BeebControl>>Stealthmaster construction


First of all, why bother?
I had a number of motives for creating StealthMaster. First of all, as an ex-designer myself, I’d always hated the Master’s clunky case design when compared with the simple classic lines of the Beeb. Secondly, I’m extremely short of desk space and having something the size of a Master lying around just for occasional firing up was not really practical. Thirdly, I happened to have acquired a jumble of odd Master and Beeb case halves and a bunch of broken, semi cannibalised inner components that would otherwise have been heading for the skip. And finally, I like a mechanical/electrical challenge.

What had to go?
The main aim of the conversion was to get the best of both worlds. That is, the extra power, memory and facilities of the Master coupled with the small footprint and classic lines of the B. But you can’t get a quart into a pint pot, so clearly something had to give.

I’ve never had much of a use for number keypads – I don’t even use a keypad keyboard with my RPC/PC – so the fact that the Master’s keypad had to go was not a problem for me. The two cartridge slots were more of an issue though and I thought long and hard about ways to incorporate them into the design. In the end I had to admit defeat and make do with a single rom extension socket in the usual Beeb style. All the Beeb case tops available already had the rom cutout present, so I would have had to make some use of it anyway. If rom cartridge support is needed in the future, I’ll make up a separate 8-way rom box using a cable that feeds into the internal cartridge slots via the rear of the Beeb case.

Other than those two features, everything else on the Master went into the Beeb.

Fitting it all in
There was surprisingly little ‘modification’ of components required to fit everything in, although extra effort was required to do it neatly and permanently.

Starting with the case
The case needed a fair amount of alteration, mostly at the rear, but only involved taking away material rather than adding it on. For instance the PSU was a mite too wide to fit inside the existing compartment (by about 1mm) and its ventilation slots assumed free air circulation at the side. This meant that the compartment side wall had to go. Similarly the PSU rear with on/off switch and power cable wouldn’t quite fit into the existing rear cutout, so the top bar on the cutout had to be removed.

The metal socket facia at the rear of the motherboard was easily shortened to fit, but the differing socket layout and other alignment issues still required a full cutout to be made at the rear to accommodate everything properly.

Finally, the long slot underneath the case needed to have its central stay removed and a strip cut away from its bottom edge to let the motherboard sit a little lower.

A number of other detail mods were needed as each component was fitted, but these were minor in comparison. All work on the plastic was carried out with a hand-held junior hacksaw blade, a selection of files, a scalpel, a ¼” wood chisel and a small drill brace.

This needed hacksaw and file modifications to the front face to clear the protruding hump that’s present in the Beeb case. I stripped the PSU innards out before this process to avoid metal dust shorting any tracks. Two round head screws on top were replaced by countersunk versions to keep the overall PSU height to a minimum as it’s a touch taller than the Beeb unit. Two new mounting holes were drilled in the underside of the plastic case.

I drilled the mounting rivets out of the rear facia brackets so that I could work on the facia separately, using 4BA bolts to remount it later. After shortening to the same width as the motherboard, it needed notches cut to clear the lid mounting brackets and two holes drilled using a drill press to allow mounting to the remaining wings of the case rear (4BA bolts again).

The motherboard itself was fine, but needed a plastic cover to protect the tracks where they protruded further through the under-slot. The protruding component wires on the underside of the board all had to be trimmed right back to provide clearance for the board and plastic sheet in the slot. I used the thin plastic card available in sheets from model shops, held in place by 6BA nuts and bolts in the outermost socket mounting holes. I also cut out a strip containing socket names from the Master's underside and repositioned it on the edge of the slot so the sockets could still be identified properly.

Various lengths of plastic support were added to the underside of the motherboard at intervals to prevent it buckling when roms and spade connectors etc are inserted.

This needed major work to shorten it down, although none of it was tricky. Principally, the number pad was cut off and clearance for the right-hand case support pillar was contoured in, plus three track links were remade with wires on the underside. A ¼” MDF riser board was also required as the Master keyboard is considerably thinner than the Beeb version. The three LEDs were re-sited (dab of superglue) into the underside of the case top and suitable extension wire runs added (the ‘cassette motor’ LED now represents ‘power on’).

The slightly different key layout on the Master keyboard required some careful modification of the keyboard cutout. I used the existing slots as guides to mark out the width for new cutouts. I scalpelled along the marked lines and carefully removed the brown decal sheet areas first, leaving the corners as 45 degree angles at this stage. Then I used the small hacksaw blade and scalpel to remove the remaining plastic, cutting at an angle so that the white slot edge wouldn’t show from the top. Finally I used a ¼” round file to finish into the corners and a flat file to round the tips.

Two remaining blank key spaces (at the break key lock position and right of the down arrow), were filled with non-operative dummy keys after the plastic key lock was levered out. I took two faulty keyswitches and fixed the actuating pillars with a dab of superglue. Then I sawed the switch bodies off so that I could superglue the remaining top slices in place as dummy mountings for keys to fill the gaps. I used the dot and comma keytops from the number pad with the symbols blacked out.

I used two ends from a redundant extension keyboard cable to wire up new keyboard leads, as the old connectors were now much too short because of the socket offset.

The two existing mounting holes in the base, next to the case pillars, were opened out all the way through ready to fix the keyboard in place with 4BA bolts. After carefully positioning the keyboard evenly in the cutout, it was wedged in place with large lumps of blu-tack to stop it moving while the case was turned over and the mounting holes spotted through lightly into the MDF. This MDF mounting also seems to get rid of that metallic twang that the Master’s keys always exhibited – it now sounds and feels much more like a Beeb keyboard. It fact I would say the ‘feel’ is a definite improvement on both of them.

I used the left over number pad keyswitches to replace any on the main board that showed as the slightest bit hesitant on screen. I modified the Master’s speaker holder to carry the battery pack too and refitted it into its old position. This new Beeb speaker position also helps stifle the usual background speaker twittering.
Now everything works perfectly.
By the way, I can recommend 'Mr Muscle Professional' and a stiff toothbrush as a case cleaning agent ;-)

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