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BBC Microcomputer Service Manual October 1985 Section 2

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Section 2 of Acorn Computers' BBC Microcomputer Service Manual (part number 0433,001, issue 1, October 1985) is the BBC Microcomputer Model B+ Service Manual, covering Acorn models ANB51 to ANB54. It is a complete service manual for the BBC Micro Model B+ in its own right, with printed pages 1 to 100. The hosted PDF has 106 pages; the text pages carry an OCR text layer and the drawings are scanned images.

Contents

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Section 2, Model B+
Chapter Covers Printed page
1–2 Introduction; packaging and installation 1–2
3 Specification The microcomputer; power supply (3.2, p. 5); video outputs; RS423; cassette; A to D; Econet; printer; audio; environment; dimensions 3
4 Disassembly and assembly 8
5 Circuit description CPU timing; reset; address decoding and memory, ROM, paged RAM and RAM; 8271 and 1770 disc interfaces; video; printer; user port; 1 MHz bus; Tube; Econet; cassette and RS423; A to D; audio; keyboard 10
6 Upgrading the PCB 1770 disc, 8271 disc, Econet, speech 27
7 Selection links 33
8 Test equipment 35
9 Fault finding Switch on (36); power supply (39); oscillator and divider (40); CPU (40); ROM (41); DRAMs (42); video (42); cassette and RS423 (45); keyboard (47); disc (48); printer (51); user port (52); 1 MHz bus (53); Tube (54); A to D (55); Econet (56) 36
Appendix 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) 57

What it settles

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  • The B+ supply specification (3.2, p. 5): input 240 V AC ±10 % at 47–53 Hz; +5 V DC ±0.1 V at 0.1–3.5 A; +12 V DC ±10 % at up to 1.25 A; −5 V DC ±10 % at up to 0.1 A; 35 W total. The supply is fully encased and built to BS 415 Class 1 (3.1).
  • Acorn's supply test (9.2, p. 39): unplug the three black 0 V, three red +5 V and one purple −5 V leads, load each red and black pair in turn with a 5 Ω 5 W resistor and expect 4.9–5.1 V with no more than 50 mV peak-to-peak noise; −5 V should read −4.75 to −5.25 V. A 10 A meter between the leads and each VCC connector in turn should show 1.5–2.2 A, depending on the upgrades fitted.
  • The fault-finding chapter gives Acorn's checks for each area by IC number: for example the 8, 4, 2 and 1 MHz clocks on pins 7, 6, 5 and 4 of the video processor IC53 and 16 MHz on its pin 8; reset on pin 40 of the 6512A CPU, IC42, with the 555 reset timer IC43 (9.3–9.4, p. 40).
  • The B+ has a 6512A processor at 2 MHz and 64 KB of RAM in eight 4164 DRAMs (IC55, IC56, IC60, IC61, IC64–IC67), allowing shadow screen modes and 12 KB of paged RAM (3.1; IC description).

What this manual does not contain

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  • Component values for the power supply. The circuit diagram on p. 99 is the only supply drawing, and Acorn treats the supply as an exchange unit.
  • The Model A and Model B, which are in Section 1, or the 64K sideways RAM upgrade, which is in Section 3.

Pages that cite this document

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Download: BBC Microcomputer Service Manual October 1985 Section 2 Model B Plus.pdf

Other sections of this manual

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