Case
The exterior of the BBC Microcomputer is a distinctive and instantly recognisable design. It is the work of Allen Boothroyd, better known as co-founder and Design Director of Meridian Audio. The case is made in two sections — upper and lower — and all the major components of the computer attached to the lower section.
There are three distinct variations of the case: coarse, fine and export. The coarse version is the earliest and is easy to distinguish due to its rougher, grainer texture and unobstructed rear and underside vents. The two later versions have a finer, more uniform texture with only minor detail differences between them.
The difference between the case textures is best seen when comparing them side-by-side, as illustrated above. You will notice that the textures of the upper and lower halves of the coarse texture case are not a good match for each other, though this is not noticeable when sitting in front of the machine.
Although the case gives an indication of the machine’s board issue but it is not always possible to tell exactly what is inside. The coarse textured case is used for all Issue 1, 2 and 3, whilst all Issue 7s are housed in the fine texture or export variant. Issue 4 boards can be found in both coarse and fine case types, depending on when they were produced. The export case does not appear to have been used with the Issue 4 board, however there might be a few early examples built by Wong’s Electronics in this configuration.
Both case types yellow with age through exposure to sunlight. By now, all cases will be yellowed to some extent but an appreciation of the original colour can be gained by opening it up and looking underneath the PCB. You might be surprised by how light it is.
The coarse texture case yellows more severely than the fine texture and can turn almost orange in extreme cases. Areas to sides of the keyboard are often worst affected, no doubt due to oils from hands and fingers helping to accelerate the process. The plastic used in this type of case seems to be softer than the fine texture case, so perhaps the composition is different.
The coarse, fine and export cases are described in detail below, with a comparison section at the end to show differences side-by-side.
Coarse Texture Case
The coarse texture case is used for Issues 1, 2, 3 and some Issue 4 machines. It is most commonly seen with the early ‘BBC microcomputer’ function key strip but a few have the later ‘British Broadcasting Corporation Microcomputer System’ design.
The upper halves of the three case types are all quite similar in design. Apart from the texture, the coarse texture case is distinguished by its open rear vent, which gives a clear view of the PCB inside when viewed from the right angle. Fine texture and export cases have a barrier at the rear of the top moulding which obstructs the opening.
The lower half of the case is where the differences are found. Looking from the rear, the lower half of the coarse texture case lacks a central pillar in the vent, whilst the Analogue In socket cutout has rounded ends. The power supply aperture is open at the top.
The power supply vents on the bottom surface are open and a metal plate is used to cover the bottom of the power supply.
Moving round to the front, there are no lugs to locate the keyboard, even though all keyboards were made with cutouts for this purpose. The keyboard is held in place only by the bolts, which, although it makes assembly slightly less easy, is not inferior once they are done up.
Some coarse texture cases use three keyboard bolts instead of two. The extra bolt is on the left when viewed from the front. This arrangement was introduced within the first year of production and discontinued when the fine texture case was introduced. It doesn’t seem to offer any advantages and, just like the lack of keyboard locating lugs, makes dismantling and re-assembly slightly less easy. Presumably the tool used to mould the lower part of the case was modified to include the extra bolt hole. Machines with three keyboard bolts were built by ICL (serial numbers 008900 onwards — Issue 3), AB Electronics (serial numbers up to 167000 — Issue 3 and 4) and Race (serial numbers up to 310500 — Issue 3 and 4). All serial numbers are approximate. There aren’t enough known Cleartone examples to say definitively whether they built any machines with three keyboard bolts, but if any are in existence they will be very uncommon and made at the very end of Cleartone’s production run, with serial numbers higher than 110000 (Issue 3).
The speaker aperture on coarse texture cases is covered by a rectangular piece of black fabric. This is glued to the case and can become detached as the glue becomes less effective with age. Various different textures of fabric were used, presumably depending on what was easily available at the time. The weave used on early examples is fairly uniform, whereas later ones have quite a distinct pattern. Fine texture and export cases cover the speaker with a plastic grille which forms part of the keyboard assembly — a much more durable design and one which retains a lot less dust.
Fine Texture Case
The fine texture case was used for some Issue 4 machines and all Issue 7s, along with the B+ and B+ 128K models.
The function key strip was almost exclusively the ‘British Broadcasting Corporation Microcomputer System’ version: this is the most commonly seen exterior treatment of the BBC Micro. Exceptions were late Issue 4 machines, some of which had the earlier ‘BBC microcomputer’ key strip, and a small number of B+ machines which featured the ‘64K’ key strip.
The fine texture case is an evolution of the original coarse texture case with minor modifications to improve electrical safety, rigidity, and to make the placement of components during assembly easier.
Electrical safety improvements took the form of redesigned vents. The most important of these was the area beneath the power supply, where the size of the apertures was significantly reduced by using offset slats. These blocked direct access to the power supply, meaning that it was no longer possible to push an object into it at 90ยบ to the case.
Whilst the modifications to the power supply vents were done to prevent harm to users, the change to the rear vent was more in the interests of preventing damage to the computer. There are no high voltages on the main PCB but there was a risk of short circuits leading to component failures if anything conductive was posted through the rear slot. The obstruction added to the rear vent didn’t eliminate this risk entirely but it made path more convoluted and reduced the size of objects which could be dropped in.
The central pillar also helped to make the vent aperture smaller and added a small amount of support and rigidity to the top of the case. However this was still not enough to support the weight of a monitor being placed on top of the case.
Further rigidity was gained with the addition of a bar at the top of the power supply aperture. This area of the coarse texture case feels particularly flimsy and flexible when the top is removed, particularly when the power supply is still in situ. The improvement brings one disadvantage: when removing the PSU, the mains lead must be pulled through the hole and the moulded mains plug must be manoeuvred at exactly the right angle to make it fit.
A small but significant improvement over the coarse texture case can be found in the keyboard area, where two small lugs engage with notches on the keyboard PCB. It is a mystery as to why these lugs are absent from the coarse texture case because all keyboards, except a handful fitted to very early Issue 1 machines, have the necessary notches. With the keyboard now partly held in place by the shape of the case, the third keyboard bolt was apparently deemed unnecessary, so all fine texture cases have only two keyboard bolts.
All fine texture cases lack the speaker cloth found on coarse texture cases. Instead, the speaker cone is protected by a black grille which is part of the keyboard assembly. This grille sits beneath the grille on the function key strip — aesthetically less pleasing but certainly more durable.
The material used for the fine texture case feels slightly harder than that of the coarse texture case. It too is prone to yellowing, though the most extreme cases are not as severe as the coarse texture.
Export Case
The export case is a variation of the fine texture case. It was used on BBC Micros built for the German and USA markets, as well as some UK-bound Issue 7 machines. The only manufacturer which built computers with this case was Wong’s Electronics.
Fine texture and export cases are indistinguishable by looking at the upper section; this part is identical in both types. There are minor differences in the bottom half: The underside has a recess for the serial number and type information sticker and there is an additional hole in the power supply area for an earthing cable.
The sticker recess is empty on UK market Issue 7 computers (pictured above). Export market machines have extensive metal shielding within the case and an extra metal strip in front of the five IDC connectors which does not leave sufficient space for the serial number label.
The earth connection for the outer shielding was made via a cable which emerged through an aperture near the auxilliary power socket. In the UK market example above, this is left empty.
Although Wong’s Electronics was the only manufacturer to use the export case, not all its output was housed in this type of case. UK market machines at the beginning of the production run used the standard fine texture case. The export case was adopted around serial number 02-B01-208000 for tape-only models and 02-ANB03-1002000 for those with a factory-fitted disc interface. These serial numbers are very approximate due to the relatively small number of known examples.
Comparison of Coarse and Fine Texture Cases
This section illustrates some of the detail differences between the coarse and fine texture cases. It is not exhaustive but does cover the main areas. The export case is similar enough to the fine texture case so it is not shown here.