BBC Micro B+ General Maintenance

BBC Model B+ 64K

Preventive maintenance for the BBC Micro Model B+. The figures and procedures are from Section 2 of Acorn's BBC Microcomputer Service Manual (October 1985), which covers the B+, and from Section 1 where the two machines share a design. Most procedures are the same as on the Model A/B.

Safety

The B+ supply is a mains-powered switch-mode unit, fully encased and built to BS 415 Class 1. Acorn says not to remove the lid with the mains connected. Its circuit diagram (Section 2, p. 99) is the same design as the Model B's Astec supply, with the primary reservoir capacitors C6 and C8 on the rectified mains; discharge them and confirm 0 V before touching the supply board. Acorn treated the supply as an exchange unit and told dealers not to repair it because of the safety regulations covering such units. The diagram gives designators but no component values. Supply capacitor information, all from community sources, is on the BBC Micro Model A/B Capacitor Replacement Guide.

Power supply specification

B+ power supply (service manual 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

Within these limits +5 V is 4.9–5.1 V, +12 V is 10.8–13.2 V and −5 V is −4.5 to −5.5 V.

Power connections

The supply connects to the main board by seven push-on leads. Connectors marked VCC take the three red wires, connectors marked 0V the three black wires, and the connector marked −5V the purple wire. Acorn asks for great care with the polarity on reassembly. The three +5 V points are connected together on the board; feeding them separately avoids very large copper tracks.

Acorn's supply test

Section 9.2 of the B+ manual tests the leads as well as the supply.

1. Switch off. Unplug the three black 0 V leads, the three red +5 V leads and the purple −5 V lead from the board.
2. Connect a 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.
3. Switch on and measure across the resistor with an oscilloscope. It must read 4.9–5.1 V with no more than 50 mV peak-to-peak noise. The resistor gets hot.
4. Repeat for the other two pairs. If one pair reads zero or very low, that lead is damaged; if all three are out of specification, change the supply.
5. With the resistor on the last pair, measure the purple −5 V lead on the other channel: it should be between −4.75 and −5.25 V.
6. Switch off and reconnect the black leads to the 0V connectors.
7. Set a multimeter to its 10 A DC range and connect it between one VCC connector on the board and all three red wires joined together. Switch on only long enough to read the current, because the board tracks heat up. The board should draw 1.5 to 2.2 A, depending on what is fitted. Repeat for each VCC connector, switching off between readings.
8. A zero or very low current at one connector means a broken lead, connector or track. Zero at all three means a short circuit that has made the supply cut out (likely) or an open circuit in the whole power network (unlikely); +5 V measured across the board then means open circuit, and 0 V means short circuit.
9. Refit the leads and finally check the supply pins of a few ICs around the board.

Opening the case

Acorn's sequence:

1. Switch off and unplug the mains lead.
2. Remove the four lid screws, two on the rear panel and two underneath. Keep the two spire clips on the lid that the rear screws locate in.
3. To remove the keyboard, undo the two screws and nuts holding it to the case bottom, noting the washers, and unplug the 17-way keyboard connector, the 2-way loudspeaker connector and, if fitted, the 10-way serial ROM connector.
4. To remove the supply, unplug its seven leads and undo the three screws under the case. Refit with the same type of screw, M3 × 6 mm.
5. To remove the main board, disconnect the two wires to SK2 (the composite video BNC), undo the seven fixing screws, slide the board forwards and lift it from the rear.

To refit the board, put the front edge in first and pull it forwards until the back edge drops in, without trapping the composite video wire. When reconnecting the keyboard, check that the connectors are fully engaged: Acorn notes that it is easy to displace the keyboard connector by one pin to the left or right.

ROM and RAM positions

ROM sockets (service manual figure 4)

Socket	ROM numbers	Link	Standard setting
IC35	2/3	S9	Molex link west, for 8 KB or 16 KB ROMs
IC44	4/5	S11	As IC35
IC57	6/7	S12	As IC35
IC62	8/9	S15	As IC35
IC68	10/11	S18	As IC35
IC71	0/1 or 14/15	S19	PCB link east, for the 32 KB OS and BASIC ROM

Each sideways socket takes an 8, 16 or 32 KB device. A 32 KB device gives two 16 KB banks and needs the extra address line A14 from IC45, which is connected by changing that socket's molex link. The eight 4164 DRAMs are IC55, IC56, IC60, IC61 and IC64 to IC67.

Inspection

1. The supply: check the rails with Acorn's test above.
2. Main board electrolytics: look for bulging, leakage and residue. Values are in the BBC Micro B+ Capacitor Replacement Guide.
3. ROM and RAM sockets: reseat the system ROM (IC71), the sideways ROMs and the DRAMs only if there is a reason to.
4. Keyboard ribbon: clean both ends with isopropyl alcohol and reseat squarely.
5. Disc connector: check the 34-way header for bent pins.

Cleaning

Clean the case with a damp cloth and very dilute mild detergent. Remove keycaps and wash them in warm water; clean the switch plungers with isopropyl alcohol on a foam swab. Use a soft brush and low-pressure air inside, and keep the ventilation under and behind the case clear.

Battery

The B+ has no battery-backed clock or memory, and Acorn's parts list contains no battery.

Tools

• Pozidriv screwdriver and an IC extraction tool
• Anti-static strap
• Digital multimeter with a 10 A range, and an oscilloscope for the supply test
• A 5 Ω 5 W resistor, Acorn's +5 V test load
• Temperature-controlled soldering iron and desoldering braid
• Foam swabs and isopropyl alcohol

See Recommended Tools.

Related pages

• BBC Micro Model B+
• BBC Micro B+ Troubleshooting Guide
• BBC Micro B+ Capacitor Replacement Guide
• BBC Micro Model A/B General Maintenance
• BBC Microcomputer Service Manual October 1985 Section 2