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BBC Micro Model A/B Power Supply Repair

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BBC Micro Model B Issue 7 interior with power supply
An issue 7 Model B with the keyboard removed. The power supply is the metal box at the top left; its red, black and purple leads run to push-on terminals on the main board (photo: Chris Whytehead, CC BY-SA 3.0)

Tynemouth Software calls the power supply the Achilles heel of the BBC Micro Model A/B.[1] Acorn's parts list gives the internal switch-mode supply as one item, "Power supply unit, ASTEC SMPS", part 103,003, and Acorn told dealers to exchange a faulty one and not to repair it.[2] Sprow lists three common faults: the X2 mains suppression capacitors C1 and C2, which crack and smoke; the start-up electrolytic C9, which dries out; and the chopping transistor Q2.[3] The BBC Micro Model B+ uses a supply of the same design, and the same repairs apply to it.[4][5]

A small number of early machines have a linear supply instead, with a mains transformer and a separate regulator for each +5 V output. The faults below are those of the switch-mode supply; the capacitor guide explains how to tell the two apart.[2]

Symptoms

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Supply faults and the part usually responsible
Symptom Usual cause
An acrid smell from the supply vents while the machine runs, then a pop and white smoke. The machine may keep running and the fuse stays intact X2 capacitor C2 (100 nF) or C1 (10 nF) has broken down[3][6]
A single straining sound at switch-on and nothing starts; under 1 V on the +5 V terminals, with or without the leads connected; a sigh at switch-off. The machine had been taking longer and longer to start Start-up capacitor C9 has dried out[3][1]
Virtually silent at switch-on and under 1 V on +5 V with the board connected; Sprow reports that with all seven leads unplugged the terminals then show their nominal +5 V and −5 V. It usually happens over a single off-on cycle of a supply that worked last time Chopping transistor Q2[3]
One pair of red and black leads reads zero or very low under load while the other two are good A damaged lead, not the supply[4]
Sound on vision, or the picture flickers when the leads are flicked with a finger Intermittent crimp on a power lead[7]
Machine runs and displays, but there is no sound and the cassette does not work −5 V missing; check the purple lead and the −5 V output[8]

If the supply is good and the machine still does not start, the fault is on the main board; see the BBC Micro Model A/B Troubleshooting Guide.

Safety

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Warning: the supply runs from the mains. The two primary reservoir capacitors, C6 and C8, sit in series across the rectified mains on Acorn's circuit diagram and stay charged for many minutes after the plug is pulled.[9][3] Unplug the machine, open the supply, discharge C6 and C8 through a resistor and measure 0 V across each before touching the board. Work on it only if you are competent with mains switch-mode supplies.

Acorn required any repaired supply, even one with only a new mains lead, to pass an earth continuity test at not less than 10 A and a 1500 V AC dielectric withstand test from live and neutral to earth.[8] Acorn notes that these tests need specialist equipment and training; refit every earth bolt and have the unit tested before it goes back into regular use.

Tools and parts

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  • X2 class capacitors: 10 nF for C1 and 100 nF for C2, rated 275 V AC (the originals were 250 V AC). Sprow specifies a 15 mm lead pitch; Tynemouth Software notes that the board has holes for either size of C2.[3][6][1]
  • Start-up capacitor C9: 220 µF 25 V, 105 °C.[3]
  • Chopping transistor Q2: the original is a 2SC1942 rated 800 V; Sprow's replacement is a BU208A in the TO-3 case, rated 700 V, and a stardot restorer fitted one.[3][10] 1024MAK found a BU508AW listed as an alternative but had not compared the data; another member fitted a BU508A in a TO-3 case.[10]
  • A test load: Acorn uses a 5 Ω 5 W resistor on each red and black pair.[4]
  • A resistor for discharging C6 and C8, a multimeter with a 10 A range, and ideally an oscilloscope. See Recommended Tools.

Procedure

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Remove the supply from the case

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  1. Switch off and unplug the mains lead.[11]
  2. Remove the four lid screws, two on the rear panel and two underneath, and lift the lid. Keep the two spire clips on the lid that the rear screws locate in.[11]
  3. Pull the seven push-on leads off the main board: three red +5 V, three black 0 V and one purple −5 V. Note which pairs of red and black leads were together.[11][4]
  4. Undo the three screws on the underside of the case and lift the supply out. Keep the screws: Acorn asks for the same type of screw on reassembly.[11]

Test before dismantling

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  1. Connect the 5 Ω 5 W resistor across one pair of red and black leads that were together on the board, and tape the other leads so they cannot short. The resistor gets hot.[4]
  2. Switch on and measure across the resistor. Acorn's limits for this supply design are 4.9 to 5.1 V with no more than 50 mV peak-to-peak noise.[4]
  3. Repeat for the other two pairs. One pair low means a damaged lead; all three low means the supply itself has failed.[4]
  4. With the load still on the last pair, measure the purple lead: −4.75 to −5.25 V.[4]
  5. Compare the behaviour with the symptom table: listen for the straining sound of a dry C9, and repeat the measurement with no load if the loaded output is under 1 V.[3]
BBC Micro Model A-B power supply circuit diagram (service manual 9.7)
Acorn's circuit diagram of the switch-mode supply (service manual Section 1, 9.7), turned to read left to right. Mains input and the X2 capacitors C1 and C2 are at the far left; DB1, C6 and C8 follow; C9, Q1 and the chopping transistor Q2 are in the centre; the outputs and the SCR1 crowbar on +5 V are at the right

Open the supply

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  1. Discharge C6 and C8 and confirm 0 V on each before going further.[3][9]
  2. Free the board from its metal case. Tynemouth Software removes several earth bolts and pushes out the floppy drive power connector and the mains switch. Push out the switch by pressing its retaining tabs flat before pulling the spade connectors off it.[1]
  3. Photograph the wiring, the earth bolts and the orientation of every part before removing anything.

Replace the X2 capacitors

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  1. Find C1 and C2 at the mains input end of the board, before the bridge rectifier DB1.[9] Look for split or cracked cases; Retro Kit's 100 nF part had split open, and its 10 nF part, which had not yet failed, showed stress lines and hairline cracks.[6]
  2. Desolder both (see Through-Hole Desoldering) and fit new X2 parts: 10 nF at C1 and 100 nF at C2. Read the value printed on each old part as well, because stardot posts disagree on which designator carries which value.[3][12] Use the hole spacing that fits the new part.[1]
  3. Replace both even if only one has failed. The machine runs without them, but they suppress interference fed back into the mains.[6]

Replace the start-up capacitor C9

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  1. Note the polarity of C9 before removing it. Where the board has no + sign, RetroClinic says a round dot marks the positive side of the position.[12]
  2. Fit a 220 µF 25 V 105 °C part, observing the polarity noted above; the general method is in Capacitor Replacement Procedure. Replace it even if the old one measures correctly: one stardot member's supply held its 5 V rail at 3 to 4 V until a C9 that measured correct capacitance and 0.3 Ω ESR was replaced.[3][10]
  3. Look at the output electrolytics while the board is out. RetroClinic replaces them only when they show a bulging vent or have discoloured the board beneath them, and says the capacitor on the 5 V output after the choke must be a low-ESR type or the supply hunts audibly under load.[12] Values and the community replacement list are in the BBC Micro Model A/B Capacitor Replacement Guide.

Replace the chopping transistor Q2

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  1. Q2 is the TO-3 transistor in the primary circuit.[3][9] Test it out of circuit as described in Testing Diodes, Transistors and Regulators; a stardot restorer found his shorted.[10]
  2. A shorted Q2 can take other parts with it. In the same supply R11 and R12 had gone open, Q1 no longer tested as a transistor, the output rectifier D7 conducted both ways, and SCR1, the crowbar thyristor across +5 V, failed out of circuit. He fitted R12 4.7 Ω 1 W, R11 0.75 Ω 1 W, Q1 PN2222A, D7 SBL2040 and SCR1 TNY612, mounting the resistors off the board.[10] Check each of these before powering a supply with a new Q2.

Reassemble

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  1. Refit the board, every earth bolt, the floppy drive power connector and the mains switch, as photographed.[1]
  2. Load-test the supply out of the case as above. One restorer ran his repaired supply on a halogen lamp for an hour before connecting it to the board.[10]
  3. Refit the supply with its three screws, and push the seven leads onto the board: red to the connectors marked VCC, black to 0V and purple to −5V.[11][4]

Testing

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  • +5 V on each pair under a 5 Ω load: 4.9 to 5.1 V, ripple no more than 50 mV peak-to-peak.[4]
  • −5 V: −4.75 to −5.25 V.[4]
  • With the board connected, Acorn's test for the B+ measures 1.5 to 2.2 A at each VCC connector, depending on what is fitted; switch on only long enough to read the current, because the board tracks heat up.[4]
  • Check +5 V on each of the three pairs at the board and −5 V; then check that the machine has sound and cassette, which need −5 V.[8]
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Dave Curran, "BBC Micro Power Supply Repair", Tynemouth Software blog, 15 May 2017 (retrieved 2026-10-25). Rifa X2 capacitors, 0.01 µF and 0.1 µF; the start-up electrolytic that causes slow or intermittent start-up; two hole spacings on the board for C2; removing the board (earth bolts, floppy drive power connector, mains switch). Community source.
  2. ↑ 2.0 2.1 Acorn Computers, BBC Microcomputer Service Manual, part no. 0433,001, issue 1, October 1985, Section 1: 3.14 "Power supply", p. 24 (outputs, seven push-on connectors, +5 V fed to three points, switch-mode and early linear supplies, exchange rather than repair); 9.8 parts list item 24 (part 103,003).
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 Sprow, "How to repair the 3 most common BBC power supply faults", Sprow's webpages, How to series, updated 24 July 2015 (retrieved 2026-10-25). Symptoms and fixes for a dry start-up capacitor C9, a failed chopping transistor Q2 and a perished filter capacitor C2 or C1; replacement parts; warning that the large capacitors store charge for many minutes. Community source.
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 Acorn Computers, BBC Microcomputer Model B+ Service Manual (BBC Microcomputer Service Manual part no. 0433,001, issue 1, October 1985, Section 2): 3.2 "Power supply", p. 5 (output specification); 9.2 "Power supply", p. 39 (test procedure); power supply circuit diagram, p. 99.
  5. ↑ Dave Curran, "BBC Micro B Plus 64K and 128K", Tynemouth Software blog, July 2019 (retrieved 2026-10-02). The B+ uses the same power supply and keyboard as the Model B. Community source.
  6. ↑ 6.0 6.1 6.2 6.3 "All about the BBC Micro X2 capacitors", Retro Kit (retrieved 2026-10-25). An original 1982 100 nF X2 capacitor rated 250 V AC with a split resin case, and a 10 nF one with stress lines and hairline cracks; modern replacements rated 275 V AC; the capacitors are for noise suppression only. Community source.
  7. ↑ Acorn Computers, BBC Microcomputer Service Manual, Section 1, 6.4 "Most common faults", p. 41: intermittent power leads causing sound-on-vision, the flick test, soldering along the crimp, and not exchanging the 0 V and +5 V connectors.
  8. ↑ 8.0 8.1 8.2 Acorn Computers, BBC Microcomputer Service Manual, Section 1, 6.3 "Fault isolation", p. 39: check +5 V on each of the three pairs of connectors; the processor, memory and display work without −5 V but the cassette, sound, speech and RS423 interfaces need it; a repaired supply must pass an earth continuity test at not less than 10 A and a 1500 V AC dielectric withstand test.
  9. ↑ 9.0 9.1 9.2 9.3 Acorn Computers, BBC Microcomputer Service Manual, Section 1, 9.7 "Power supply circuit diagram" (PDF p. 89): F1, R1, C1, T1, C2, C3, C4, 230/115 V link, DB1, L1, C5, C6, C8, start-up components R2, R3 and C9, Q1, chopping transistor Q2 with R11 and R12 in its circuit, transformer T2, output rectifiers D6 to D8, outputs +12 V, +5 V, COM and −5 V, crowbar SCR1 and Z1 on +5 V, output adjust resistors R25, R26 and R30.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 10.5 stardot.org.uk, "BBC Micro Power Supply advanced repair" (April 2020 to July 2021, retrieved 2026-10-25). dmantione: a supply with C1, C2, R12, R11, Q2, Q1, D7 and SCR1 failed; replacements R12 4.7 Ω 1 W, R11 0.75 Ω 1 W, Q2 BU208A (original 2SC1942, 800 V; spikes up to 540 V measured at the collector), Q1 PN2222A (original 2SC210), D7 SBL2040 (original S10SC3M), SCR1 TNY612; one-hour test on a halogen lamp. tricky: 28 of more than 30 supplies needed only C1, C2 and C9. GoodToGo!: a low 5 V rail cured only by replacing C9, which had measured correct capacitance and 0.3 Ω ESR. 1024MAK: BU508AW listed by one supplier as an alternative to the BU208A, data not compared. dsdf323: BU508A in a TO-3 case. Community source.
  11. ↑ 11.0 11.1 11.2 11.3 11.4 Acorn Computers, BBC Microcomputer Service Manual, Section 1, 1.4 "Mechanical assembly of case etc", p. 5.
  12. ↑ 12.0 12.1 12.2 stardot.org.uk, "Beeb PSU capacitors" (March to April 2013, retrieved 2026-10-25). leenew changes C1, C2 and C9 on every supply he refurbishes; flaxcottage's and "IBM portable PC"'s RS part numbers give the two X2 values against opposite designators; RetroClinic replaces C9 every time and the others only when needed, judges overheating by a bulging vent or a discoloured board underneath, and requires a low-ESR capacitor on the 5 V output after the choke; RetroClinic: where the board has no + sign, a round dot marks the positive side of an electrolytic. Community source.


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