RetroTechCollection

BBC Micro B+ General Maintenance

This guide documents preventive maintenance for the BBC Micro Model B+ (Acorn 1985 enhanced Model B with shadow RAM and the WD1770 floppy controller). Most procedures are shared with the Model B, with B+-specific procedures noted where they differ.

Safety Warning

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The B+ uses an Astec linear power supply module mounted inside the case. Mains-side capacitors retain charge after disconnection from the wall — particularly the C9 primary bulk electrolytic and the C1 / C2 X-class mains-suppression capacitors on the PSU.[1]

Before any service work:

  1. Power off and unplug the mains lead.
  2. Wait at least 30 seconds.
  3. Discharge the PSU bulk capacitor across a 1 kΩ / 5 W resistor.
  4. Verify with a multimeter that all PSU outputs and the primary bulk cap read 0 V.

A failed RIFA capacitor will release acrid white smoke and a distinctive fishy smell — if you smell either while operating, power off immediately at the wall and let the unit cool before opening it.

Documentation Set

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  • Acorn BBC Microcomputer Service Manual (October 1985), Sections 1 and 2.[2] Section 2 covers the B+ specifically.
  • Advanced User Guide for the BBC Micro (Bray, Dickens, Holmes) — applicable to most B+ hardware (excluding shadow RAM and 1770 sub-circuits).
  • BBC Microcomputer Advanced Reference Manual (Acorn).

Opening the Case

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  1. Power off and unplug all leads.
  2. Turn the machine upside-down on a soft surface.
  3. Remove the four Pozidriv #2 case screws around the bottom edge. (B+ keyboards do not have the additional Acorn-only fixed screws found on Issue 7 Model Bs.)
  4. Lift the top cover and disconnect the keyboard ribbon cable from the main board connector PL13 by squeezing the ribbon header tabs.
  5. Disconnect the speaker connector (PL14).
  6. Set the keyboard / top assembly aside.

The PSU sits in a metal shielding can on the right-hand side of the main board. Removing the PSU for service requires unscrewing four chassis screws and unplugging the four-pin output connector.

Inspecting the System

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In service order:

  1. PSU shielding — open and inspect C1 and C2 (X2 mains-suppression caps on the live input). RIFA-branded caps are an automatic preventive replacement regardless of visual condition.[3]
  2. C9 PSU primary bulk electrolytic — visually inspect for bulging or leakage; this part is responsible for cold-start reliability on aged PSUs.
  3. Main board electrolytics — three large capacitors near the PSU connector and a row of smaller axial caps along the rear edge. Inspect for bulging, leakage and crusty residue.
  4. DRAM and ROM sockets — reseat the eight 4164 DRAMs in the top half of the board and the five sideways ROMs in the top-left corner (these are easy to access on the B+ because Acorn relocated the ROM sockets there).
  5. Keyboard ribbon — clean both ends of the ribbon cable with IPA and a foam swab; reseat firmly.
  6. Floppy connector — inspect the 34-pin floppy header (PL5) for bent pins; clean the edge connector on attached drives with deoxidising contact cleaner.

Regular Cleaning

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  • External case — beige ABS plastic. Use a mild detergent (washing-up liquid) at very low concentration on a damp microfibre; rinse with clean water on a second cloth. Avoid IPA on ABS — it can cause crazing.
  • Keyboard — keycaps are double-shot moulded and dishwasher-safe at 40 °C with no detergent; air-dry thoroughly before refitting. The key-switch plungers underneath should be cleaned with IPA on a foam swab.
  • Internals — soft-bristle brush plus low-pressure compressed air. Avoid blow-out on the open WD1770's surface — its lid is welded but accumulated dust under the lid can be problematic later.

PSU Voltage Checks

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Expected rails at the four-pin Molex connector to the main board (refer to the PSU schematic):

  • +5 V — should read 4.95 V to 5.10 V under load.
  • +12 V — should read 11.7 V to 12.3 V under load.
  • −5 V — should read −4.85 V to −5.15 V under load.

The B+ does not have a switch-mode PSU — all rails are regulated linearly from the Astec brick, so the +5 V rail will dip mildly under heavy disc load. A dip below 4.9 V indicates ageing PSU electrolytics or a tired regulator.

ROM Updating

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B+ MOS is on a 32 K EPROM (HN613256) at IC51. Updating MOS requires:

  1. Remove the MOS EPROM with an IC extraction tool.
  2. Reprogram a 27C256 / 27SF512 with the new MOS image.
  3. Refit and run the standard self-test (*FX 0).

For sideways ROMs, simply lift the existing ROM with an extraction tool and fit the replacement in the same numbered socket (the B+ has five sideways sockets, numbered 4–8 left-to-right). Avoid the common mistake of inserting a sideways ROM backwards — the notch on the chip points to the rear of the case.

Battery

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The B+ does not have a real-time clock or a backup battery — there is nothing to replace in this regard. (The BBC Master 128 has a CMOS RAM battery on its main board which does fail and should be replaced; this is the predecessor to the issue that became infamous on Macintoshes and IBM PS/2s.)

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  • Pozidriv #2 screwdriver (Acorn cases).
  • IC extraction tool / DIL puller.
  • Anti-static strap.
  • Digital multimeter.
  • Fine-tip soldering iron + solder wick (for PSU recap work).
  • Foam swabs and IPA (for key switches).
  • Deoxidising contact cleaner.
  • PSU bench-load (a 2.5 Ω / 25 W resistor on the +5 V rail is a good test load).
  • Acorn BBC Microcomputer Service Manual PDF.
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References

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  1. ↑ "BBC PSU recap and RIFA caps", Stardot Forums
  2. ↑ Acorn BBC Microcomputer Service Manual, Chris's Acorns
  3. ↑ "RIFA capacitor replacement on Acorn machines", dalbystoys.co.uk
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