 MHz colour subcarrier) | 0699,004 |
C27 is the 220 pF part from the 1770 DRQ line to ground that only some boards need; the fault-finding section says there will be no place for it on boards that do not.[12] Acorn's later fixes raise C85 and C86 from 100 pF to 10 nF for machines that crash when warm, and add 100 pF across C30 for Teletext break-up on machines bought before 3 February 1986.[10]
Co-processor boards
[edit | edit source]- The 80186 co-processor board of the BBC Master 512 (drawing 0143,200/C) has four electrolytics: C7, C8 and C9, 100 µF 10 V on the +5 V supply, and C1, 10 µF 10 V in the reset circuit of the Tube ULA.[2]
- Simon Inns's 2019 redraw of the 65C102 board used in the Master Turbo and Master AIV shows only 100 nF decouplers and small ceramics, with no electrolytics.[13]
Power supply
[edit | edit source]Acorn's service manual has no circuit, parts list or ratings for the supply; its fault-finding simply says to try another one.[14] Repairers on stardot report at least two, probably three, designs, from Astec, San Hua Tien and a supply badged Acorn; some have the start-up capacitor on a small daughter board, and some have no RIFA capacitors at all. The supply's designators are its own: its C1, C2 and C9 are not the main board's C1, C2 and C9, which are small ceramic parts.[1][15][16]
X2 suppression capacitors
[edit | edit source]The common Astec supply follows the BBC Micro arrangement, with C1 10 nF and C2 100 nF class X2 across the mains.[3] Replace them with class X2 parts rated 250 V AC or more, not necessarily RIFA, and check the lead spacing before ordering: the Master board has only one set of holes for C2, so a narrower 100 nF part that fits a BBC Micro supply may not fit.[17][5] Owners have found different parts in these positions: 0.22 µF in C1 on one Astec type, and on an Astec 13830 a single 100 nF part rated 400 V DC with no 10 nF part.[3][5] Read the value on the old part and keep the same capacitance.
Electrolytics
[edit | edit source]No designator list for the Master supply has been published. These are the values repairers have read from their own supplies:
| Supply | Start-up (primary side) | Primary reservoirs | Secondary |
|---|---|---|---|
| Astec, no daughter board | C9, 220 µF 25 V | Not reported | Not reported |
| Astec with a side-mounted daughter board | C101, 220 µF, on the daughter board | Not reported | Not reported |
| San Hua Tien (board marked 194V0L6) | Not listed (C9 already replaced) | C7, C8: 100 µF 250 V | C12, C13: 220 µF 25 V; C15: 2200 µF 25 V; C18, C19, C20: 2200 µF 16 V; one 470 µF 25 V |
| Badged Acorn | 220 µF 25 V | Two 100 µF, 250 V as originally fitted | Three 2200 µF 16 V; one 2200 µF 25 V; one 470 µF 25 V; two 220 µF 25 V |
Sources: the threads cited in this section.[15][4][16][7] The two primary reservoir capacitors are in series across the rectified mains, and each should read about 170 V DC with the supply running.[16]
MJS, who fitted 400 V parts in the primary reservoir positions, notes that if one of the bleeder resistors goes open circuit the full rectified mains can appear across one 250 V capacitor.[7]
Start with the start-up capacitor. Repairers on stardot replace it first, with a 105 °C part, because an aged one causes slow starting and low or unstable outputs. A supply that holds 5 V unloaded but ticks or cycles once connected usually needs its output capacitors, and in one supply the ticking after a full recap was a cracked leg on the voltage reference IC1 on the daughter board.[18][19] If the supply still starts slowly with a new C9, measure the 180 kΩ resistors R2 and R3 that charge it; one went from 180 kΩ to 275 kΩ in a supply that then started instantly once both were replaced.[20]
Procedure
[edit | edit source]- Unplug the machine. Turn it over, remove the four screws marked FIX (the rear two are longer) and lift off the lid.[14][21]
- Pull the seven supply leads off the main board, noting them: three red to the connectors marked +5V, three black to 0V and the purple to −5V. Acorn warns that the supplies short-circuit if any two are reversed on refitting.[14]
- Remove the three supply screws under the case, the screw holding the keyboard to the supply and the screw holding the main board's back plate to the supply, then lift the supply out. Removing the keyboard first makes this easier. Refit with the same M3 × 6 mm screws.[14][19]
- Discharge the primary reservoir capacitors through a resistor and confirm 0 V with a meter before touching the primary side.
- The DC output wires pass through a strain-relief bush in the case; owners use strain-relief bush pliers to remove and refit it.[22] To change only the X2 capacitors, some owners cut the cable tie on the mains wires and pull the two spade terminals off the switch instead.[17]
- Record each capacitor's value, voltage and polarity, then replace it with the same value at the same or a higher voltage rating.
- Test the supply on a load before reconnecting the board: a 12 V 21 W car bulb across one red and one black lead should hold +5 V between 4.85 and 5.15 V.[16] Acorn's test kit for the Master lists a 5 Ω 5 W resistor for the same purpose.[23]
See Capacitor Failure Symptoms and Recommended Tools.
Related pages
[edit | edit source]- BBC Master 128
- BBC Master 128 Maintenance Guide
- BBC Master 128 Troubleshooting Guide
- BBC Micro Model A/B Capacitor Replacement Guide, for the BBC Micro supply of the same pattern
- BBC Master Series Documentation
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 Acorn Computers, British Broadcasting Corporation Master Series Microcomputer Service Manual, part no. 0443,004, issue 1, April 1986, parts list, pp. 57–61 (capacitor items 82–120 on pp. 58–59, with Acorn part numbers), and component locations, pp. 62–69. The PDF's text was produced by OCR.
- ↑ 2.0 2.1 2.2 2.3 2.4 Acorn Computers, Master 128 main PCB circuit diagram, drawing 0143,000/C, sheets 1 and 2, and 80186 co-processor drawing 0143,200/C, in File:Acorn BBC Master Series Technical Drawings.pdf (PDF pages 1, 2 and 6), described on BBC Master Series Documentation. Sheet 1: C21 47 µF tantalum on the power-on reset (IC19, IC39), C60 10 µF tantalum on the modulator supply. Sheet 2: C11 10 µF tantalum on the 146818 supply; C14 4.7 µF, C15 and C17 10 µF tantalums around IC9; C18 47 µF electrolytic between the LM386 (IC13) and the speaker; C32 1 µF tantalum on the cassette input; C39 47 µF tantalum not fitted; the supply input filters PL15 to PL23 with L2 to L10 (6.8 µH), C61 to C69 (270 pF) and C70 to C73 (47 µF electrolytic, C70 on +12 V not fitted).
- ↑ 3.0 3.1 3.2 stardot.org.uk, "BBC Master 128 Astec PSU - Cap Advice" (June 2022, retrieved 2026-10-02). 1024MAK: C1 10 nF X2 and C2 100 nF X2, any reputable maker, check the lead pitch. Gelpack found 0.22 µF in the C1 position of two supplies of one Astec type; danielj: one version of the Master supply has no paper RIFA capacitors and can be left alone if it works. Secondary source.
- ↑ 4.0 4.1 stardot.org.uk, "BBC Master PSU - No C9" (December 2020, retrieved 2026-10-02). An Astec Master supply with the start-up capacitor on a daughter board numbered C101 (220 µF); 1024MAK: daughter boards restart their numbering in the next hundred, and the start-up capacitor is the one not on the secondary side. Secondary source.
- ↑ 5.0 5.1 5.2 stardot.org.uk, "How to recap Master PSU, Astec 13830" (January–February 2019, retrieved 2026-10-02). Posts by 1024MAK: the X2 capacitors sit at the input fuse end of the board; not every Master supply uses RIFA parts, and it is only RIFA-made ones that normally need replacing; 10 nF may be marked 0.01 µF and 100 nF 0.1 µF; a replacement must be class X2, 250 V AC or more. VectorEyes found one 100 nF part in this supply, a 400 V DC type, and no 10 nF part; the start-up capacitor is on a small board mounted at right angles to the main board. Secondary source.
- ↑ Whytehead, Chris. "Master Turbo", Chris's Acorns (retrieved 2026-10-02). His Master's supply failed with "a sizzling sound, a cloud of evil smelling smoke".
- ↑ 7.0 7.1 7.2 stardot.org.uk, "BBC Master Power Supply Questions" (December 2020, retrieved 2026-10-02). 1024MAK: three bleeder resistors in series discharge and balance the primary capacitors and feed the start-up capacitor C9 (220 µF); R2 or R3 (180 kΩ) sometimes fail. MJS: R2 found open circuit; annotated photograph of the electrolytics in his Acorn-branded Master supply, with electrolyte found under the orange secondary capacitors. Secondary source.
- ↑ 8.0 8.1 stardot.org.uk, "Master 128 Mainboard Replacement Caps" (November 2022, retrieved 2026-10-02). wiggy: main board electrolytics and tantalums are under no significant stress and, with the possible exception of the audio circuit, not critical; OCR copies of the service manual parts list differ from one another. danielj: three electrolytics on his board; tantalum failures in a Master are rare enough that he has not heard of one. 1024MAK: none of the electrolytics sees unusual heat or ripple; C18, the speaker coupling capacitor, carries AC only occasionally, so its life is not significantly affected. Secondary source.
- ↑ Acorn Computers, Master Series Microcomputer Service Manual, 0443,004, 7.5.1, p. 44: "Has it got an incorrect or faulty integration capacitor C49?"; about 1.8 V reference across IC49 pins 8 and 9.
- ↑ 10.0 10.1 Acorn Computers, Master Series Hardware Specification, hosted as File:Acorn BBC Master Series Hardware Specification.pdf, PDF p. 50, "Additional information relating to Master Repair Guide": loss of configuration (C11 to 100 µF 0610,100, D8 to 1N4001 0794,001, 1K8 0502,182 between base and emitter of Q3); break-up in mode 7 (100 pF added to C30, machines bought before 3.2.86); crashes after a period of time, found by heating the 74S04 IC43 to about 55 °C (C85 and C86 from 100 pF to 10 nF, 0629,010); no colour on composite video (470 pF between the east legs of R137 and R153).
- ↑ Acorn Computers, Master Series Microcomputer Service Manual, 0443,004, 7.1.9, p. 36: "On some early computers, C11 needs to be 100uF instead of 10uF, D8 should be changed to IN4001, and Q3 should have a 1k8 resistor between its base and emitter."
- ↑ Acorn Computers, Master Series Microcomputer Service Manual, 0443,004, 7.8.3 to 7.8.5, p. 45: C27, 220 pF from DRQ to ground, fitted on some computers; issue 1 boards need a small capacitor there.
- ↑ Simon Inns, Acorn 65C102 Second Processor, KiCad schematic, 5 sheets, rev. 1.0, 2 April 2019, CC BY-SA 4.0, hosted as File:Acorn 65C102 Internal Co-Processor Schematic (Simon Inns).pdf. Sheet 1: Tube ULA IC1, 65SC102 IC8, 16 MHz crystal with C1 and C2 (15 pF); sheet 4: decouplers A1 to A21, 100 nF.
- ↑ 14.0 14.1 14.2 14.3 Acorn Computers, Master Series Microcomputer Service Manual, 0443,004, 3 "Disassembly and assembly", pp. 6–7, and 7.1.1, p. 32 ("Try replacing the power supply anyway as the fuse may have blown").
- ↑ 15.0 15.1 stardot.org.uk, "Master PSU variants" (July–August 2023, retrieved 2026-10-02). 1024MAK: "at least two designs, likely three", similar to the BBC Micro supply, with a minimum load affecting the unregulated +12 V and −5 V outputs, a crowbar SCR and zener on +5 V, one main switching transistor, and the start-up capacitor equivalent to C9 in the BBC Micro supply (220 µF 25 V). Ukwebb: two variants, one with RIFA (or later polyester) capacitors and one without, with different side-mounted boards. Secondary source.
- ↑ 16.0 16.1 16.2 16.3 stardot.org.uk, "Master 128 and the long slow beep of doom" (May–October 2019, retrieved 2026-10-02). 1024MAK: test the supply with a 12 V 21 W car bulb across a red and a black lead, aiming for 5.0 V and accepting 4.85–5.15 V; renew RIFA capacitors in C1 and C2 and the start-up capacitor C9; R2 and R3 (180 kΩ) can go high; the two primary capacitors should each read about 170 V DC when running; the main transistor Q2 is often a Hitachi 2SC2928. FelisSapien: a San Hua Tien supply (board marked 194V0L6) with C7 and C8 100 µF 250 V, C12 and C13 220 µF 25 V, C15 2200 µF 25 V, C18 to C20 2200 µF 16 V and one 470 µF 25 V. Secondary source.
- ↑ 17.0 17.1 stardot.org.uk, "BBC Master PSU caps blown" (January 2012, retrieved 2026-10-02). danielj: the BBC Micro supply has an extra hole for C2 so a narrower part fits, but the Master supply does not; replace C9 at the same time. station240: the X2 capacitors can be reached by cutting the cable tie on the mains wires and pulling the two spade terminals off the switch. Secondary source.
- ↑ stardot.org.uk, "BBC Master Power Supply Ticking" (September 2024 – January 2025, retrieved 2026-10-02). 1024MAK: replace the start-up capacitor C9 (normally 220 µF) first, with a 105 °C part, as it causes low outputs, slow starting and unstable outputs; then the secondary capacitors. Ukwebb: ticking is down to the output capacitors; a supply that holds 5 V unloaded but cycles on load usually needs them. somerville535: ticking after a full recap cured by resoldering a cracked leg on IC1, the voltage reference on the daughter board. Kepler: seven secondary capacitors, five of them orange, three of 2200 µF. Secondary source.
- ↑ 19.0 19.1 stardot.org.uk, "Replacing the caps in a Master PSU" (December 2019 – January 2020, retrieved 2026-10-02). tricky's removal sequence (seven board connectors, three screws underneath, the screw to the keyboard and the screw to the back plate); derek replaced C1, C2 and C9; tricky reports more than 40 Master supplies opened. Secondary source.
- ↑ stardot.org.uk, "Master 128 PSU schematic" (September–October 2017, retrieved 2026-10-02). dominicbeesley: slow start-up cured by replacing R2 and R3, one of which had drifted from 180 kΩ to 275 kΩ, after replacing them with 3 W parts. Secondary source.
- ↑ Acorn Computers, Master Series Battery Unit, part no. 0443,900, issue 3, May 1993, hosted as File:Acorn BBC Master Series Battery Unit 0443,900 Issue 3.pdf.
- ↑ stardot.org.uk, "Master PSU recap" (October 2020, retrieved 2026-10-02). philb replaced the three primary electrolytics and two X capacitors; VectorEyes and KenLowe on the strain-relief bush and the pliers that remove it; component leads bent over on a solder-flooded underside. Secondary source.
- ↑ Acorn Computers, Master Series Microcomputer Service Manual, 0443,004, test equipment, p. 30: screwdrivers, pliers, cutters, 10 A multimeter, RGB monitor, logic probe and a 5 Ω 5 W resistor; a 100 MHz oscilloscope, frequency counter and known good computer speed the repair.