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 RGB video 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.

DIP-40

MOS 6502

CPU — 8-bit processor. The heart of every BBC Micro (6502A at 2 MHz, 65C02 in the Master).

View Pinout →

DIP-40

MC6845 CRTC

CRT Controller — generates all video timing, horizontal and vertical sync, cursor, and light pen input.

View Pinout →

DIP-40

Intel 8255A PPI

Programmable Peripheral Interface — keyboard scanning, DIP switches, and centronics printer port on some systems.

View Pinout →

DIP-28

WD1770 / 1772 Disc Controller

Single-rail (+5 V) FM/MFM floppy controller fitted to Model B+/Master 128 and Model B 1770 DFS upgrades.

View Pinout →

DIP-28

TI TMS5220 Speech

IC99 — Linear Predictive Coding speech synthesizer paired with the Kenneth Kendall TMS6100 PHROM (IC98).

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 & Tube 34-Pin IDC

See also our dedicated 6-Pin DIN RGB SCART, 7-Pin DIN Cassette, 5-Pin Domino RS-423, and 34-Pin Shugart Floppy pages. 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

Retro File Utilities (Rust/Wasm) — 6502/6502A disassembler & Sideways ROM .BIN CRC32 verifier/patcher
IC Pinout Finder — search all retro ICs
Connector Pinout Directory — 6-Pin DIN RGB, 1MHz Bus & Tube, 7-Pin DIN Cassette, 5-Pin Domino RS-423, 34-Pin Shugart Floppy
Recap & Repair Reference — capacitor replacement guide
Ohm's Law Calculator
Resistor & Capacitor Code Calculator
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
Amstrad CPC 464 Hub — CPC 464 & Datacorder reference
C128 Machine Hub — C128, C128D & DCR reference
SX-64 Machine Hub — Executive Portable C64 reference
Atari ST Hub — 520ST, 1040STFM & STE reference
Apple IIe Hub — Unenhanced, Enhanced & Platinum reference
Sinclair QL Hub — 68008, Microdrive & ULA reference
Acorn Atom & Electron Hub — Atom & Electron reference
Osborne 1 & Kaypro II Hub — CP/M luggable reference
Oric-1 & Atmos Hub — Oric 6502A & ULA reference
Amstrad CPC 6128 Hub — 128K & 3″ disc reference
MSX Machine Hub — MSX1, MSX2, MSX2+ & Turbo R reference
Big-Box Amiga Hub — A1000, A2000, A3000 & A4000 reference
Atari 8-Bit Hub — 400/800, 800XL & 130XE reference
VIC-20 Machine Hub — Commodore VIC-20 reference
Commodore PET Hub — PET 2001, 4000 & 8000 reference
Plus/4 & C16 (TED) Hub — C16, C116 & Plus/4 reference
TRS-80 & CoCo Hub — Model I/III/4 & CoCo 1/2/3 reference
TI-99/4A Machine Hub — 16-bit TMS9900 & PEB 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

Acorn Computers 1981 · 8-Bit Computers

BBC Micro (B / B+ / Master 128) — Unified Workbench Reference

RIFA PME271M mains capacitor alert, Model B/B+/Master 128 selector, Sideways ROM socket map, 6-pin DIN RGB SCART, and Tube/1MHz bus pinouts.

Switch System Hub (36):

CPU / Architecture MOS 6502A / 65SC12 @ 2 MHz

Memory (RAM / VRAM) 32 KB – 128 KB SRAM/DRAM

Capacitor Mount Through-Hole

Custom Chipset & Key ICs

6502A 2MHz HD6845 CRTC SAA5050 Teletext SN76489 PSG 6522 VIA 8271 / WD1770

Capacitor & Power Stage Reference

BBC Micro / Master — Capacitor & Recap Reference

Board revisions, electrolytic capacitor lists, voltage/farad ratings, and Through-Hole servicing notes.

BBC Micro / Master — Printable Board Recap & Capacitor BOM ReferenceFormat this system's capacitor replacement list, diagnostic flowchart, and IC pinouts into a high-contrast bench PDF.

BBC Micro & Master 128 — Model B (Issue 4 / Issue 7), Master 128 & Astec Switched-Mode PSU

Acorn 6502A mainboards & Astec AA10050 PSU • Mount: Through-hole • Difficulty: Beginner friendly

Full Printable Recap Sheet →

Astec Switched-Mode PSU (BBC Model B & Master 128) (6 Capacitors)

Ref	Capacitance	Voltage	Package / Type	Circuit Location & Bench Notes
`C1`	0.1 uF (100 nF)	275 V AC	Class X2 Film (15mm pitch)	Replaces cracking amber RIFA PME271M smoke bomb!
`C2`	0.01 uF (10 nF)	275 V AC	Class X2 Film (10mm pitch)	Replaces smaller RIFA PME271M cap.
`C9`	220 uF	25 V	Radial (105 C Low-ESR)	When C9 dries out, the PSU refuses to start from cold.
`C13`	1000 uF	10 V	Radial
`C14`	1000 uF	10 V	Radial
`C16`	220 uF	16 V	Radial

BBC Micro Model B Mainboard (Issue 4 / Issue 7) (6 Capacitors)

Ref	Capacitance	Voltage	Package / Type
`C1`	220 uF	10 V	Axial
`C2`	47 uF	16 V	Axial
`C7`	10 uF	25 V	Radial
`C15`	10 uF	25 V	Radial
`C19`	100 uF	10 V	Axial
`C36`	47 uF	16 V	Radial

Printable Diagnostic Flowchart & Voltage CheckpointsFormat this step-by-step fault isolation tree into a high-contrast bench reference sheet.

BBC Micro / Master — Interactive Diagnostic Flowchart & Voltage Checkpoints

Enable JavaScript to run the client-side interactive fault-isolation flowchart engine.

Complete Diagnostic Flowchart: Capacitive Load, ESR & Power Rail Ripple Diagnostic Tree (Universal Retro Mainboards (C64, Amiga, ZX Spectrum, Game Gear, Consoles))

STEP 1: Measure 100Hz/120Hz Peak-to-Peak AC Ripple on the +5V / +12V Bulk Filter Cap [node-1]

Set your oscilloscope to AC Coupling, 200 mV/div, and 2.5 ms/div (20 MHz bandwidth limit ON). Probe directly across the main bulk filter capacitor (e.g. C19 2200µF / C88 1000µF on C64, or C303/C304 on Amiga) under full system load. Measure the peak-to-peak sawtooth ripple amplitude (mVpp).

Expected Reading: < 50 mVpp clean regulated rail (post-regulator) or < 350 mVpp on raw unregulated bulk DC before the 7805/7812.

STEP 2: Check Linear Regulator (7805 / 7812) Input-to-Output Dropout Margin [node-2]

Switch oscilloscope or multimeter to DC Volts and measure Pin 1 (Input) of the 7805 (+5V) or 7812 (+12V) regulator at the bottom of the ripple trough. Verify that Input Voltage remains at least 2.0V above the regulated output (e.g. ≥7.2V into a 7805, ≥14.2V into a 7812).

Expected Reading: 7805 Input ≥ 7.2V DC; 7812 Input ≥ 14.2V DC. Below this dropout threshold, ripple punches straight through to the logic rail.

STEP 3: Check Local IC Decoupling & Audio/Video AC Coupling Capacitors [node-3]

Probe audio output coupling caps (e.g. C91 100µF on C64 SID Pin 27, or C324/C334 22µF SMD on Amiga Paula) — compare upstream vs. downstream signal amplitude. Inspect SMD electrolytic pads (Amiga 600/1200, Game Gear, DMG-01) under magnification for fishy odor, dull grey solder corrosion, or lifted copper traces.

Expected Reading: Full audio/video swing across the coupling capacitor with zero DC leakage on the output side and clean, shiny solder pads.

[ISOLATED FAULT: RES-BULK-CAP] Fault Isolated: High-ESR Bulk Filter Capacitor (>100 mVpp Rail Ripple) VERDICT

Excessive 100/120Hz sawtooth ripple on the DC rail confirms that the primary reservoir/filter electrolytics have dried out and lost effective capacitance.

[ISOLATED FAULT: RES-REG-DROPOUT] Fault Isolated: Linear Regulator Dropout / Bridge Rectifier Diode Open VERDICT

The pre-regulator DC rail is sagging below the minimum 2V dropout headroom required by the 7805/7812 regulator.

[ISOLATED FAULT: RES-COUPLING-CAP] Fault Isolated: Open/Leaky Audio or Video Coupling Capacitor & Trace Corrosion VERDICT

Signal attenuation, muffled audio, or DC offset hum across the coupling stage indicates dried-out or leaking coupling capacitors.

[ISOLATED FAULT: RES-RAILS-HEALTHY] Verification Passed: Power Rails, Ripple & Coupling Capacitors Within Spec VERDICT

All measured DC supply rails, AC ripple levels (<50 mVpp), and coupling stages pass bench specification. If performing a preventive recap, use the verified board BOM below.

Interactive Pinouts, Visualizers & Disassemblers

BBC Micro / Master — System Bench Tools & IC Pinouts

Click any chip below to open its interactive DIP/PLCC pinout diagram, pin-by-pin signal table, and oscilloscope test waveforms.

BBC Micro / Master — Printable IC Pinout & Chipset SummaryPrint this system's custom IC pinout summary or open any chip card below for its full DIP/PLCC pinout sheet.

DIP-14 74-Series Gates

74LS00

SN74LS00N / DM74LS00N

Quad 2-input NAND gate — the most common glue-logic part in retro computers.

BBC Micro Machine Hub

RIFA Mains Filter Capacitor — REPLACE BEFORE POWERING ON

1 Identify the RIFA Caps

2 Remove & Replace

! Why This Matters

Model Selector

BBC Micro Model B Issue 3 – Issue 7 • 1981–86

Key ICs

Board Revisions & Common Issues

BBC Micro Model B+ 64K & 128K • 1985–86

Key ICs

Key Differences from Model B

BBC Master 128 1986–93

Key ICs

Master 128 Specifics

EPROM & Option ROM Socket Map

Model B ROM Sockets IC35–IC52 / IC88–IC101

Master 128 ROM Sockets IC24 / IC37 + Cartridges

EPROM Substitution Guide 27C128 / 27C256

Video & Display Guide

6-Pin DIN RGB Pinout All Models

BBC Micro to SCART Cable Wiring

HDMI Converter Options

Diagnostic Checklist

Power Supply Rail Checks

Common PSU Faults

Econet & User Port Testing

Econet Tests (System VIA / IC3)

User Port Tests (User VIA / IC69)

Key IC Fault Symptoms

Key IC Pinouts

MOS 6502

MC6845 CRTC

Intel 8255A PPI

WD1770 / 1772 Disc Controller

TI TMS5220 Speech

Connector Pinouts

User Port 20-Pin IDC

Econet Port 5-Pin DIN

1 MHz Bus & Tube 34-Pin IDC

Resources & Further Reading

Smiggy's BBC Micro Blog Posts

Bench Tools

External Resources

BBC Micro (B / B+ / Master 128) — Unified Workbench Reference

BBC Micro / Master — Capacitor & Recap Reference

BBC Micro & Master 128 — Model B (Issue 4 / Issue 7), Master 128 & Astec Switched-Mode PSU

Astec Switched-Mode PSU (BBC Model B & Master 128) (6 Capacitors)

BBC Micro Model B Mainboard (Issue 4 / Issue 7) (6 Capacitors)

Complete Diagnostic Flowchart: Capacitive Load, ESR & Power Rail Ripple Diagnostic Tree (Universal Retro Mainboards (C64, Amiga, ZX Spectrum, Game Gear, Consoles))

BBC Micro / Master — System Bench Tools & IC Pinouts

74LS00

74LS04

74LS138

74LS139

74LS153

74LS244

74LS245

74LS373

74LS74

74LS86

CD4011

CD4013

MOS 6502

WDC W65C02S

Zilog Z80

MOS 6522 VIA

Motorola MC6850 ACIA

WD1770 / WD1772 Floppy Controller

Motorola MC6845 CRTC

TI SN76489

TI TMS5220 / TMC0285 Speech

2764 8K x 8 EPROM

27128 / 27256 EPROM & CPC 40009 System ROM

4164 64K x 1 DRAM

6264 8K x 8 SRAM

Maxim / TI MAX232

NE555

Signal & Trace Visualizer (Scope)

Retro File Utilities (Rust/Wasm)