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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* 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''' — 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 ==

== Related Pages ==

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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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 * 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 ==