BBC Micro Model B (Issue 4 motherboard)

This particular computer was produced during a manufacturing change between the earlier BBC Micro computers and the later Issue 4 and Issue 7 machines. As such, the BBC Micro is housed in the older design case which has several differences compared to the later design.

The BBC Micro Model B - Early Issue 4 release

The case texture itself is rougher on the older design and these machines can appear much dirtier than their later counterparts over time due to the deeper crevices that can hold the dirt. The branding too was changed for later models to avoid trademark infringement in the United States as the term "BBC" was trademarked by an American company in the US as opposed to the British Broadcasting Corporation here in the UK.

A detail shot of the BBC Microcomputer branding 

Expansions

This BBC Micro is relatively unexpanded. It contains the following upgrades.

  • An original Intel 8271 Disc Interface
  • BeebMaster 32Kb Sideways RAM

Repair/Change History

This BBC Micro was bought through eBay in November 2010. The machine was in a poor state of repair with several sub systems being faulty. It had apparently been stored unprotected in a loft for many, many years and the poor storage conditions had taken their toll. The machine was internally covered in a thin layer of mould and externally was very grubby indeed.

The machine was stripped down and thoroughly cleaned with the case being soaked for several hours before being treated to a deep clean with a toothbrush and fairy liquid. The case was then dried and cleaned again with an antiseptic solution to kill off any remaining mould spores. The motherboard was cleaned with Isopropyl Alcohol, gently removing all traces of mould.

Once cleaned, the machine was rebuilt and switched on for testing and although it powered on and worked, there were many areas of the BBC Micro that had failed resulting in an extensive test programme being carried out to fully diagnose the issues found with subsequent parts being repaired or replaced.

  • IC3 was failing intermittently causing the machine to simply hang on the low level beep when powering on. The 6522 VIA chip in socket IC69 was switched into the IC3 socket to fix the boot time and keyboard issues. A replacement for IC69 was sourced later to restore the printer and user port functions.
  • The disc interface, Analogue port and Tube interface were all in various states of malfunction. IC’s 20 - 26 were replaced, all disc interface chips were replaced and several sockets were replaced on the disc interface too. IC's 14 and 15 were replaced to eliminate any potential RAM buffer issues; finally IC77 which was found to be faulty and its socket which was worn out were replaced restoring the Disc Interface, Analogue port and Tube to full function.
  • The BASIC I ROM was replaced with the later BASIC II ROM to update the machine to a more common configuration.
  • A BeebMaster 32K sideways RAM upgrade was installed allowing for the soft loading of ROMs.
  • An 8 way DIL switch was added to the keyboard for the ease of setting the computers initial configuration settings.
  • The cassette motor LED solder pad had parted from the PCB and required that the track leading to the LED was repaired. A small piece of wire bypassing the fault was used to re-attach the LED pad to the broken track.
  • The PSU X2 Capacitors were preemptively replaced.
  • The heat sink grease was removed and replaced with new grease and the heat sink clasps were tightened so the heat sink would perform more efficiently.
  • Plastic weld was used on the clear trim to repair several cracks around the speaker grille.
  • In August 2011, the PSU failed. It started producing only 1V even when not under load where it should have been producing 5V. It turned out that rather than the more usual failures, a coil failed due to metal fatigue and had to be re-wound.

The photo shows the faulty coil (L1) which was stripped and re-wound (double winding) with 980mm of 24SWG Enamelled Copper Wire. The resin needed to be removed a piece at a time using fine wire cutters and pliers to cut off and break up the resin.

Inductor coil showing the failed leg due to metal fatigue

  • On re-assembly after fixing the PSU, several keys on the keyboard failed. The problem was eventually traced to a cracked track near to the pins for the down arrow key. Simply re-flowing the solder on the pin connected to the track connecting the down arrow key to the up arrow key fixed the problem.

Translate this site

Acorn BBC Micro Manuals

Whilst the BBC Micro Service Manual is useful, RetroClinic produce a fully revised and updated Service Manual that covers the BBC Micro A/B,B+,Master and Master Compact which you can usually find for sale on ebay.

BBC Micro maintenance

Further information

The BBC Micro case was designed by Allen Boothroyd

Here are some links to further information and resources for the BBC Micro that may be of interest.

The 8 bit Acorn Webring