SECTION 2 BBC Microcomputer Model B +
Acorn · BBC Micro Model B+ 64K · 107 pp.
Acorn BBC Micro Model B+ 64K Service Manual PDF — Repair & Fault Finding Guide
Document text
BBC Microcomputer Model B+ Service Manual Contents 1 - Introduction 1 2 - Packaging and installation 2 3 - Specification 3 3.1 The microcomputer 3 3.2 Power supply 5 3.3 Video outputs 5 3.4 RS423 5 3.5 Cassette interface 6 3.6 Analogue to digital convertor 6 3.7 ECONET 6 3.8 CENTRONICS compatible printer interface 6 3.9 Audio output 6 3.10 Environment 7 3.11 Dimensions 7 4 - Disassembly and assembly 8 5 - Circuit description 10 5.1 General 10 5.2 CPU timing 11 5.3 Reset circuitry 13 5.4 Address decoding and memory 13 5.4.1 ROM operation 15 5.4.2 Paged RAM operation 16 5.4.3 RAM.
access 16 5.4.4 RAM circuitry 18 5.5 Disc interface 19 5.5.1 8271 FDC 19 5.5.2 1770 FDC 21 5.6 Video circuitry 21 5.6.1 RGB 21 5.6.2 Composite video 21 5.6.3 UHF 22 5.7 CENTRONICS compatible printer interface 23 5.8 User port 23 5.9 1MHz extension bus 23 5.10 TUBE interface 23 5.11 ECONET 24 5.12 Cassette and RS423 ports 24 5.13 Analogue to digital convertor 25 5.14 Audio circuitry 25 5.15 Keyboard 26 6 - Upgrading the PCB 27 6.1 1770 disc option 27 6.2 8271 disc option 28 6.3 ECONET 30 6.4 Speech 31 7 - Selection links 33
8 - Test equipment 35 9 - Fault finding 36 9.1 Switch on 36 9.2 Power supply 39 9.3 Oscillator and divider circuitry 40 9.4 CPU 40 9.5 ROM 41 9.6 DRAMs 42 9.7 Video 42 9.8 Cassette interface and RS423 45 9.9 Keyboard 47 9.10 Disc interface 48 9.11 Printer port 51 9.12 User port 52 9.13 1MHz extension bus 53 9.14 TUBE interface 54 9.15 Analogue to digital conversion 55 9.16 ECONET 56 Appendix 57 Connector pinouts 58 Parts list 63 Glossary 69 IC description 71 Final assembly 89 Circuit block diagram 91 PCB circuit diagram 93 PCB layout 95 Keyboard circuit diagram 97 Power supply circuit diagram 99
WARNING: THE COMPUTER MUST BE EARTHED IMPORTANT: The wires in the mains lead for the apparatus are coloured in accordance with the following code: Green & Yellow Earth Blue Neutral Brown Live The moulded plug must be used with the fuse and fuse carrier firmly in place. The fuse carrier is of the same basic colour (though not necessarily the same shade of that colour) as the coloured insert in the base of the plug. Different manufacturers' plugs and fuse carriers are not interchangeable. In the event of loss of the fuse carrier, the moulded plug MUST NOT be used.
Either replace the moulded plug with another conventional plug wired as detailed below, or obtain a replacement fuse carrier from an authorised ACORN dealer. In the event of the fuse blowing it should be replaced, after clearing any faults, with a 3 amp fuse that is ASTA approved to BS1362. If the socket outlet available is not suitable for the plug supplied, the plug should be cut off and the appropriate plug fitted and wired as previously noted. The moulded plug which was cut off must be disposed of as it would be a potential shock hazard if it were to be plugged in with the cut off end of the mains cord exposed.
As the colours of the wires may not correspond with the coloured markings identifying the terminals in your plug, proceed as follows: The wire which is coloured green and yellow must be connected to the terminal in the plug which is marked by the letter E, or by the safety earth symbol ± , or coloured either green or green and yellow. The wire which is coloured blue must be connected to the terminal which is marked with the letter N, or coloured black. The wire which is coloured brown must be connected to the terminal which is marked with the letter L, or coloured red.