BBC Micro Model A/B Troubleshooting Guide: Difference between revisions
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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. | 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. | 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. | ||
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If any of these are missing, proceed to the next section. | 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. | The BBC Micro provides clear visual and audio cues at startup. Use these to narrow down faults. | ||
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* UHF output (RF modulator) – test with analogue TV, channel 36 (UK). | * 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 and ROM failures are common in ageing BBC Micros, especially with original DRAM chips. | ||
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*ROMs are typically 27128 or 27128-compatible EPROMs; always observe correct orientation.* | *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. | Loose or oxidised sockets are a frequent cause of intermittent faults. | ||
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* Check for broken solder joints on the power socket and video connectors. | * Check for broken solder joints on the power socket and video connectors. | ||
== | == Component-level Tests == | ||
=== Clock & Reset === | === Clock & Reset === | ||
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* Check for activity on the NMI and IRQ lines (CPU pins 6 and 4). | * Check for activity on the NMI and IRQ lines (CPU pins 6 and 4). | ||
== | == Audio & I/O Failures == | ||
{| class="wikitable styled-table" | {| class="wikitable styled-table" | ||
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|} | |} | ||
== | == Storage & Expansion Issues == | ||
=== Floppy Disk Problems (Model B or upgraded A) === | === Floppy Disk Problems (Model B or upgraded A) === | ||
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* "No ROM at ..." or language not found: check sideways ROM sockets, orientation, and supply voltage. | * "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. | The BBC Micro uses beep codes for basic error reporting at power-on. | ||
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* Never operate the BBC Micro with missing or incorrect voltage rails, especially -5V on early DRAM boards. | * Never operate the BBC Micro with missing or incorrect voltage rails, especially -5V on early DRAM boards. | ||
== | == ⚠️ Power supply — recap before switching on == | ||
* [[BBC Micro RAM Replacement Guide]] | 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''' — 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).<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 — the '''ULA (IC6)''', the '''6502 CPU (IC1)''' and the '''ROMs (IC51/IC52)''' — 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 V rail down to as low as 4.7 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 ROM Upgrades]] | ||
* [[BBC Micro Keyboard Repairs]] | * [[BBC Micro Keyboard Repairs]] | ||
* [[BBC Micro Video Output Troubleshooting]] | * [[BBC Micro Video Output Troubleshooting]] | ||
[[Category:BBC Micro]] | |||
[[Category:Acorn Computers]] | [[Category:Acorn Computers]] | ||
[[Category: | [[Category:Troubleshooting Guides]] | ||