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BBC Master 128 Maintenance Guide

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Acorn BBC Master 128 with top cover removed
BBC Master 128 with the top cover removed: power supply at left, battery pack at the foot of the expansion gap, Econet module at top right (photo: Chris Whytehead, CC BY-SA 3.0)

Preventive maintenance for the BBC Master 128 and the machines built on its board: the Master Turbo, Master 512, Master AIV and Master ET. The procedures are from Acorn's Master series service manual, its Master Series Hardware Specification and its battery leaflets, with repairers' reports from stardot where Acorn says nothing.[1] The two main jobs are the CMOS battery, which leaks, and the power supply's suppression capacitors, which crack and smoke.

Safety

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The supply is a mains switch-mode unit inside the case. Acorn says the computer must be earthed, that its moulded plug takes a 3 A fuse to BS 1362, and that the lid must not be removed with the mains connected.[1] The supply's primary reservoir capacitors sit across the rectified mains, about 170 V each in use; discharge them and check for 0 V before touching the primary side.[2] Capacitor work on the supply is in the BBC Master 128 Capacitor Replacement Guide.

Opening the case

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  1. Disconnect the mains and all peripherals.
  2. Turn the computer upside down and remove the four screws marked FIX, two at the front and two at the rear. The rear screws are longer.[3]
  3. Hold the two halves together, turn the machine back over and lift the top straight off.

Inside are four sub-assemblies: the keyboard, the speaker and battery unit (the cartridge nest), the power supply and the main board.[1]

Removing the sub-assemblies (service manual section 3)
Part How it comes out
Keyboard Unplug its two connectors from the main board; undo three screws to the case bottom and one to the power supply
Speaker and battery unit Lift the moulding upwards and unplug its two connectors (battery PL8, speaker PL9)
Power supply Pull off the seven push-on leads; undo three screws under the case. Refit with the same M3 × 6 mm screws
Main board Comes out with its metal back plate: undo the four screws that pass through the cartridge connectors into the case and the one screw from the back plate to the supply, lift the rear slightly and slide the board backwards

When refitting:[1]

  • Put the main board in front edge first and pull it forwards until the back plate drops into place, with every lug in the case engaged.
  • Reconnect the supply: three red leads to the connectors marked +5V, three black to 0V and the purple to −5V. Acorn warns that the supplies short-circuit if any two are reversed.
  • PL8 (battery) and PL9 (speaker) can go either way round, but the battery plug must cover all three pins of PL8.
  • Check that the keyboard ribbon sockets are on every pin; Acorn notes it is easy to shift them one pin left or right.

Battery

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Acorn BBC Master 512 with top cover removed
A Master 512 with its battery pack wrapped in yellow tape beside the co-processor board (photo: Chris Whytehead, CC BY-SA 3.0)

The battery keeps the 146818 clock running and holds the 50 bytes of configuration in CMOS RAM. Acorn's 1986 service manual describes a lithium manganese dioxide cell beside the speaker; Acorn's battery leaflet of January 1987 says later Masters have a 4.5 V unit instead of the 3.0 V lithium cell.[4][5] There are two styles of that unit:[3]

Acorn battery units (leaflet 0443,900)
Style Where it is Fixing
Old style: a battery holder on a metal plate Between the left edge of the main board and the power supply Two screws, lead to PL8
New style: three shrink-wrapped cells Pushed into the recess between the left of the keyboard and the case side None; lead routed between the keyboard and the supply to PL8

The Master AIV is different: its three cells sit in a nest under the computer, behind a cover with one large-headed screw, near the auxiliary power socket.[6]

The board charges any battery connected to PL8. Replacement packs therefore have a diode and a current-limiting resistor in the lead; RetroClinic, which sells packs, says bare non-rechargeable cells would otherwise be charged and can overheat.[7] 1024MAK on stardot gives Acorn's parts as a 1N4001 diode and a 120 Ω ¼ W resistor, and says any triple-AA holder with the same layout and three 1.5 V alkaline cells will do.[8]

To replace the battery:

  1. Save the configuration first: select a filing system, type MODE 0, *SPOOL CONFIG, *STATUS and *SPOOL.[3]
  2. Open the case, unplug the lead from PL8 noting its route, and lift out the old unit (two screws for the old style).[3]
  3. Inspect PL8, the lead and the board around them. Alkaline electrolyte travels up the wire inside the insulation and turns the plug contacts to green deposits; replace the lead and the plug along with the cells.[8][7] One repairer on stardot reflows the solder on the resistors next to the plug as well, because the corrosion reaches their pads.[9] Clean any damage as described in Battery Explosion, Capacitor or Corrosion Damage.
  4. Fit the new pack with its diode and resistor, push its plug over all three pins of PL8, and mount it where a future leak cannot reach the board.[3]
  5. Switch on holding R until a message appears, then restore the settings with *CONFIGURE.[3]
  6. With the mains off there should be at least 2.6 V at the clock chip; if not, and the battery is good, check FET Q4 and the parts around it.[4]

Acorn's 1993 leaflet expects about a year of life from the battery and says it should in any case be replaced at least every 18 months to avoid leakage damage.[3] More on packs and chemistry is in Battery Refurbishment.

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The system ROM (IC24) holds the MOS, BASIC and the built-in applications. Three 28-pin sockets, IC27, IC37 and IC41, take extra ROMs. IC37 and IC41 share their address space with the sideways RAM: plug links LK19 (for IC37) and LK18 (for IC41) select RAM in the west position or ROM in the east position, and each socket used for a ROM removes 32 KB of sideways RAM.[10][11] Acorn recommends IC27 for the ANFS ROM when an Econet module is fitted.[11]

Links worth knowing (service manual section 4)
Link Type As shipped Purpose
LK4 Plug Made Connects the 146818 alarm interrupt to the CPU IRQ line
LK5 PCB track East Composite sync polarity (negative)
LK6 Four-pin plug A–B 16 MHz clock from the board; B–D takes it from the cartridge slot, which must then supply a clock or the DRAMs can be destroyed
LK7 PCB track East RGB polarity (true)
LK12 Plug B Cartridge machine detect (tells a cartridge it is in a Master); A connects CSYNC for genlock
LK13 PCB track West A-to-D reference from the on-board 1.8 V source
LK15 PCB track West PAL colour on the UHF output
LK18, LK19 Plug West Sideways RAM in slots 6–7 and 4–5; east gives the slots to ROM sockets IC41 and IC37

Cleaning and ventilation

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Acorn asks for the machine not to be used in extreme heat, cold, humidity or dust, and for the ventilation under and behind the computer to be kept clear.[1] With the case open, brush out dust and check the cartridge slot connectors (SK3 and SK4) for worn or bent contacts, which Acorn's fault-finding treats as a reason to replace the connector.[12]

Fitting internal options

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  • Econet module: two plastic support posts go into the main board, the module's PL1 into SK5 (the two westmost holes of SK5 are unused) and PL2 into SK6, then the ANFS ROM goes in IC27. The station number must be set before the machine joins a network.[11]
  • Internal co-processors: the 65C102 (Turbo) and 80186 (512) boards plug into SK1 and SK2 on support posts and are enabled with *CONFIGURE TUBE and *CONFIGURE INTUBE. See Acorn Master Turbo, Acorn 65C102 Co-Processor User Guide and BBC Master 512.

Checklist

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  1. Inspect the supply's X2 capacitors; replace RIFA paper types that are cracked, and the start-up capacitor if the supply starts slowly. See BBC Master 128 Capacitor Replacement Guide.
  2. Load-test the supply: +5 V between 4.85 and 5.15 V across a 12 V 21 W bulb.[2]
  3. Remove or replace the battery pack, check PL8 and its wiring, and fit a pack with a diode and resistor.
  4. Check that all seven supply leads are on the right connectors and firmly pushed home.
  5. Reseat the keyboard ribbon connectors on every pin.
  6. Save the configuration with *STATUS before any work that may lose it.

Tools

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Acorn's list for Master fault-finding is a set of screwdrivers, pliers, cutters, a 10 A multimeter, an RGB monitor, a logic probe and a 5 Ω 5 W resistor; a 100 MHz dual-beam oscilloscope, a frequency counter and a known good Master make the work quicker.[13] Repairers use strain-relief bush pliers to get the supply's DC wiring out of its case.[14] See also Recommended Tools.

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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 Acorn Computers, British Broadcasting Corporation Master Series Microcomputer Service Manual, part no. 0443,004, issue 1, April 1986: mains plug and 3 A fuse, p. 2; packaging and installation (240 V AC 50 Hz, ventilation), p. 5; 3 "Disassembly and assembly", pp. 6–7.
  2. ↑ 2.0 2.1 stardot.org.uk, "Master 128 and the long slow beep of doom" (May–October 2019, retrieved 2026-10-02). 1024MAK: discharge and check the two high-voltage capacitors, each about 170 V DC when running; the 12 V 21 W bulb load test; R2 and R3; C9. Secondary source.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Acorn Computers, Master Series Battery Unit, part no. 0443,900, issue 3, May 1993, hosted as File:Acorn BBC Master Series Battery Unit 0443,900 Issue 3.pdf. Battery life about one year; replace at least every 18 months to avoid leakage; save the CMOS settings with *SPOOL and *STATUS first; four FIX screws, rear ones longer; old-style unit between the main board and the power supply with two screws, new-style three shrink-wrapped cells pushed into the recess left of the keyboard; lead routed to PL8; connector either way round but over all three pins; power-on reset with R held until a message appears.
  4. ↑ 4.0 4.1 Acorn Computers, Master Series Microcomputer Service Manual, 0443,004, pp. 21–22 (146818 battery back-up, lithium manganese dioxide cell, optional rechargeable battery on the keyboard, charging at about 30 mA for 15 minutes then 1 mA) and 7.4.3, p. 40 (PL8 with all three pins engaged, battery in its holder next to the speaker, at least 2.6 V at the clock chip with the mains off, FET Q4).
  5. ↑ Acorn Computers, Master Series - Battery Unit, part no. 0443,900, issue 1, January 1987, hosted as File:Acorn BBC Master Series Battery Unit 0443,900 Issue 1.pdf: black lead to the north pin of PL8, red to the centre pin, south pin not connected.
  6. ↑ Acorn Computers, Special Instructions for Replacing Batteries on the BBC Master AIV Computer, part no. 0461,004, issue 1, October 1986, hosted as File:Acorn BBC Master AIV Battery Replacement 0461,004.pdf. Three 1.5 V cells in place of the 3.0 V lithium cell; nest under the computer near the auxiliary power outlet; holder designed for Duracell MN1500 cells.
  7. ↑ 7.0 7.1 RetroClinic, "BBC Master 128 – Replacement CMOS battery packs" (retrieved 2026-10-02). Vendor source: alkaline packs leak, mostly over the case and sometimes onto the board; alkali creeps up the cable to the board connector, so a leaking holder should be replaced with its cable; the Master board charges any battery connected, so packs carry a diode and current-limiting resistor; a three-AA alkaline pack or a single 3.7 V lithium thionyl chloride cell.
  8. ↑ 8.0 8.1 stardot.org.uk, "'Metal plate' type BBC Master battery pack (ADT16)" (March 2026, retrieved 2026-10-02). 1024MAK: the lead from the three-pin connector carries a 1N4001 (or any higher-voltage 1N400x) and a 120 Ω ¼ W carbon film resistor, value not critical up to about 330 Ω; any triple-AA holder of the same layout; alkaline liquid can travel inside the wire insulation and corrode the wire and whatever it connects to. maniacminer: the contacts of old metal-plate packs are usually dissolved by electrolyte. Secondary source.
  9. ↑ stardot.org.uk, "Corroded Master 128 battery plug" (August 2020, retrieved 2026-10-02). philpem: the plug is a Molex KK 0.1-inch three-way, and green deposits mean electrolyte has run along the wire; 8271: corrosion also tracks to nearby solder pads, so reflow the resistors next to the plug. Secondary source.
  10. ↑ 11.0 11.1 11.2 Acorn Computers, Master Series Hardware Specification, hosted as File:Acorn BBC Master Series Hardware Specification.pdf. Fitting an ECONET module, PDF pp. 46–49: two support posts, PL1 to SK5 with the two west holes of SK5 unused, PL2 to SK6, ANFS ROM preferably in IC27, LK18 and LK19 for IC41 and IC37, pin 1 to the west. Additional information relating to Master Repair Guide, p. 50.
  11. ↑ Acorn Computers, Master Series Microcomputer Service Manual, 0443,004, 7 "Fault finding", pp. 29–49: first checks and hints, p. 29; cartridge port 7.16, p. 49.
  12. ↑ Acorn Computers, Master Series Microcomputer Service Manual, 0443,004, test equipment, p. 30.
  13. ↑ stardot.org.uk, "Master PSU recap" (October 2020, retrieved 2026-10-02). VectorEyes and KenLowe on the strain-relief bush and the pliers that remove it. Secondary source.


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