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Acorn BBC Micro & Electron

The British educational reference (1981): a 6502, robust, expandable, ubiquitous in UK schools (Elite was born on it). The Electron (1983) is its budget version. Standard 8-bit repair: recap, PSU RIFA, keyboard.

BBC Micro Model B

BBC Micro Model B — photo by Stuart Brady, domaine public (Wikimedia Commons).

Identification

Model Year CPU Note
BBC Model A / B 1981 6502 @ 2 MHz the school standard (B = 32 KB)
BBC Master 1986 65C102 evolution, more RAM/ROM
Acorn Electron 1983 6502 budget version (ULA, slower)

SN76489 sound; RGB/composite output; a rich expansion bus ("Tube", 1 MHz bus).

Repair (technician)

  • Recap + RIFA (PSU mains filter) → Recap; the RIFA smokes/explodes with age.
  • Won't boot / unstable: regulator / RAM / socketed ROM → substitution.
  • Keyboard: contacts / keys → clean.
  • Floppy drive (5.25″ via interface): mechanism → clean / emulator.
  • Electron: the ULA is the weak point (like the Spectrum) → cool/replace.

Procedure — a dead computer, step by step ✅

What this adds

Ordered for a home computer: what kills these is its power supply, its battery, its capacitors, its socketed RAM and its keyboard — roughly in that order, and well before the CPU. → PCB diagnosis

1. The power supply — measured before it is plugged in. This is the one step not to swap around. On a machine of this age a supply does not simply stop: a 5 V regulator that fails high puts 7, 9, 12 V onto the bus and takes the RAM and the custom chips with it. Measure every rail off the machine, under a dummy load if you can, and only then plug it in. This page documents RIFA capacitors on this machine: the mains filter caps that crack, then let go in smoke on the first power-up. Replace them before you switch on, not after.

2. The battery, if it has one. Any RTC or backup cell soldered to a board is a future leak. If yours has one and it has never been changed, take it out before it takes the tracks with it. → Backup batteries

3. Capacitors. Recap is the default on these machines, not a last resort — and on the supply first, since that is what protects everything else. → Recap

4. RAM, on its sockets. These machines were built to be repaired: the RAM, and often the logic, sit in sockets. Reseat everything first — thirty years of oxide is a classic no-boot. Then swap chips: a bad RAM chip usually gives a repeatable pattern of garbage or a screen frozen at a fixed character.

5. The keyboard. A console does not have one; a computer's is a wear part. A membrane keyboard fails as whole dead rows or columns — that is the membrane, not the machine. A mechanical one wants its contacts cleaning, not replacing. Check this page: the failure mode is often documented per model.

6. Storage — tape or disk. Both often come down to the same things: a dirty head, and for tape the azimuth and a belt turned to tar. ⚠️ On the floppy side, check whether the mechanism actually has a belt before looking for one: many do, but some drives are direct drive, and hunting a belt that isn't there means stripping a drive for nothing. A machine that boots but loads nothing is a drive fault, not a logic fault — do not go looking on the board for it.

7. Video output. RF, composite, RGB or a dedicated monitor, depending on the machine (see Identification). Test with something else before you conclude: a period modulator and a modern TV disagree far more often than either is broken.

8. Still nothing. Now the logic: rails on the board, reset, clock, bus. → PCB diagnosis

Common faults

Symptom Likely cause Lead
Smoke at power-on PSU RIFA Replace the RIFA
Won't boot / unstable regulator / RAM / ROM Recap, substitute
Dead keys keyboard Clean
No picture (Electron) ULA Cool / replace

Modernisation & mods — what to buy, how

PSU RIFA + recap

The BBC's PSU has RIFA caps that blow into smoke with age → replace them systematically on acquisition. Recap + check the regulator.

Modern storage - BBC: Gotek + FlashFloppy or HxC 🟡 (5.25″/3.5″ floppy emulation); native SD solutions (MMFS, Datacentre / SmartSPI boards) 🟡 that expose an SD card as a drive → load .ssd/.dsd. - Electron: ElkSD 🟢 (SD via the cartridge port), Plus 1 + modern interfaces.

Video (the emblematic case) - The BBC's TTL RGB output is the original target of RGB2HDMI 🟡 (hoglet's Raspberry Pi project, born on the BBC) → crisp, zero-lag HDMI; otherwise SCART RGB 🟢 + OSSC/RetroTINK.

Expansion - PiTubeDirect 🟢: a Raspberry Pi on the Tube port emulates a second processor (fast 6502, Z80, ARM…) → a huge speed boost. - ROM/RAM boards (sideways ROM sockets) 🟢, Econet networking.

⚠️ Do not host ROMs/games — method only.

Archived technical documentation ✅

Manufacturer documentation archived locally: schematics, block diagrams, expected voltages, parts lists. The reference when a symptom matches no known fault.

  • BBC Micro B — PDF
  • BBC Micro B+ — PDF
  • BBC Master — PDF
  • BBC Master ARM — PDF (scanned by "chris whytehead", 2005)
  • BBC Master Compact — PDF
  • Acorn Electron — PDF
  • BBC Master 512 — PDF

Status

Manufacturer documents kept for preservation (machines out of production for decades), with no explicit licence — the same position as for the cabinet manuals already archived here.

See also

Sources & attribution

Review & corrections

How this space is used

Spotted a wrong value, an outdated procedure, a chip reference that does not match your board? Say so here, with what you observed (model, board revision, serial number, measurement). Every report is cross-checked against a source before anything changes — an unverifiable correction is published as “reported by …, not cross-checked” rather than silently applied.

Reading is open to everyone; posting requires signing in with Discord. Reports from the wiki's declared reviewers are handled first; anyone else's are read too, but go through a human before anything is changed.