BBC Micro Machine Hub
Model B, B+ & Master 128 repair reference — RIFA cap warnings, EPROM guides, RGB video, diagnostics and connector pinouts.
RIFA Mains Filter Capacitor — REPLACE BEFORE POWERING ON
Every BBC Micro contains one or more RIFA PME 271 M (or equivalent) X-class mains filter capacitors soldered directly to the PSU board. After 40+ years these capacitors crack, absorb moisture, and fail explosively when mains voltage is applied — producing thick acrid smoke, a loud bang, and potentially showering the PSU with debris. This is not a "might happen" — it will happen on any unserviced machine.
1 Identify the RIFA Caps
- Open the case and locate the power supply board (top-right on Model B, separate module on Master 128).
- Look for yellow or orange rectangular capacitors marked RIFA PME 271 M or similar. Typical values: 100 nF, 47 nF, or 10 nF, rated 250 V AC (X2 class).
- Inspect for visible cracks in the epoxy casing — hairline fractures are enough to allow moisture ingress and catastrophic failure.
- There are typically 2–3 RIFA caps per BBC Micro PSU. The Model B Issue 7 has two; the Master 128 may have three.
2 Remove & Replace
- Disconnect from mains and discharge any residual voltage before working on the PSU.
- Desolder the RIFA caps with a soldering iron. They are through-hole parts and come out easily.
- Replace with modern X2-rated 275 V AC film capacitors of the same value (e.g., Kemet R46, Vishay MKP, or Epcos B32922). Must be X2 or Y2 safety-rated for mains use.
- Do not use standard polyester or ceramic caps as replacements — only safety-rated film caps are appropriate across the mains.
- While the PSU is open, visually inspect the mains bridge rectifier, smoothing electrolytics, and voltage regulator heatsink for any signs of heat damage.
! Why This Matters
- The RIFA PME 271 M uses a hygroscopic epoxy encapsulation that cracks with age, allowing moisture to reach the metallised film inside.
- When mains voltage is applied to a moisture-compromised capacitor, the internal film arcs, generating intense heat and rapid gas expansion.
- Failure mode is a loud bang, flash, and thick white/yellow smoke that fills the room. The capacitor may split open or eject fragments.
- While X-class caps are designed to fail safely (they should not cause fire), the event is dramatic, damages the PCB around the cap, and the smoke is unpleasant and potentially irritating.
- Rule: never power on a BBC Micro without first checking and replacing the RIFA caps.
Model Selector
Select your BBC Micro model to view board details, key ICs, and component notes.
BBC Micro Model B Issue 3 – Issue 7 • 1981–86
The iconic BBC Micro used in UK schools and homes. 32 KB RAM (16 KB available in MODE 7, up to 20 KB in MODE 0–6 depending on screen mode). MOS Technology 6502A CPU at 2 MHz. Analog RGB and composite video via 6-pin DIN. Teletext Mode 7 via the SAA5050 chip. Multiple expansion interfaces: Econet, 1 MHz Bus, User Port, Printer Port, Tube interface for second processors.
Key ICs
- CPU: MOS 6502A @ 2 MHz
- CRTC: Motorola 6845 — CRT controller, all video timing
- Video ULA: Custom Acorn IC — pixel serialiser, palette, screen mode generation
- Teletext: SAA5050 — Mode 7 character generator
- System VIA: 6522 — keyboard, sound, slow peripheral bus
- User VIA: 6522 — User Port, printer, CB1/CB2 I/O
- Serial ULA: Custom — cassette interface, RS-423
- Sound: SN76489 — 3 tone channels + 1 noise
- ROM: OS ROM (16 KB) + BASIC ROM (16 KB) + sideways ROM sockets
Board Revisions & Common Issues
| Issue | Years | Notes |
|---|---|---|
| Issue 3 | 1981–82 | Early production. Some have 6502 (not A). Known for solder bridge issues on the keyboard connector. |
| Issue 4 | 1982–83 | Most common early board. Improved layout. Floppy disc interface optional (extra chip + connector). |
| Issue 7 | 1984–86 | Final revision. Onboard 1770 floppy disc controller. Improved PSU regulation. Best starting point for new owners. |
Common failures: RIFA caps (PSU), 4164 DRAM (16×1-bit, 8 chips), keyboard membrane degradation, ULA failure (custom chip, no modern replacement), and clock crystal drift.
BBC Micro Model B+ 64K & 128K • 1985–86
An upgraded Model B with 64 KB RAM (B+64) or 128 KB (B+128) using paged "shadow" and "sideways" RAM. Largely compatible with Model B software but some timing-sensitive programs fail. Same 6502A CPU at 2 MHz. Improved board layout with fewer chips thanks to integration. Same 6-pin DIN video output.
Key ICs
- CPU: MOS 6502A @ 2 MHz
- CRTC: Motorola 6845
- Video ULA: Custom Acorn (same as Model B)
- Teletext: SAA5050
- System VIA: 6522
- User VIA: 6522
- Sound: SN76489
- FDC: WD1770 onboard
- RAM: 64 KB or 128 KB (4264/41256 DRAM)
- ROM: OS 2.0 ROM + BASIC II + sideways ROM/RAM sockets
Key Differences from Model B
| Feature | Model B | Model B+ |
|---|---|---|
| Main RAM | 32 KB | 64 KB (B+64) or 128 KB (B+128) |
| Shadow RAM | No | Yes — 20 KB shadow screen RAM frees main RAM |
| Sideways RAM | No (ROM only) | Yes (B+128: 4×16 KB banks) |
| FDC | Optional (Issue 4) or 1770 (Issue 7) | WD1770 onboard |
| Econet | Optional | Onboard (active with jumper) |
Compatibility note: Some Model B games that use direct hardware access or rely on exact memory-map timing will not work on the B+ without patches. The shadow RAM paging mechanism at &FE34 is B+ specific.
BBC Master 128 1986–93
The final and most capable BBC Micro. 128 KB RAM with sophisticated paging (sideways RAM, shadow screen, filing system workspace). 65C02 CPU at 2 MHz (CMOS, lower power, extra instructions). Onboard ADFS ROM, WD1770 FDC, cartridge slots, and battery-backed CMOS configuration RAM. Numeric keypad. Same 6-pin DIN RGB output.
Key ICs
- CPU: 65C02 @ 2 MHz (CMOS)
- CRTC: HD6845S / UM6845R — CMOS CRTC variant
- Video ULA: Custom Acorn (same as Model B)
- Teletext: SAA5050
- System VIA: 65C22 (CMOS 6522)
- User VIA: 65C22
- Sound: SN76489
- FDC: WD1770 onboard
- RAM: 128 KB (8×4464 or 2×41464)
- ROM: MOS 3.20 + BASIC IV + ADFS + Edit + View + ViewSheet (in two 128 KB mask ROMs)
- CMOS RAM: 50 bytes battery-backed (DS2401 or MC146818)
Master 128 Specifics
| Feature | Details |
|---|---|
| Cartridge Slots | 2× cartridge slots (top of machine) for ROM cartridges. Each holds up to 32 KB of ROM. |
| CMOS Settings | Battery-backed CMOS RAM stores boot config (language, filing system, display mode, etc.). Battery: CR2032 or 3×AA pack. |
| Internal Tube | Internal Tube connector for second processor boards (65C102, Z80, 32016, ARM). Active when no external Tube device connected. |
| Econet | Onboard, active with jumper/link. ADFS and Econet filing system in ROM. |
| HAZEL RAM | 8 KB "HAZEL" RAM at &C000–&DFFF for filing system workspace. Frees main RAM. |
Common failures: RIFA caps (PSU), CMOS battery leakage (replace immediately), keyboard membrane, and 65C22 VIA failure. The CMOS CRTC variants (HD6845S) behave slightly differently to the original MC6845 — some software tests may give different results.
EPROM & Option ROM Socket Map
Sideways ROM socket assignments, jumper configuration, and EPROM substitution guide.
Model B ROM Sockets IC35–IC52 / IC88–IC101
The Model B has 4 sideways ROM sockets (some Issue 7 boards have more). Each socket holds one 16 KB ROM (or half of a 32 KB EPROM with address line strapping).
| Socket | Bank | Default Use | Notes |
|---|---|---|---|
| IC52 / IC88 | 15 | OS ROM (MOS) | Fixed — always selected at startup. Do not remove. |
| IC51 / IC101 | 14 | BASIC ROM | Default language. Can be replaced with BASIC II or Hi-BASIC. |
| IC50 / IC100 | 13 | Empty / DFS | Disc Filing System ROM if fitted. Otherwise empty for user ROMs. |
| IC49 / IC99 | 12 | Empty | Available for user sideways ROMs (word processors, assemblers, etc.). |
Lower bank numbers have lower priority. Higher-numbered banks are selected first at startup. The OS ROM (bank 15) always has highest priority.
Master 128 ROM Sockets IC24 / IC37 + Cartridges
The Master 128 uses two 128 KB mask ROMs plus two cartridge slots. Internal sideways RAM banks are available for loaded software.
| Source | Banks | Contents |
|---|---|---|
| IC24 (128K ROM) | 0–3 | BASIC IV, ADFS, Edit, View |
| IC37 (128K ROM) | 4–7 | ViewSheet, Terminal, MOS 3.20 (banks 6–7) |
| Cartridge Slot 0 | 8–9 | User cartridges (up to 32 KB each) |
| Cartridge Slot 1 | 10–11 | User cartridges |
| Sideways RAM | 4–7 | Shared with ROM banks; RAM takes priority when enabled |
Sideways RAM is battery-backed on some configurations. Use *SRLOAD and *SRSAVE to load software into RAM banks.
EPROM Substitution Guide 27C128 / 27C256
Modern UV EPROMs and EEPROMs can replace original mask ROMs. Key compatibility notes:
| EPROM | Size | Fits | Notes |
|---|---|---|---|
| 27C128 | 16 KB | Direct drop-in | Pin-compatible with original 16 KB mask ROMs. Most common replacement. |
| 27C256 | 32 KB | With address jumper | Pin 27 (A14) must be connected to a link/jumper selecting the upper or lower 16 KB half, or directly to the bank-select logic. |
| 28C256 (EEPROM) | 32 KB | With adapter | Electrically erasable. Pin 1 (/WE vs Vpp) needs attention — tie /WE high (+5 V) to disable writes. Pin 27 jumper as above. |
| W27C512 | 64 KB | With adapter board | Four 16 KB banks accessible via A14/A15 jumpers. Useful for multi-ROM carrier boards. |
Tip: Always program EPROMs with Vpp at the correct voltage for the device (12.5 V for older 27128, 12 V for 27C128). The 27C256 is the most practical single-ROM replacement. When using 32 KB EPROMs, some boards require cutting a track and adding a wire to route A14 correctly.
Video & Display Guide
6-pin DIN RGB pinout, SCART wiring, and HDMI converter options for the BBC Micro.
6-Pin DIN RGB Pinout All Models
All BBC Micros output analog RGB through a 6-pin DIN socket. The signal is active-high TTL-level RGB (0–5 V logic levels from the Video ULA), not 75 Ω analog like the Amiga.
| Pin | Signal | Notes |
|---|---|---|
| 1 | Green | TTL-level (0 V / ~4.5 V) |
| 2 | Composite Sync | Active-low CSYNC (directly usable for SCART pin 20) |
| 3 | +5 V DC | Directly from main rail. Low current only. |
| 4 | GND | Signal ground |
| 5 | Red | TTL-level |
| 6 | Blue | TTL-level |
Important: The BBC Micro outputs TTL-level RGB (5 V swings), not the 0.7 V p-p analog RGB that SCART TVs expect. You must use voltage divider resistors on each RGB line to bring the level down. A typical divider is 330 Ω in series + 470 Ω to ground on each channel.
BBC Micro to SCART Cable Wiring
Wire a 6-pin DIN plug to a SCART plug with resistor dividers on each RGB line:
| DIN Pin | Signal | Resistor | SCART Pin |
|---|---|---|---|
| 5 (Red) | Red | 330 Ω series + 470 Ω to GND | 15 |
| 1 (Green) | Green | 330 Ω series + 470 Ω to GND | 11 |
| 6 (Blue) | Blue | 330 Ω series + 470 Ω to GND | 7 |
| 2 (CSYNC) | Sync | Direct (already ~1 V logic-low swing) | 20 |
| 4 (GND) | Ground | — | 4, 5, 9, 13, 17, 18, 21 |
| 3 (+5V) | Blanking | 180 Ω to SCART pin 16 | 16 (force RGB mode) |
The +5 V on DIN pin 3 can drive SCART pin 16 (blanking) through a 180 Ω resistor to force the TV into RGB mode. Without this, most SCART TVs default to composite.
HDMI Converter Options
The BBC Micro's 15 kHz RGB output (with TTL levels) needs conversion for modern displays. Options:
- RetroTINK 5X / 4K SCART input (via RGB cable with resistor dividers). Best overall quality and compatibility. Multiple scaling modes, minimal latency.
- OSSC (Open Source Scan Converter) SCART input. Line-multiplication with zero added latency. Requires a display tolerant of non-standard resolutions.
- RGB2HDMI (active converter board) A Raspberry Pi-based digital capture board that reads the TTL RGB signals directly from the BBC Micro's video output (no analog conversion). Pixel-perfect output via HDMI. Fits internally or connects externally. The best dedicated solution for BBC Micro video.
- Cheap SCART-to-HDMI boxes Low-cost passive converters vary hugely in quality. Many introduce significant latency and poor scaling. Use as a last resort.
Best option: The RGB2HDMI board is the gold standard for BBC Micro video — it samples the TTL RGB directly, bypassing all analog conversion, giving pixel-perfect HDMI output. Actively maintained open-source project.
Diagnostic Checklist
Power supply rail checks, Econet and User port testing, and key IC fault symptoms.
Power Supply Rail Checks
After replacing the RIFA caps, verify all PSU output rails before reconnecting to the main board.
| Rail | Spec | Tolerance | Where to Measure |
|---|---|---|---|
| +5 V | 5.0 V DC | ±5% (4.75–5.25 V) | PSU output connector or IC1 (6502) pin 8 |
| -5 V | -5.0 V DC | ±10% | DRAM supply pin (Model B only, 4116/4164 dependent) |
| +12 V | 12.0 V DC | ±10% | PSU header or DRAM supply pin (early boards with 4116 RAM) |
Common PSU Faults
- Completely dead: check mains fuse, RIFA caps, and bridge rectifier.
- Low +5 V (below 4.6 V): failing 7805 voltage regulator. Replace and add fresh thermal paste to heatsink.
- Excessive ripple on +5 V: dried-out smoothing electrolytics. Recap with 105 °C low-ESR parts.
- Missing -5 V or +12 V: check the 7912/7812 regulators and associated electrolytics.
Econet & User Port Testing
The Econet interface and User Port are driven by the two 6522 VIA chips. Failures in either VIA cause specific, testable symptoms.
Econet Tests (System VIA / IC3)
- Econet not responding: check 6854 ADLC (IC86) and clock module (IC81).
- No network detected: verify termination resistors (120 Ω) and cable continuity.
- Station number not saved: CMOS battery dead (Master 128) or Econet configuration link not set (Model B).
- Econet causes system hangs: 6854 ADLC failure or clock module not oscillating. Verify clock output with a scope.
User Port Tests (User VIA / IC69)
- User Port completely dead: User VIA (6522 at IC69) failure. Swap with known-good chip.
- Partial I/O failure: individual port B data lines (PB0–PB7) stuck high or low. Test with a logic probe.
- CB1/CB2 lines not toggling: VIA timer/shift register fault or wrong DDR (Data Direction Register) config.
- Printer port dead: shares User VIA on Model B. Check printer strobe line (active-low via CA2).
The 6522 VIA is a DIP-40 IC and is socketed on most boards. Swap the System VIA and User VIA to isolate which chip is at fault.
Key IC Fault Symptoms
Quick reference for identifying the most commonly failing ICs in the BBC Micro.
| IC | Chip | Failure Symptoms |
|---|---|---|
| IC1 | 6502A CPU | Completely dead machine (no beep, no video). Check clock on pin 37 (Φ0 in) — should be 2 MHz. No clock = oscillator or divider fault. |
| IC18 | 6845 CRTC | No video output, blank screen, or corrupted display timing (wrong number of lines, off-centre image). Video ULA may still show colour bars if CRTC is dead. |
| IC6 | Video ULA | Missing colours, wrong palette, garbled pixels in modes 0–6 (Mode 7 unaffected — it uses the SAA5050). Custom chip with no modern replacement. |
| IC2 | SAA5050 | Mode 7 teletext display corrupt or blank. Modes 0–6 unaffected. Relatively easy to find as spares. |
| IC3 | System VIA (6522) | Keyboard dead, no startup beep, sound chip not responding, slow bus peripherals not working. |
| IC69 | User VIA (6522) | User Port dead, printer not working, CB1/CB2 I/O lines not toggling. |
| IC22 | SN76489 | No sound output. Beep on startup absent. Verify VIA CA2 connection to sound chip enable. |
| IC4–IC19 | 4164 DRAM | Continuous beep on startup (RAM test failure), garbled display, or random crashes. Use a logic probe or piggyback known-good RAM to identify the faulty chip. |
Tip: A continuous tone on startup indicates a RAM test failure. Count the number of beeps (if distinct) — some OS versions give a count indicating which bank has failed. Use an oscilloscope on the DRAM data and address lines to isolate the faulty chip.
Key IC Pinouts
Direct links to full pinout reference pages for the major ICs in the BBC Micro.
MOS 6502
CPU — 8-bit processor. The heart of every BBC Micro (6502A at 2 MHz, 65C02 in the Master).
View Pinout →MC6845 CRTC
CRT Controller — generates all video timing, horizontal and vertical sync, cursor, and light pen input.
View Pinout →Intel 8255A PPI
Programmable Peripheral Interface — keyboard scanning, DIP switches, and centronics printer port on some systems.
View Pinout →Connector Pinouts
User Port, Econet, and 1 MHz Bus connector references.
User Port 20-Pin IDC
A general-purpose parallel I/O port directly connected to the User VIA (6522, IC69). Used for joystick adapters, MIDI interfaces, printers, and custom hardware.
| Pin | Signal | Pin | Signal |
|---|---|---|---|
| 1 | +5 V DC | 2 | CB1 |
| 3 | +5 V DC | 4 | CB2 |
| 5 | GND | 6 | PB0 |
| 7 | GND | 8 | PB1 |
| 9 | GND | 10 | PB2 |
| 11 | GND | 12 | PB3 |
| 13 | GND | 14 | PB4 |
| 15 | GND | 16 | PB5 |
| 17 | GND | 18 | PB6 |
| 19 | GND | 20 | PB7 |
PB0–PB7 are directly mapped to VIA Port B (address &FE60). CB1/CB2 are control lines. Set DDR at &FE62 before use.
Econet Port 5-Pin DIN
Acorn's proprietary local area network using a 6854 ADLC and a clock module. Allows file sharing, printing, and remote booting across connected BBC Micros.
| Pin | Signal | Notes |
|---|---|---|
| 1 | Clock | Network clock (active-low, differential) |
| 2 | GND | Signal ground |
| 3 | Data+ | Data line (positive, differential) |
| 4 | Clock | Clock return (differential pair with pin 1) |
| 5 | Data− | Data return (differential pair with pin 3) |
Econet uses 120 Ω termination at each end of the network. Maximum cable length ~500 m. The clock must be generated by a dedicated clock box or an Econet bridge/server.
1 MHz Bus 34-Pin IDC
A general expansion bus running at 1 MHz (active during the Φ2 low phase of the 2 MHz clock). Used for Teletext adapters, music synthesisers, IEEE-488 interfaces, and more.
| Pin | Signal | Pin | Signal |
|---|---|---|---|
| 1 | +5 V | 2 | R/W |
| 3 | +5 V | 4 | 1MHzE |
| 5 | GND | 6 | A0 |
| 7 | GND | 8 | A1 |
| 9 | GND | 10 | A2 |
| 11 | GND | 12 | A3 |
| 13 | GND | 14 | A4 |
| 15 | GND | 16 | A5 |
| 17 | GND | 18 | A6 |
| 19 | GND | 20 | A7 |
| 21 | GND | 22 | D0 |
| 23 | GND | 24 | D1 |
| 25 | GND | 26 | D2 |
| 27 | GND | 28 | D3 |
| 29 | GND | 30 | D4 |
| 31 | GND | 32 | D5 |
| 33 | GND | 34 | D6 / D7 (/NRST on some pins) |
Address range: &FC00–&FCFF (active when /NPGFC active-low). The 1MHzE signal is the active enable — directly gates peripheral access. Active during the 1 MHz phase of the 2 MHz clock.
Resources & Further Reading
Blog posts, bench tools, and community links for BBC Micro repair and restoration.
Smiggy's BBC Micro Blog Posts
Bench Tools
- IC Pinout Finder — search all retro ICs
- Recap Planner — capacitor replacement guide
- Ohm's Law Calculator
- Capacitor Code Decoder
- C64 Machine Hub — Commodore 64 reference
- Amiga Machine Hub — A500/A600/A1200 reference
- ZX Spectrum Hub — 16K/48K & +2/+3 reference
- Dragon 32/64 Hub — Dragon 32 & 64 reference
External Resources
- StarDot Forums — the largest active BBC Micro community
- BeebWiki — BBC Micro encyclopaedia and technical reference
- 8-Bit Software — BBC Micro software archive
- BBC Micro at NVG — programs, ROMs, and documentation archive
- RGB2HDMI (GitHub) — pixel-perfect HDMI output project