Editing BBC Micro Model A/B Troubleshooting Guide

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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 ===

{| class="wikitable styled-table"
! 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 ===
# Switch on the unit.
# Confirm the red power LED lights.
# Listen for a single "beep" from the speaker.
# 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.

{| class="wikitable styled-table"
! 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 ===

{| class="wikitable styled-table"
! 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 ===

{| class="wikitable styled-table"
! 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 ==

{| class="wikitable styled-table"
! 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.

{| class="wikitable styled-table"
! 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.

== Related Pages ==
* [[BBC Micro Model A/B Capacitor Replacement Guide]]
* [[BBC Micro ROM Upgrades]]
* [[BBC Micro Keyboard Repairs]]
* [[BBC Micro Video Output Troubleshooting]]
[[Category:BBC Micro]]
[[Category:Acorn Computers]]
[[Category:Troubleshooting Guides]]