BBC Micro Model A/B Troubleshooting Guide

This guide details systematic troubleshooting for the BBC Micro Model A and Model B home computers. These classic 8-bit systems are robust but now decades old, so methodical diagnosis is essential for reliable restoration and repair.

Preliminary & Power-up Checks

Begin by confirming the BBC Micro receives correct voltages and basic startup conditions. Many faults stem from power or socket issues rather than failed chips.

Visual & Basic Checks

• Remove the top cover; inspect for burnt components, corrosion, or bulging capacitors.
• Check all socketed ICs are firmly seated.
• Inspect the power supply for leaking capacitors or burnt smell.
• Confirm the keyboard ribbon cable is securely connected.

Power Supply Voltages

Test Point	Expected Voltage	Notes
5V rail (IC pin 14/40, e.g. 6502 CPU)	+5 V DC (±5%)	Main logic supply
12V rail (disk interface, if fitted)	+12 V DC	Only present on Model B or upgraded A
-5V rail (RAM, if fitted)	-5 V DC	Required for some DRAMs (early Issue 3/4 boards)
RESET line (IC pin 40, 6502)	Low → High	Should pulse low at power-on, then remain high

• If any rail is missing or out of tolerance, recap or repair the PSU before further diagnosis.*

Power-up Sequence

1. Switch on the unit.
2. Confirm the red power LED lights.
3. Listen for a single "beep" from the speaker.
4. Observe the display for a white or black screen with a cursor and "BBC Computer" banner.

If any of these are missing, proceed to the next section.

Display & Chime Diagnostics

The BBC Micro provides clear visual and audio cues at startup. Use these to narrow down faults.

Symptom	Probable Cause(s)	Diagnostic Action
No display, no beep	No power, PSU failure, blown fuse, shorted IC	Check PSU voltages, inspect fuses, feel for hot chips
No display, but beep present	Video circuit failure, ULA (IC6), bad socket	Check video output, swap ULA if possible
Display present, no beep	Speaker, sound IC (IC18), CPU not running code	Check speaker, test with external speaker, probe CPU activity
"Scrolling" or unstable display	Wrong video mode, bad crystal, ULA fault	Try MODE 7, check 16 MHz/8 MHz clock, swap ULA
Garbage or random characters	RAM fault, CPU fault, ROM not read	See memory/ROM section, check CPU clock/reset
"Language?" prompt or "No ROM at ..."	OS ROM or language ROM missing/faulty	Reseat ROMs, replace as needed

Video Output Types

• Composite video (BNC or phono) – test with a known-good monitor.
• RGB output (6-pin DIN) – check cable and monitor compatibility.
• UHF output (RF modulator) – test with analogue TV, channel 36 (UK).

Memory & ROM Faults

RAM and ROM failures are common in ageing BBC Micros, especially with original DRAM chips.

RAM Faults

Symptom	Likely Cause	Diagnostic Steps
No boot, continuous beep	Lower 16K RAM failure	Touch DRAMs for excess heat, piggy-back known-good 4116/4164, check -5V rail
"Bad RAM" message, random crashes	Upper 16K RAM fault (Model B)	Swap upper bank DRAMs, check for dry joints
Corrupt screen, random characters	Address/data bus fault, RAM chip	Logic probe address/data lines, swap suspected DRAM

• Early Model A: only lower 16K fitted. Model B: both banks (32K total).*

ROM Faults

Symptom	ROM Involved	Action
"Language?" prompt	BASIC ROM missing/faulty	Reseat or replace BASIC ROM (IC52)
"No ROM at ..."	Any sideways ROM	Reseat or replace named ROM
No boot, black screen	OS ROM (IC51)	Swap with known-good OS ROM

• ROMs are typically 27128 or 27128-compatible EPROMs; always observe correct orientation.*

Connector & Socket Issues

Loose or oxidised sockets are a frequent cause of intermittent faults.

• Reseat all socketed ICs, especially the ULA (IC6), CPU (IC1), and ROMs (IC51/IC52).
• Clean edge connectors (user port, 1 MHz bus, Tube, etc.) with isopropyl alcohol.
• Inspect keyboard ribbon and connectors for cracks or poor contact.
• Check for broken solder joints on the power socket and video connectors.

Component-level Tests

Clock & Reset

• 6502 CPU (IC1) pin 37: should show a stable 2 MHz clock.
• ULA (IC6): generates system clocks; check for activity on pins 35 (16 MHz), 39 (8 MHz).
• RESET (CPU pin 40): should pulse low at power-on, then remain high.

Piggy-back & Substitution

• Carefully piggy-back a known-good DRAM on top of each suspect chip.
• Swap socketed chips (ULA, CPU, ROMs) one at a time with known-good parts.

Logic Probing

• Probe address/data lines for stuck-high or stuck-low signals.
• Check for activity on the NMI and IRQ lines (CPU pins 6 and 4).

Audio & I/O Failures

Symptom	Probable Cause	Action
No beep or sound	Faulty speaker, sound IC (IC18), ULA	Test speaker, swap sound IC, check ULA
Keyboard dead	Faulty keyboard, ribbon, 6522 VIA (IC3)	Test keyboard on another BBC, check ribbon, swap VIA
Cassette not working	Faulty relay, op-amp (IC7), socket	Listen for relay click, check op-amp, clean sockets
Joystick/user port not working	6522 VIA (IC3), bad port	Swap VIA, inspect port soldering

Storage & Expansion Issues

Floppy Disk Problems (Model B or upgraded A)

• No drive activity: check 1770/8271 disk controller, drive power, ribbon cable.
• "Drive not ready" or "No drive": check drive select jumpers, cable orientation.
• Disk errors: clean drive heads, try known-good drive.

Cassette Interface

• "Loading" message but no progress: check cassette relay, op-amp (IC7), cable.
• No relay click: check relay driver transistor (Q2), 5V rail.

Cartridge/ROM Expansion

• "No ROM at ..." or language not found: check sideways ROM sockets, orientation, and supply voltage.

Error & Beep Code Table

The BBC Micro uses beep codes for basic error reporting at power-on.

Beep Pattern	Meaning	Likely Cause
Continuous tone	RAM failure (lower 16K)	One or more DRAMs faulty
Single beep	Normal startup	System healthy
No beep	CPU, ULA, or ROM fault	Check CPU, ULA, OS ROM

Final Notes

• Always start with power and visual checks before replacing chips.
• Fit sockets when replacing DRAMs or major ICs for future serviceability.
• Many faults are due to poor connections or aged sockets—reseating often cures intermittent issues.
• Never operate the BBC Micro with missing or incorrect voltage rails, especially -5V on early DRAM boards.

⚠️ Power supply — recap before switching on

The BBC Micro's switch-mode PSU is notorious for its RIFA X2 mains-suppression capacitors (a 10 nF/0.01 uF and a 100 nF/0.1 uF across the mains input), which crack and fail with age, sometimes explosively, with plumes of acrid smoke. Standard practice is to replace them before the machine is switched on for the first time in years. A small 220 uF electrolytic in the supply also causes intermittent or failed start-up; recap the whole PSU.

Continuous tone, no display

A BBC that emits a continuous tone with no picture — often first seen after a PSU recap once the supply is healthy again — is commonly a faulty video ULA (the Ferranti video processor). Reseat it first; if that does not help, the ULA itself is suspect (a known failure part, with replacements and modern recreations available).

Core logic

With a healthy PSU, work the BBC's socketed logic: reseat the 6502 CPU, the OS and BASIC ROMs (sideways ROM sockets), the RAM, the 6845 CRTC and the SAA5050 teletext generator (Mode 7). A garbled or wrong-colour display points to the 6845, the SAA5050 or the video ULA; a dead machine with good rails points to the CPU, the ROMs or the system clock. Cross-check the +5 V and other rails at the PSU connector first.

Related Pages

• BBC Micro Model A/B Capacitor Replacement Guide
• BBC Micro ROM Upgrades
• BBC Micro Keyboard Repairs
• BBC Micro Video Output Troubleshooting