4 Disassembly and assembly To service the BBC Microcomputer B Plus, first disconnect the power supply plug from the mains and remove all peripheral connections from the computer. To disassemble The lid of the microcomputer case may be removed after undoing four fixing screws, two on the rear panel and two underneath. Take care not to lose the two spire clips pushed onto the case lid, into which the rear fixing screws locate. DO NOT remove the lid with the mains power connected. Inside the microcomputer are three main sub-assemblies: power supply unit, keyboard and the main printed circuit board.
To remove the keyboard, undo the two screws and nuts holding it to the case bottom, taking care to note the positions of the associated washers. Unplug the 17-way keyboard connector and the 2-way loudspeaker connector from the main printed circuit board, and the 10- way serial-ROM connector, if fitted. The power supply unit is connected to the main circuit board by seven push-on connectors which may be unplugged. Three screws on the underside of, the case are undone allowing the unit to be removed. On reassembly, ensure that the same type of screw is used (M3x6mm). The main printed circuit board can be removed after the two wires to SK2 (composite video BNC socket) have been disconnected.
Undo the seven fixing screws and remove the circuit board from the case by sliding it forwards and then lifting it from the rear. To reassemble Replace the main printed circuit board by putting the front edge (with connector headers) in first and pulling it forwards as far as possible until the back edge drops in. Be careful not to trap the composite video wire to the BNC connector if this was removed. Replace the PCB fixing screws. Reconnect the composite video and its connector. Reconnect the power supply, being careful to route the wires neatly, and connect the wires (seven) to the push-on connectors on the PCB, being very careful to get the polarity right.
PCB connectors marked VCC must have a red wire attached(three) PCB connectors marked OV must have a black wire attached (three) The connector marked -5V has the purple wire attached (one). Replace the keyboard and reconnect the loudspeaker to the main PCB. Be careful to reconnect the keyboard ribbon socket so that all the pins are engaged; it is easy to displace the connector one pin to right or left. Replace the 10-way serial-ROM connector if fitted. Replace the nuts and bolts holding the keyboard in place. 8