BBC Micro B+ Troubleshooting Guide: Difference between revisions
Create BBC Micro Model B+ page set |
Expand troubleshooting: RIFA PSU recap (before power-on), video-ULA continuous-tone fault, core logic reseat; cited (Tynemouth/BreakIntoProgram/Retro-Kit) |
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# If banner OK but B+128 reports B+64 memory: re-seat sideways RAM modules in sockets 4 and 6. | # If banner OK but B+128 reports B+64 memory: re-seat sideways RAM modules in sockets 4 and 6. | ||
# Always recap PSU '''before''' chasing intermittent boot failures on a 40-year-old machine. | # Always recap PSU '''before''' chasing intermittent boot failures on a 40-year-old machine. | ||
== ⚠️ 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> | |||
== References == | |||
<references /> | |||
== Related Pages == | == Related Pages == | ||