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BBC Master Compact Capacitor Guide

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Master Compact main board in the keyboard unit (photo: Chris Whytehead, CC BY-SA 3.0)

The BBC Master Compact has capacitors in two places: the main board in the keyboard unit, and the power supply board in the disc unit (monitor stand). The main board values below are from the parts list in Acorn's Compact service manual.[1] Acorn published no values for the supply, and the scan of the service manual has no circuit diagram for either board, so the supply figures come from repairers.

Visual inspection

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  • In the disc unit, look at the X2 suppression capacitor on the mains input of the supply board. RIFA paper types crack and fail with smoke.[2]
  • Look for bulging or leaking electrolytics on the supply's output side.[2]
  • The Compact has no battery, so there is no battery leakage to look for on the main board.[3]

Main board

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Main board aluminium electrolytics (parts list)
Designators Value Type Acorn part
C1, C8, C19, C20, C21, C25, C32 22 µF 16 V Axial 0622,220
C6 47 µF 10 V Axial 0621,470
C16 47 µF 16 V Radial 0635,470

The parts list has no tantalum capacitors. Acorn's hints say the north end of C1, in the north-west corner of the board, makes a convenient 0 V point for an oscilloscope probe, and warn that the large copper area in the south-west of the board looks like an earth plane but is +5 V.[4] Acorn gives no circuit positions for the other electrolytics in the hosted scan; check polarity on the board before removing each one.

Main board non-polarised capacitors (parts list)
Value and type Designators Acorn part
10 pF plate ceramic C27 0631,010
22 pF plate ceramic C13, C18 0631,022
33 pF plate ceramic C26 0631,033
47 pF plate ceramic C24, C30 0631,047
100 pF plate ceramic C17 0631,100
220 pF plate ceramic C10, C23 0631,220
1 nF plate ceramic C4 0630,100
2.2 nF plate ceramic C2, C5 0630,220
10 nF plate ceramic C29 0629,010
47 nF decoupler C3, C7, C15, C31 0680,004
100 nF axial decoupler 43 unnumbered positions marked "A" 0680,101

C2 sets the write cycle time of the EEPROM IC24 together with R1; Acorn's last check for a configuration memory fault is this RC timing circuit.[3][5] One owner replaced C3 and C7 with ceramic parts while repairing a Compact; 1024MAK pointed out that they are not polarised on Acorn's schematic and are not tantalums.[6]

Power supply

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The supply board is in the disc unit. Acorn's service guide covers only replacing the board, its cables and the fuse, and told dealers not to fault-find on the supply.[7] Repairers recap it like any other switch-mode supply:[2][8]

Compact supply capacitors reported on stardot (not Acorn data)
Designator Value Notes
C1 0.1 µF (100 nF) 275 V AC, class X2 RIFA paper type on 1024MAK's supply; replace with a class X2 film part
C10 220 µF 10 V Primary-side start-up capacitor; replacements of 220 µF 16 V or 25 V, 105 °C, have been fitted

A start-up capacitor that has lost value makes the supply slow to start: one measured 65 µF and gave a one- to two-minute delay, and the supply started at once with a new part. If a new C10 does not cure slow starting, measure the high-value resistors (100 kΩ or more) that charge it.[8] Other output-side electrolytics have been found bulging, but no list of their values has been published.[2]

Procedure

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Warning: the disc unit contains mains voltage even with its switch off while the lead is plugged in; Acorn says to unplug it before dismantling.[7]

  1. Switch off and unplug the mains lead from the wall and from the disc unit. Unplug the 5 V lead and the disc data cable from the keyboard unit.
  2. Pull off the plastic front panel (thumbs underneath, fingers on top, pull towards you and down), then the back panel and the two side panels.
  3. Turn the unit over and remove the outermost screws on the underside (four or six, depending on the model). Turn it back, slide the top backwards and pull the main unit clear, without damaging the disc data cable or the 5 V cable.
  4. To take out the supply board, undo the four screws on top of the board and unplug the mains input connector, the DC output connector and the earth tag. Refit any metal screen that was fitted around the supply.
  5. The two larger primary capacitors at the top right of the board can hold a high-voltage charge after power-off, especially if their discharge resistors have failed. Discharge them and confirm 0 V before working on the board. In use each carries about 162–170 V DC, 325–340 V across the pair.[8]
  6. Replace C1 and C10, and any bulging output capacitor, with the same value and an equal or higher voltage rating.
  7. Reassemble with the ventilation slots of the top at the back. Acorn required an earth continuity test (under 0.1 Ω) and a 1.5 kV AC insulation test after any work on mains parts.[7]
  8. Check the 5 V output before reconnecting the keyboard unit.

To recap the main board, remove the keyboard unit's four underside screws, lift the top from the back, rock the keyboard connector off the board, take out the five board screws and the expansion connector cover, and lift the board out from the left-hand end.[9]

See Capacitor Failure Symptoms and Recommended Tools.

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References

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  1. ↑ Acorn Computers, British Broadcasting Corporation Master Series Compact Microcomputer Service Manual, part no. 0458,002, issue 1, October 1986, hosted as File:Acorn BBC Master Compact Service Manual 0458,002.pdf, parts list items 60–87, printed p. 44: capacitor values, types and Acorn part numbers for the 0258,000 main board. The PDF's text was produced by OCR.
  2. ↑ 2.0 2.1 2.2 2.3 stardot.org.uk, "Refurb process for Master Compact Base Unit PSU" (February–March 2025, retrieved 2026-10-02). 1024MAK: the minimum service is the RIFA X2 capacitor and the small primary-side start-up electrolytic; on his supply C1 is 0.1 µF 275 V AC class X2 and C10 is 220 µF 10 V, replaced with 220 µF 16 V 105 °C; he uses metallised polypropylene X2 parts; photographs after "the white smelly smoke". egel: at least two types of supply with different refurbishment. 1980s_john: a cracked RIFA and a bulging electrolytic on the output replaced on his Compact. Secondary source.
  3. ↑ 3.0 3.1 Acorn Computers, Master Series Compact Microcomputer Service Manual, 0458,002, "Non volatile memory", printed pp. 19–20: the PCD8572 EEPROM IC24 means battery back-up is not required; its write cycle time is set by R1 and C2.
  4. ↑ Acorn Computers, Master Series Compact Microcomputer Service Manual, 0458,002, 7 "Hints and Tips", printed p. 39.
  5. ↑ Acorn Computers, Master Series Compact Microcomputer Service Manual, 0458,002, 6.11 "Non Volatile Memory", printed p. 38.
  6. ↑ stardot.org.uk, "Master Compact - Faulty - FIXED" (December 2023 – February 2024, retrieved 2026-10-02). DaveLecky's repair: two DRAMs, a faulty MOS socket and the configuration EEPROM replaced; the remaining instability was the external power supply he was using. 1024MAK: C3 and C7 are not polarised on the schematic. maniacminer: a dead Compact whose RAM had failed, probably after RAS and CAS were held low. tom_seddon: the Compact MOS appears able to boot with default settings if the EEPROM does not answer. Secondary source.
  7. ↑ 7.0 7.1 7.2 Acorn Computers, Master Series Compact Microcomputer Service Manual, 0458,002, section 2, monitor stand service guide, PDF pp. 50–58: safety tests; first-line servicing; dismantling; mains fuse anti-surge 3.15 A HBC 250 V; changing the supply board (four screws on the board, mains input connector, DC output connector, earth tag); a metal screen around the supply on some units that must be refitted; a second type of supply with the same voltages and different cable layout.
  8. ↑ 8.0 8.1 8.2 stardot.org.uk, "BBC Master Compact PSU very slow to start" (January 2022, retrieved 2026-10-02). steve3000: the supply took one to two minutes to reach its output; C10, a 220 µF 10 V electrolytic, measured 65 µF; with a 220 µF 25 V part fitted it starts at once. 1024MAK: the start-up capacitor is the smallest of three electrolytics at the top right of the board (wires to the left); check the larger resistors of 100 kΩ or more that charge it; the two larger primary capacitors can hold a charge after power-off, and in use carry about 162–170 V DC each, 325–340 V across both. Secondary source.
  9. ↑ Acorn Computers, Master Series Compact Microcomputer Service Manual, 0458,002, 3 "Disassembly and assembly", printed p. 8.


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