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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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 ==

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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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 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 ==