BBC Master Compact Maintenance
Routine maintenance for the BBC Master Compact. The Compact is two units: the keyboard unit, which holds the main board, and the disc unit (Acorn's "monitor stand"), which holds the 3.5-inch drive and the mains power supply. The procedures are from Acorn's Compact service manual, with repairers' reports from stardot where Acorn gives none.[1] Unlike the Master 128 it has no battery to leak: its configuration is held in an EEPROM.[2]
Safety
[edit | edit source]The disc unit contains the mains supply. Acorn's guide says it must be earthed, that mains voltages are present inside even with its switch off while the lead is plugged in, and that it must be unplugged before it is taken apart. After work on mains parts Acorn required an earth continuity test (no more than 0.1 Ω, excluding the lead) and a 1.5 kV AC insulation test for at least ten seconds.[3] Capacitor work on the supply is in the BBC Master Compact Capacitor Guide.
Opening the keyboard unit
[edit | edit source]- Unplug the 5 V lead and all peripherals.
- Undo the four screws under the case and lift the top, which carries the keyboard, carefully from the back.
- Remove the keyboard connector from the board by rocking it gently left and right. The keyboard itself is held in the top by six screws.
- The speaker plugs into PL1 and lifts straight out.
- To remove the main board, undo its five screws, take off the expansion connector cover and lift the board from the left-hand (west) end.
Refit the board right-hand edge first, with the expansion connector, and push it until it drops into the case. Make sure every pin of the keyboard connector is engaged and that the connector is not trapped when the top goes back on.[4] Check that no case stand-off presses on a component: one owner traced a Compact that failed only in its case to a stand-off crushing two pins of IC22.[5]
Opening the disc unit
[edit | edit source]- Switch off and unplug the mains lead from the wall and the unit; unplug the 5 V lead and the disc data cable from the keyboard unit.
- With the front towards you, put your thumbs under the unit and fingers on top and pull the plastic front panel towards you and down. Remove the back panel the same way, then the two side panels.
- Turn the unit over and undo the outermost screws underneath (four or six, depending on the model).
- Turn it the right way up, slide the top backwards and pull the main unit clear without catching the disc data cable or the 5 V cable.
On reassembly the ventilation slots of the top go to the back.[3]
| Part | Check |
|---|---|
| Mains lead | Continuity; 5 A fuse in the moulded plug |
| Mains fuse | On the back of the unit: anti-surge 3.15 A HBC 250 V, type T |
| Mains switch, mains input and auxiliary output | Push-on connections remade as shown in Acorn's figure 3 |
| 5 V lead to the keyboard unit | Part of the DC cable assembly; under 100 mΩ per wire at full load |
| Disc data cable | Try a known working cable if disc errors keep occurring |
| Supply board | Four screws on top; unplug the mains input, DC output and earth tag. Refit any metal screen fitted around it |
| Disc drive | Sony F6 3W, switch set to drive 0; four support pillars |
Source: Acorn's monitor stand service guide.[3]
Configuration memory
[edit | edit source]The 128 bytes of configuration live in the PCD8572 EEPROM, IC24, which is socketed because it has a finite number of write cycles.[2] To reset the configuration, switch on holding R, then press CTRL F BREAK.[6] Useful settings on the Compact are *CONFIGURE LANG 14 for BASIC and, if a disc drive will not spin, *CONFIGURE FILE 13 with *INSERT 13.[7] The Compact has no clock chip, so the date is always 31 December 1999 unless set over Econet.[7]
Links
[edit | edit source]| Link | As shipped | Purpose |
|---|---|---|
| PL7 | Not fitted | Connects the light pen strobe to the CRTC |
| PL9 | North (PCB track) | Normal video; south inverts it |
| PL10 | East (PCB track) | RGB CSYNC inverted; west for monitors that need it non-inverted |
| PL11 | North (shunt) | ROM banks 0 and 1 to the expansion connector; south gives them to IC38 |
| PL12 | North (shunt) | 64 KB system ROM; south for a 128 KB system ROM |
Optional positions: VR1 for a speaker volume control, FS1 for a fuse and L1 and L2 for extra filtering, each fitted by cutting a track first.[1]
Keyboard
[edit | edit source]The Compact's keyboard is a membrane type. Acorn's manual says individual keys cannot be replaced and the whole keyboard is changed if no track fault can be found.[8] Owners on stardot have restored keys by taking the keyboard apart and brushing dust off the conductive pads; 1024MAK suggests isopropyl alcohol before anything stronger, tested on an unimportant part first.[9]
Upgrades
[edit | edit source]- Econet: the module is the same daughter board as on the Master 128 and needs the ANFS ROM in one of the internal sockets.[10]
- RS232: fitted by plugging the parts into sockets already on the board, without soldering or removing the board. With it fitted, TP1 should read −5 to −10 V and TP2 +5 to +10 V, typically 9 V each.[10][1]
- Second disc drive: Acorn's kit replaces the data cable with a dual cable (twisted to orient the connectors, clamp refitted), and the new drive is switched to drive 1 and mounted on four pillars beside the first.[3]
Checklist
[edit | edit source]- Inspect the disc unit supply's X2 capacitor and start-up capacitor (see BBC Master Compact Capacitor Guide).
- Check the 5 V lead and its plug; measure 5 V at the keyboard unit.
- Check the mains fuse rating (anti-surge 3.15 A).
- Reseat the keyboard connector and the socketed ICs (system ROM IC49, EEPROM IC24).
- Clean the membrane keyboard pads if keys are unreliable.
See also Recommended Tools.
Related pages
[edit | edit source]- BBC Master Compact
- BBC Master Compact Troubleshooting
- BBC Master Compact Capacitor Guide
- BBC Master Compact Service Manual
References
[edit | edit source]- ↑ 1.0 1.1 1.2 Acorn Computers, British Broadcasting Corporation Master Series Compact Microcomputer Service Manual, part no. 0458,002, issue 1, October 1986, hosted as File:Acorn BBC Master Compact Service Manual 0458,002.pdf: 2 "Packaging and installation", printed p. 7 (box contents, ventilation); 3 "Disassembly and assembly", p. 8; 4 "Selection links and test points", pp. 9–10.
- ↑ 2.0 2.1 Acorn Computers, Master Series Compact Microcomputer Service Manual, 0458,002, "Non volatile memory", printed pp. 19–20: 128 bytes in the PCD8572 EEPROM IC24, socketed because of its finite life, so no battery back-up is needed.
- ↑ 3.0 3.1 3.2 3.3 Acorn Computers, Master Series Compact Microcomputer Service Manual, 0458,002, section 2, monitor stand service guide and disc upgrade instructions, PDF pp. 50–63: safety warnings and tests; dismantling; 5 A plug fuse; mains switch, auxiliary output, fuse and input; changing the supply; the +5 V lead and DC cable assembly (under 100 mΩ per wire at full load); the disc data cable; changing the drive (Sony F6 3W, drive 0); fitting a second drive (dual data cable, drive 1, four support pillars).
- ↑ Acorn Computers, Master Series Compact Microcomputer Service Manual, 0458,002, 3 "Disassembly and assembly", printed p. 8.
- ↑ stardot.org.uk, "BBC Master Compact - faulty when in case [SOLVED"] (December 2024, retrieved 2026-10-02). Secondary source.
- ↑ Acorn Computers, Master Series Compact Microcomputer Service Manual, 0458,002, 6.1.5, printed p. 29, and 6.11 "Non Volatile Memory", p. 38.
- ↑ 7.0 7.1 Sprow, "The most common beeb power on problems", Sprow's webpages, updated 9 November 2010 (retrieved 2026-10-02). Community source: *CONFIGURE LANG 14 and *CONFIGURE FILE 13 on the Compact; the fixed 31 December 1999 date.
- ↑ Acorn Computers, Master Series Compact Microcomputer Service Manual, 0458,002, 6.1.8, printed p. 30.
- ↑ stardot.org.uk, "Fixing a Master Compact keyboard" (2014 and 2021, retrieved 2026-10-02). richardtoohey: removing a key's grey plunger by levering its two side tabs; jms2: dust on the conductive pads brushed off without solvents; tricky: a cleaned key failing again a year later; 1024MAK: try IPA first and test anything else on an unimportant area. Secondary source.
- ↑ 10.0 10.1 Acorn Computers, Master Series Compact Microcomputer Service Manual, 0458,002, 5.1 "Introduction", printed pp. 11–12: Econet daughter board as on the Master 128 with the ANFS ROM; RS232 by plugging ICs into sockets provided, without removing the board; Atari-type joystick port; 50-way expansion connector.