BBC Micro B+ General Maintenance

Preventive maintenance for the BBC Micro Model B+ — Acorn's 1985 enhanced Model B with shadow RAM and the WD1770 floppy controller. Every electrical figure and every procedure on this page is Acorn's own, from Section 2 of the BBC Microcomputer Service Manual (October 1985), which covers the B+ specifically, and from Section 1 where the two machines share a circuit.^[1]^[2]

Most procedures are shared with the Model B; the B+-specific ones are noted.

Safety

The B+ has a fully encased internal power supply manufactured to BS 415 Class 1, and it is mains-powered. Acorn's own instruction is blunt: do not remove the lid with the mains power connected.^[1]

The mains side stores charge after the plug is pulled. Before any work inside the supply, discharge the reservoir capacitor through a resistor and confirm 0 V with a meter. What can kill you is the primary side and the reservoir; the secondary rails cannot.

Acorn's position on the supply itself is that it is an exchange module. Its words: "it is not recommended that attempts should be made to repair this power supply, instead it should be treated as a module to be exchanged. This is because of the stringent safety regulations relating to such units."^[2] That is Acorn's advice to its dealers, not a prohibition — you may recap it — but it is why Acorn publishes no component values or designators for it. Earlier versions of this page named a "C9 primary bulk electrolytic" and "C1 / C2 X-class" capacitors on the PSU. Those designators are not in Acorn's documentation and have been removed; read the value and type off each fitted part.

A mains-filter capacitor that fails releases acrid smoke and an unmistakable smell. If that happens, switch off at the wall and let the unit cool before opening it.

Which supply do you have

Most B+ machines have a switched-mode supply — an ASTEC SMPS, which is what Acorn's own parts list names. A small number of early machines have a linear supply with a conventional mains transformer and regulator circuit.^[2]

An earlier version of this page said the B+ "does not have a switch-mode PSU" and that all rails were "regulated linearly from the Astec brick". Both halves are wrong: the usual supply is switched-mode, and it is an internal encased unit, not an external brick.

The distinction matters when diagnosing. On the linear supply the three outputs come from separate regulators, so Acorn notes that power can appear on two of the three pairs of connectors while the third is dead.^[2]

Acorn's power-supply specification

**BBC Micro B+ power supply, Service Manual section 3.2**
Item	Specification
Input voltage	240 V AC RMS ±10 %
Input frequency	47–53 Hz
+5 V output	+5 V DC ±0.1 V; 0.1 A minimum, 3.5 A maximum
+12 V output	+12 V DC ±10 %; 1.25 A maximum
−5 V output	−5 V DC ±10 %; 0.1 A maximum
Total output power	35 W

Note the +12 V and −5 V tolerances: ±10 %, not the ±2 or ±3 % bands that circulate. Earlier versions of this page gave +12 V as 11.7–12.3 V and −5 V as −4.85 to −5.15 V. Both are far tighter than Acorn's specification, and a reader working to them would condemn a supply Acorn would pass. The −4.85 to −5.15 V figure in particular is an Apple band that has been copied onto machines across this wiki; it does not belong here.

The Model B's own figures, for comparison: 5 V at 3.75 A and −5 V at 100 mA for the main board, with +12 V at 1.25 A available on the external connector — how much of that is actually available depends on what is fitted internally (Econet, disc interface, sideways ROMs).^[2]

The connectors — there are seven, not four

The power supply connects to the main board by seven push-on connectors, and the +5 V is fed to three different points across the board. Acorn distributed it that way to avoid needing very large copper tracks; the three points are electrically common.^[2]

Three connectors marked VCC take the red wires.
Three connectors marked 0V take the black wires.
One connector marked −5V takes the purple wire.^[1]

Get this right on reassembly. Acorn warns specifically about the polarity of these seven connectors, and the Model B manual warns against exchanging the 0 V and +5 V connectors.^[1]^[2]

Acorn's supply test procedure

This is Acorn's section 9.2, and it is worth following exactly because it tests the leads as well as the supply.^[1]

Power off. Unplug the three black 0 V leads, the three red +5 V leads and the single purple −5 V lead from the board.
Connect a 5 Ω 5 W resistor across one pair of red and black leads — a pair that were together on the board — and tape the other leads so they cannot short.
Power on and measure across the resistor with an oscilloscope. The voltage must be 4.9 to 5.1 V, with a maximum of 50 mV peak-to-peak noise. The resistor gets hot.
Repeat for the other two pairs.
If any one pair reads zero or very low, that lead is damaged. If all three are out of specification, change the supply.
With the resistor still on the last pair, put the other scope channel on the −5 V (purple) lead. Ground both probes, superimpose the traces, then measure: the two traces deflect in opposite directions and −5 V should be between −4.75 and −5.25 V.
Power off and reconnect the black leads to the 0V connectors.
Set a multimeter to a 10 A DC range and connect it between one VCC connector on the board and all three red wires joined together. Power on only long enough to read the current — any longer may overheat the board tracks. The board should draw 1.5 to 2.2 A, depending on its upgrade state. Repeat for each of the three +5 V connectors, powering off between each.
If the current at any one connector is zero or very low, that connection or its track is at fault.

That last step is the useful one and it is rarely done: it separates a tired supply from a board that is not drawing what it should, and it localises a broken +5 V distribution track.

Opening the case

Acorn's own sequence:^[1]

Power off and unplug the mains lead. Do not remove the lid with the mains connected.
Undo the four lid fixing screws — two on the rear panel and two underneath.
Do not lose the two spire clips pushed onto the lid, which the rear screws locate into.
To remove the keyboard: undo the two screws and nuts holding it to the case bottom, noting the washer positions. Unplug the 17-way keyboard connector, the 2-way loudspeaker connector and, if fitted, the 10-way serial-ROM connector.
To remove the supply: unplug its seven push-on connectors and undo three screws on the underside of the case. Use M3 × 6 mm screws on reassembly — Acorn specifies the type.
To remove the main board: disconnect the two wires to SK2 (the composite video BNC), undo seven fixing screws, and slide the board forwards before lifting it from the rear.

On reassembly, put the front edge of the board in first, pull it forwards as far as it will go until the back edge drops in, and mind the composite video wire. And reconnect the keyboard ribbon carefully — Acorn warns that it is easy to displace the connector one pin to the left or right.^[1]

ROM and RAM positions

Acorn's own designators, which are worth having before you pull anything:^[1]

Five sideways ROM sockets: ICs 35, 44, 57, 62 and 68. Each takes an 8, 16 or 32 KB device. A 32 KB device in one socket is decoded as two 16 KB sideways slots, using address line A14 from the sideways ROM select latch IC45.
A sixth ROM socket, IC71, holds the 32 KB system ROM — the machine's MOS together with BASIC. Link S19 is hard-wired east in the 32 K position as standard.
Eight RAM devices: ICs 55, 56, 60, 61, 64, 65, 66 and 67. Of that RAM, 32 KB is always accessible to the processor and 12 KB can be paged into the sideways area.

The system ROM is IC71, not IC51. An earlier version of this page gave IC51 and a specific EPROM type; the position is wrong and no source was found for the part type. Note also that because the MOS and BASIC share one device, a badly seated system ROM prevents the machine from powering up at all rather than producing a "Language?" prompt.^[1]

When fitting a sideways ROM, the notch points to the rear of the case. Fitting one backwards is the most common self-inflicted fault on these machines.

Inspection order

The supply. Acorn's own view is that it is an exchange item. If you are recapping it, read the values off the fitted parts and treat the primary side as lethal.
Main board electrolytics. Inspect for bulging, leakage and residue. The BBC Micro B+ Capacitor Replacement Guide has the values Acorn does publish.
ROM and RAM sockets. Reseat the five sideways ROMs (ICs 35, 44, 57, 62, 68), the system ROM (IC71) and the eight RAM devices.
Keyboard ribbon. Clean both ends with isopropyl alcohol on a foam swab and reseat squarely — one pin out is a whole dead row or column.
Floppy header. Check the 34-way header for bent pins and clean the edge connector on attached drives.

Cleaning

Case — a very dilute mild detergent on a damp microfibre cloth, then clean water on a second cloth. Avoid isopropyl alcohol on the ABS, which can craze it.
Keycaps — remove them and wash in warm water. Do not put them in a dishwasher. An earlier version of this page suggested a 40 °C dishwasher cycle; nothing supports it and the risk to double-shot caps and to any printed legends is not worth taking. Clean the switch plungers with isopropyl alcohol on a foam swab.
Internals — a soft brush and low-pressure compressed air. Do not blast air at the exposed surface of the WD1770.

Battery

The B+ has no real-time clock and no backup battery. There is nothing to replace. The BBC Master 128 does have a battery-backed CMOS RAM and that battery does fail.

Tools

Pozidriv #2 screwdriver
IC extraction tool
Anti-static strap
Digital multimeter, and an oscilloscope for the supply test above
A 5 Ω 5 W resistor — Acorn's specified +5 V test load
Fine-tip soldering iron and solder wick
Foam swabs, isopropyl alcohol, deoxidising contact cleaner
Section 2 of the Service Manual, hosted on this wiki

See Recommended Tools.

Related pages

References

↑ ^1.0 ^1.1 ^1.2 ^1.3 ^1.4 ^1.5 ^1.6 ^1.7 ^1.8 Acorn Computers, BBC Microcomputer Service Manual, October 1985, Section 2 — Model B Plus, hosted on this wiki as BBC Microcomputer Service Manual October 1985 Section 2 Model B Plus.pdf. Source for the section 3.2 power-supply specification (240 V AC RMS ±10 %, 47–53 Hz, +5 V DC ±0.1 V at 0.1–3.5 A, +12 V DC ±10 % at 1.25 A maximum, −5 V DC ±10 % at 0.1 A maximum, 35 W total), the fully encased BS 415 Class 1 supply, the section 9.2 test procedure (5 Ω 5 W resistor per red/black pair, 4.9–5.1 V with 50 mV p-p maximum noise, the −5 V check at −4.75 to −5.25 V, and the 10 A DC board-current test giving 1.5–2.2 A per VCC connector), the disassembly sequence (four lid screws — two rear and two underneath — the two spire clips, the two keyboard screws, the 17-way keyboard, 2-way loudspeaker and 10-way serial-ROM connectors, the seven push-on PSU connectors and three underside screws with M3 × 6 mm on reassembly, the SK2 composite-video wires and the seven main-board screws), the reassembly polarity (three red to VCC, three black to 0V, one purple to −5V) and the warning about displacing the keyboard ribbon by one pin, and the ROM and RAM positions (five sideways sockets at ICs 35, 44, 57, 62 and 68 taking 8, 16 or 32 KB devices decoded as two 16 KB slots via A14 from latch IC45; the 32 KB system ROM at IC71 with link S19 hard-wired east; eight RAM devices at ICs 55, 56, 60, 61, 64, 65, 66 and 67 with 32 KB always accessible and 12 KB pageable).
↑ ^2.0 ^2.1 ^2.2 ^2.3 ^2.4 ^2.5 ^2.6 Acorn Computers, BBC Microcomputer Service Manual, October 1985, Section 1, hosted on this wiki as BBC Microcomputer Service Manual October 1985.pdf. Source for section 3.14, the power supply: 5 V at 3.75 A and −5 V at 100 mA for the main board with +12 V at 1.25 A on the external connector; the seven push-on connectors with +5 V fed to three electrically common points across the board to avoid large copper tracks; Acorn's statement that most machines have a switched-mode supply and that it should be treated as an exchange module rather than repaired because of the safety regulations governing such units; the note that a small number of early machines have a linear supply whose three outputs come from separate regulators, so power can appear on two of the three pairs; the ASTEC SMPS in the parts list; and the warning against exchanging the 0 V and +5 V connectors.

[sm2-1] 1.0 ^1.1 ^1.2 ^1.3 ^1.4 ^1.5 ^1.6 ^1.7 ^1.8 Acorn Computers, BBC Microcomputer Service Manual, October 1985, Section 2 — Model B Plus, hosted on this wiki as BBC Microcomputer Service Manual October 1985 Section 2 Model B Plus.pdf. Source for the section 3.2 power-supply specification (240 V AC RMS ±10 %, 47–53 Hz, +5 V DC ±0.1 V at 0.1–3.5 A, +12 V DC ±10 % at 1.25 A maximum, −5 V DC ±10 % at 0.1 A maximum, 35 W total), the fully encased BS 415 Class 1 supply, the section 9.2 test procedure (5 Ω 5 W resistor per red/black pair, 4.9–5.1 V with 50 mV p-p maximum noise, the −5 V check at −4.75 to −5.25 V, and the 10 A DC board-current test giving 1.5–2.2 A per VCC connector), the disassembly sequence (four lid screws — two rear and two underneath — the two spire clips, the two keyboard screws, the 17-way keyboard, 2-way loudspeaker and 10-way serial-ROM connectors, the seven push-on PSU connectors and three underside screws with M3 × 6 mm on reassembly, the SK2 composite-video wires and the seven main-board screws), the reassembly polarity (three red to VCC, three black to 0V, one purple to −5V) and the warning about displacing the keyboard ribbon by one pin, and the ROM and RAM positions (five sideways sockets at ICs 35, 44, 57, 62 and 68 taking 8, 16 or 32 KB devices decoded as two 16 KB slots via A14 from latch IC45; the 32 KB system ROM at IC71 with link S19 hard-wired east; eight RAM devices at ICs 55, 56, 60, 61, 64, 65, 66 and 67 with 32 KB always accessible and 12 KB pageable).

[sm1-2] 2.0 ^2.1 ^2.2 ^2.3 ^2.4 ^2.5 ^2.6 Acorn Computers, BBC Microcomputer Service Manual, October 1985, Section 1, hosted on this wiki as BBC Microcomputer Service Manual October 1985.pdf. Source for section 3.14, the power supply: 5 V at 3.75 A and −5 V at 100 mA for the main board with +12 V at 1.25 A on the external connector; the seven push-on connectors with +5 V fed to three electrically common points across the board to avoid large copper tracks; Acorn's statement that most machines have a switched-mode supply and that it should be treated as an exchange module rather than repaired because of the safety regulations governing such units; the note that a small number of early machines have a linear supply whose three outputs come from separate regulators, so power can appear on two of the three pairs; the ASTEC SMPS in the parts list; and the warning against exchanging the 0 V and +5 V connectors.

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