Editing 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 [[BBC Micro Model A/B|Model B]], with B+-specific procedures noted where they differ.

== Safety Warning ==

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.<ref>[https://stardot.org.uk/forums/viewtopic.php?t=18493 "BBC PSU recap and RIFA caps", Stardot Forums]</ref>

Before any service work:

# Power off and unplug the mains lead.
# Wait at least 30 seconds.
# Discharge the PSU bulk capacitor across a 1 kΩ / 5 W resistor.
# 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 ==

* '''Acorn BBC Microcomputer Service Manual''' (October 1985), Sections 1 and 2.<ref>[https://chrisacorns.computinghistory.org.uk/docs/Acorn/Manuals/Acorn_BBCMicroServiceManual.html Acorn BBC Microcomputer Service Manual, Chris's Acorns]</ref> 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 ==

# Power off and unplug all leads.
# Turn the machine upside-down on a soft surface.
# 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.)
# Lift the top cover and disconnect the keyboard ribbon cable from the main board connector PL13 by squeezing the ribbon header tabs.
# Disconnect the speaker connector (PL14).
# 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 ==

In service order:

# '''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.<ref>[http://www.dalbystoys.co.uk/bbc/rifa.html "RIFA capacitor replacement on Acorn machines", dalbystoys.co.uk]</ref>
# '''C9 PSU primary bulk electrolytic''' — visually inspect for bulging or leakage; this part is responsible for cold-start reliability on aged PSUs.
# '''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.
# '''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).
# '''Keyboard ribbon''' — clean both ends of the ribbon cable with IPA and a foam swab; reseat firmly.
# '''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 ==

* '''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 ==

Expected rails at the four-pin Molex connector to the main board (refer to the [[BBC Micro Model B+#PCB Schematics and Service Manual|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 ==

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

# Remove the MOS EPROM with an IC extraction tool.
# Reprogram a 27C256 / 27SF512 with the new MOS image.
# 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 ==

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.)

== Recommended Tools ==

* 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.

== Related Pages ==

* [[BBC Micro Model B+]]
* [[BBC Micro B+ Troubleshooting Guide]]
* [[BBC Micro B+ Capacitor Replacement Guide]]
* [[BBC Micro Model A/B General Maintenance]]
* [[Recommended Tools]]

== References ==

<references />

[[Category:Acorn Computers]]
[[Category:BBC Micro]]
[[Category:Maintenance Guides]]