Editing BBC Micro Model A/B Troubleshooting Guide

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[[File:BBC Micro Model A-B (photo).jpg|thumb|right|300px|BBC Micro Model A/B. Source: Wikimedia Commons.]]

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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.

== ⚠️ 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.<ref name="bbc">[http://www.breakintoprogram.co.uk/hardware/recapping-a-bbc-model-b-power-supply Recapping a BBC Model B Power Supply], L Break Into Program; [http://blog.tynemouthsoftware.co.uk/2017/05/bbc-micro-power-supply-repair.html BBC Micro Power Supply Repair], Tynemouth Software; and [https://www.retro-kit.co.uk/page.cfm/content/BBC-Micro-PSU-X2-capacitors/ Replacing the BBC Micro X2 capacitors], Retro-Kit. Source for the RIFA X2 mains-cap failure (smoke), the 220 uF electrolytic causing intermittent start-up, the recap-before-power-on practice, and the video-ULA continuous-tone no-display fault.</ref>

== Continuous tone, no display ==

A BBC that emits a '''continuous tone with no picture''' &mdash; often first seen after a PSU recap once the supply is healthy again &mdash; 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).<ref name="bbc">[http://www.breakintoprogram.co.uk/hardware/recapping-a-bbc-model-b-power-supply Recapping a BBC Model B Power Supply], L Break Into Program; [http://blog.tynemouthsoftware.co.uk/2017/05/bbc-micro-power-supply-repair.html BBC Micro Power Supply Repair], Tynemouth Software; and [https://www.retro-kit.co.uk/page.cfm/content/BBC-Micro-PSU-X2-capacitors/ Replacing the BBC Micro X2 capacitors], Retro-Kit. Source for the RIFA X2 mains-cap failure (smoke), the 220 uF electrolytic causing intermittent start-up, the recap-before-power-on practice, and the video-ULA continuous-tone no-display fault.</ref>

== 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.<ref name="bbc">[http://www.breakintoprogram.co.uk/hardware/recapping-a-bbc-model-b-power-supply Recapping a BBC Model B Power Supply], L Break Into Program; [http://blog.tynemouthsoftware.co.uk/2017/05/bbc-micro-power-supply-repair.html BBC Micro Power Supply Repair], Tynemouth Software; and [https://www.retro-kit.co.uk/page.cfm/content/BBC-Micro-PSU-X2-capacitors/ Replacing the BBC Micro X2 capacitors], Retro-Kit. Source for the RIFA X2 mains-cap failure (smoke), the 220 uF electrolytic causing intermittent start-up, the recap-before-power-on practice, and the video-ULA continuous-tone no-display fault.</ref>

== Component-level faults (deep dive) ==

=== Recap the PSU before first power-on ===

Every BBC up to the B+ uses the same power supply, and its '''RIFA X-class mains-filter capacitors''' dry out and tend to fail (a puff of smoke and an acrid smell) shortly after the first power-up following long storage. Replace them before switching a long-stored machine on.<ref name="bbcflt">[https://8bs.com/mag/37/bfaults.txt BBC Model B Computer Faults/Cures], 8BS; and Tynemouth Software, [http://blog.tynemouthsoftware.co.uk/2019/06/bbc-micro-issue-3-model-b-repair.html BBC Micro Issue 3 Model B Repair]. Source for the RIFA mains-filter failure, the socketed-IC reseating (ULA IC6, CPU IC1, ROMs IC51/IC52), the 6845 warm-up display loss, the 74LS245 (IC14) garbage-screen fault, and the PSU spade-connector voltage drop.</ref>

=== Reseat sockets first ===

Loose or oxidised sockets are a frequent cause of intermittent faults. Reseat the socketed ICs &mdash; the '''ULA (IC6)''', the '''6502 CPU (IC1)''' and the '''ROMs (IC51/IC52)''' &mdash; before condemning any chip.<ref name="bbcflt">[https://8bs.com/mag/37/bfaults.txt BBC Model B Computer Faults/Cures], 8BS; and Tynemouth Software, [http://blog.tynemouthsoftware.co.uk/2019/06/bbc-micro-issue-3-model-b-repair.html BBC Micro Issue 3 Model B Repair]. Source for the RIFA mains-filter failure, the socketed-IC reseating (ULA IC6, CPU IC1, ROMs IC51/IC52), the 6845 warm-up display loss, the 74LS245 (IC14) garbage-screen fault, and the PSU spade-connector voltage drop.</ref>

=== Display faults ===

* Loss of display '''after the machine warms up''' is a classic '''6845 CRTC''' failure.<ref name="bbcflt">[https://8bs.com/mag/37/bfaults.txt BBC Model B Computer Faults/Cures], 8BS; and Tynemouth Software, [http://blog.tynemouthsoftware.co.uk/2019/06/bbc-micro-issue-3-model-b-repair.html BBC Micro Issue 3 Model B Repair]. Source for the RIFA mains-filter failure, the socketed-IC reseating (ULA IC6, CPU IC1, ROMs IC51/IC52), the 6845 warm-up display loss, the 74LS245 (IC14) garbage-screen fault, and the PSU spade-connector voltage drop.</ref>
* Garbage or strange characters on screen commonly trace to '''IC14 (74LS245)''' on the data bus.<ref name="bbcflt">[https://8bs.com/mag/37/bfaults.txt BBC Model B Computer Faults/Cures], 8BS; and Tynemouth Software, [http://blog.tynemouthsoftware.co.uk/2019/06/bbc-micro-issue-3-model-b-repair.html BBC Micro Issue 3 Model B Repair]. Source for the RIFA mains-filter failure, the socketed-IC reseating (ULA IC6, CPU IC1, ROMs IC51/IC52), the 6845 warm-up display loss, the 74LS245 (IC14) garbage-screen fault, and the PSU spade-connector voltage drop.</ref>

=== RAM ===

Original DRAM failures are common on ageing boards and give corruption or a failed startup; reseat and substitute suspect chips.<ref name="bbcflt">[https://8bs.com/mag/37/bfaults.txt BBC Model B Computer Faults/Cures], 8BS; and Tynemouth Software, [http://blog.tynemouthsoftware.co.uk/2019/06/bbc-micro-issue-3-model-b-repair.html BBC Micro Issue 3 Model B Repair]. Source for the RIFA mains-filter failure, the socketed-IC reseating (ULA IC6, CPU IC1, ROMs IC51/IC52), the 6845 warm-up display loss, the 74LS245 (IC14) garbage-screen fault, and the PSU spade-connector voltage drop.</ref>

=== Power connections ===

Check the PSU spade connectors: pre-soldered wires develop high-resistance joints that pull the 5&nbsp;V rail down to as low as 4.7&nbsp;V and cause erratic faults. Clean the spades and ensure a tight fit.<ref name="bbcflt">[https://8bs.com/mag/37/bfaults.txt BBC Model B Computer Faults/Cures], 8BS; and Tynemouth Software, [http://blog.tynemouthsoftware.co.uk/2019/06/bbc-micro-issue-3-model-b-repair.html BBC Micro Issue 3 Model B Repair]. Source for the RIFA mains-filter failure, the socketed-IC reseating (ULA IC6, CPU IC1, ROMs IC51/IC52), the 6845 warm-up display loss, the 74LS245 (IC14) garbage-screen fault, and the PSU spade-connector voltage drop.</ref>

== References ==

<references />

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