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I now have a number of keyboards available for my System Computer and Atom replica projects some of which reuse original Acorn computer keyboards which require some modification.
Disassembly
The first thing to do is disassemble your donor keyboard. One of those plastic keycap removers is a useful tool for pulling the keycaps off. Then you need to desolder the switches from the PCB, this is generally quite straight forward especially as Acorn only used single sided PCB's for their keyboards. It should be sufficient to remove the solder with a solder sucker and then prise the switches away from the PCB. Its then a good idea to clean up any excess solder from the switch pins so that they drop straight into the new PCB later, solder braid is good for this.
Its then worth spending the time giving each switch a clean with some contact cleaner and then checking they work OK with a continuity checker or multimeter, they are difficult to take out later if they don't work ! If you need to disassemble any switches to clean/repair them they come apart quite easily, just unscrew the contact pins with a pair of pliers and pull off the bottom cover.
Metal Frame
The metal plates will then need some modification to match the layout of the new keyboard. Electron switches need 13mm holes whilst BBC use 16mm. Ideally this is done with a punch in a press as the additional holes overlap existing holes making drilling difficult. Of course not everybody has access to such equipment (including me) so you have to resort to drilling.
To make this easier I have made a 3D printable drilling guide that push fits into two adjacent holes and allows an additional hole to be drilled precisely 19mm away. The guide was designed in OpenSCAD, I have actually only used the 16mm BBC version but have also produced a design for a 13mm version for the Electron.
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OpenSCAD Files
STL Files
I printed mine with solid infill, you may need to tweak the diameter of the spigots to get a good tight fit so that the guide does not move when fitted.
After drilling the additional switch holes the Electron switches require a diagonal alignment slot which I made use a needle file, the BBC switches need a couple of 2mm holes for the alignment pins to go into.
If you use the drilling methods then you will almost certainly need to do a bit of fettling with files to get the switches to line up nicely - its worth spending the time getting it right as its impossible to fix after you've made the keyboard and find the keycaps are wonky.
When finished I give the plate a spray with some satin black paint.
Assembly
The switches are then pushed through the metal frame and soldered in but make sure you fit the space bar holding rings before fitting the C and > keys !
I start off fitting the top right and top left switches, the space bar and C and >, soldering just one pin to start with and making sure they are all pushed right down before soldering the second pin.
I then go ahead and fit the remaining switches again just soldering one pin to start with and making sure they are pushed fully in, also check the switches fitted in the drilled/punched holes are sitting in-line before finally soldering the second pins.
Finishing
Some of the keycaps from a BBC don't quite have the right legends on them though some are close enough and of course the PC keycaps for an MX version are missing several legends. My plan is to print some labels and use a Cricut to cut them out.