Chapter 6: Keyboards
A method for cleaning keyboard's keycaps is similar as with other computer parts, but I just put them to bowl of water, add some washing powder (only for moulded keycaps, for printed ones normal dishwashing liquid is enough) and stir them using a brush on longer stick. After quarter or 2 they are clean and can be rinsed and dried (put them to old pillowslip, put on warm surface and move from time to time). In the meantime it is usually needed to remove layers of dirt from the keyboard's base plate.
Keyboard mechanism itself (contacts) should be disassembled if and only if there is a problem with it. There are many designs and constructions and there may be a problem if it falls apart. There are few types of keyboard mechanisms:
1. Magnetic relay (reed switch)/hallotron based keyboards (e.g.
Meritum, many Eastern-block computers)
Pressing a key moves a small magnet towards Hall detector or reed switch. In most constructions they are soldered horizontally to base PCB. Problems may include broken reed switch, missing magnet or broken solderings. Some Italian computers use vertically mounted reed switches, they fail mechanically (glass tube cracks) very easily. Remember that if you want to desolder reed switch, use solder pump and don't bend tube connectors as they may break off with tube. These keyboards are quite durable and hitting keys usually doesn't lead to excessive wear of contacts.
|2. Mechanical switch keyboards (e.g. early PCs,
Keys move switches which mechanically join contacts. The most typical problems are broken solders, sometimes even broken tracks if someone slams the key too hard, in rare cases the switches themselves need replacing. It is generally impossible to disassemble it entirely as switches are directly soldered to PCB. You can identify it by removing a keycap - switch will be shown, or seeing a PCB from below, in this case you will see lots of soldered switches, sometimes centered using a bolt in hole.
The problems are fixed by replacing single switches or fixing broken PCB using a soldered wire.
Remember that some very old keyboards (1970s or so) may have an old, mercury-based switches, so be careful while trying to disassemble a switch. Most keyboards from early 1980s and later have normal, mechanical switches.
You usually won't be able to remove the plastic base plate without desoldering all switches from the PCB.
Two types of mechanical keyboard contacts
3. Conductive track (e.g. Atari ST)
PCB is still used, but contains normal tracks (covered with masking lacquer) and exposed gold-plated contacts, usually as concentric spirals or ladders. Key presses a carbon-coated rubber connecting gold-plated contacts. In some rare cases slightly concave metal discs are used. If this keyboard doesn't work, PCB has to be removed and cleaned using denaturated alcohol or IPA. Then using soft cloth dipped in alcohol, every conductive rubber has to be slightly cleaned. Don't wash the conductive layer off the rubber. During disassembly it is essential to keep the rubbers in their places. They may easily fall apart.
It may not be exactly conductive, but sometimes it forms a capacitor, so checking with ohmmeter may not give results. Generally capacitive keyboards have silver contacts looking like alluminium foil.
4. Conductive mat (e.g. some IBM
Similar as 3, but there is a piece of insulation with holes in places of gold-coated contacts. On it, there is a conductive mat. Key presses the piston, which presses the mat to contacts. Because mat may be fragile, it may be pressed not directly with piston, but with spring buckling smaller hammer. This technique has been used in some IBM's keyboards. Such keyboard cannot be easily disassembled as hammers will fall off, but is quite durable as key pressure is not directly transported to contacts. A nice description and illustration is shown in this Wikipedia page.
|5. Membrane with keys printed-in (e.g.
Two layers of conductive foil are divided by matrix with holes. On this sandwich, another layer with printed keys is placed. Keys are not pressed very deep, usually it is needed only to press the foil sheets toegther to make a short. Sometimes 3 layers are used, simulating pressing 2 keys. These layers are separated by two sheets of foil with holes. Pressing a button causes two topmost layers to be shorted first, then bottom of second layer shorts with the lowest layer.
Is is very hard to repair such membrane, because the conductive foil cannot be soldered (melts in lower temperature than solder) and it easily breaks. It can break near frequently used keys, in ribbon cables or printed layer may just wear off and break in pressed areas.
If the ribbon cable is broken, it can be cut and, using a socket, patched with another cable. For example in ZX81 only about 1cm of good ribbon left, the rest was damaged by crack. I decided to unsolder the socket, glue it to the bottom of the casing near the end of a ribbon and plug the remaining ribbon to it. Next, wires were used to connect socket to its place on the mainboard. Repair was successful. Such repair is shown below:
6. Membrane with separate keys (e.g. ZX Spectrum Plus)
As above, but with solid plastic keys pressing the membrane. Repairs are also difficult because it ends with the same membrane. This keyboard is the most popular in later PCs (mid-1990s and onwards). If the keys are made of rubber, like in ZX Spectrum, bad membrane can be replaced with thin PCB and conductive rubbers may be added to the rubber part. Such DIY membrane for ZX Spectrum has been made and works well.
In these keyboards 3-layer version is commonly used so it is not so easy to make the substitute using thin PCB etching.
7. Membrane with rubbers (e.g.
A mix of 6. and 3. Single foil with tracks, exposing gold-plated contacts. Also hard to repair.
Of course different computers usually have different types of keyboards and connecting e.g. robotron PC1715 keyboard to robotron A5120 is not a good idea (leading to permanent keyboard malfunction) even if their plugs are the same. Common PC keyboards have DIN plugs, and they are usually not compatible with 8-bit microcomputers with DIN keyboard connectors. About PC keyboard - there are 2 standards, XT and AT. It's better to have keyboard with a switch allowing to change mode.
Cleaning and restoration
Washing the keyboard
Some people wanted me to describe this. Mostly for "keyboard soup" pictures probably :).
First, check two things which prevent this procedure:
|1. If printing on keys holds poorly and falls off, do
not do this.
2. If you cannot remove a key in a reversible way, i.e. you cannot put it back, definitely do not do this. Old plastic in keyboards may become fragile and latches may break. After this procedure, the plastic may even become more fragile. If you have such fragile keyboard, the better cleaning method will be to use a small paintbrush first to sweep all things from under keys and then cleaning with wet cloth using something flat to go between keys.
If the painting holds on keys well (or they
are double-moulded keycaps) and it is possible to remove and put back
keys without destroying, you're good to go.
|The procedure in fact is simple, and can be described
the following way: Disassemble, wash, rinse, dry, reassemble. However,
as many things may go wrong, take as many photos as you can to put parts
back as they were originally.
1. Remove keycaps and store them in one
container. All other accompanying things like springs, plastic tubes or
latches go to another container. They may be cleaned with a brush,
alcohol or window-washing liquid if they are extremely dirty, but they
rarely get as dirty as keys.
|3. Clean the cover. Separated cover can be cleaned in
running water, even with dish washing powder and some brush.
4. Now the keys. Fill a bowl with warm water, add 1-2 teaspoons of washing powder, put keys in, slowly mix with brush to make keys "click" against each other. For a typical dirt resulting from using and storage in normal conditions this is totally sufficient and after 10 minutes you will get quite clean keycaps.
5. If keys are still dirty, take a few keys by hand and clean with brush in water, then repeat.
6. Whent hey are clean enough, remove all keys to another container. Preferably something with small holes like an old sieve or colander. I use an old metal basket from decomissioned deep fryer.
7. Check one more time that no keys or their parts were left in water. Water after washing is not transparent anymore, so empty the bowl slowly and preferably through drain holes smaller than key or their parts. Sometimes wide keys have a plug-in guides for metal parts - it's better to make sure that all of them are in place.
|8. Rinse the bowl, fill with new water, rinse the keys
by moving them the same way as washing.
9. Repeat the procedure a few times, this is important as the key may loose smooth move if not rinsed well.
10. Put keys in old pillowslip or just wrap in old cloth, move them a little for some time to remove excess water, change the cloth. Do it a few times.
11. Because in the smallest parts there is still water, dry keys for a few hours in a sunny day or ca. 2 days in room conditions.
12. Re-assemble the keyboard.
Repairing reed switch / Hall detector keys:
In fact, there is not much to break in them. Check is magnet in right position and is the reed switch not broken, if anything is broken, replace. In silent environments you can slowly press the key and listen for "click" of the reed switch to check it. Most pushing force is directed not to contact, but right to base board, so these are durable. Reed switches contacts display no mechanical fatigue, as their deformation is just too low. They however have glass casings so they may break when really too big force is applied, e.g. keyboard is thrown with switches exposed. While replacing reed switches do not try to bend their pins holding only the glass envelope as this will break. The proper method is to hold (with pliers) a pin right near the envelope and bend it on the other side. When desoldering a working reed switch, use some pump or wick to remove alloy and then take the switch without bending pins.
Restoring mechanical contact switches, if the contact cannot be disassembled, goes in the following order of fixes:
1. Resoldering the contact to base PCB.
2. Blowing compressed air inside, through the "key shaft" pressed in.
3. Spraying with contact cleaner (usually IPA, Isopropyl alcohol) with keycap removed, it should flow inside, then press it lots of times, wait until evaporates.
4. If all else fails, replacing contact.
I have not found effective method for repairing these transparent foil keymats. When tracks are broken, it's impossible to solder them as melting point of foil is too low, maybe conductive, silver paint? They are so fragile that some liquids (like wine for example) may dissolve them.
Restoring the capacitive keyboard contacts
|You usually have to re-build the contacts. Contacts are
round "sandwiches" which have following layers:
1. A stiff plastic locking the contact in key plunger.
2. A few millimeters of sponge as a buffer.
3. A very thing conductive layer as capacitor's layer.
4. An insulating layer as capacitor's insulator.
So they are not electrical "contact" like in a switch, but capacitive contact, as printed circuit board's tracks contacting the insulating layer with conductive layer behind work as capacitor, passing only some higher frequencies. This may look complicated to detect and generate at first, but technically many times an ordinary multi-bit binary counter chip is used to generate different frequencies (even shift registers can be used). Some op-amp nearby amplifies weak signals passed through capacitance enough to measure their timings by the microcontroller and know which frequency has passed through which contact.
|Unfortunately when the capacitive pad breaks, it is
needed to make new "sandwiches". It is hard to salvage something, as
usually the sponge is worn, the conductive layer could break in half and
stiff plastic has remains of sponge on it. If the contacts on PCB are a
bit worn, as in photo on the left, it's still usually not a problem.
This requires some specific materials, I will try to describe what I
used successfully and where to get them:
|2. The stiff plastic. I found the transparent
plastic for bound documets cover totally sufficient. With one sheet you
can do many pieces.
3. The sponge. Not these closed-cell foam, but the sponge which can be compressed and acts as a buffer for key pressed in. You need a proper thickness. Too thin and layer will not reach the contact on circuit board. Too thick and it will contact a whole time. I found in my keyboards it was a bit smaller than 5mm. The best way is to have a proper thickness measured from old parts (use these which have sponge in one piece). If not, read further, but be careful with the process of reducing thickness.
4. The capacitive layer. Test different foils, it must be silvery and not conductive (check with ohmmeter) - it means that the aluminum layer is there, but is coated with transparent insulator. To test an unknown material in keyboard's printed circuit board, do not put a whole sheet as this will not work, but cut a rough circle of more or less proper diameter, put it rather with some plastic tools instead of fingers as fingers also have some capacity, and monitor does the keyboard register the keypress. I have successfully used the following things:
- A foil used to seal juice boxes, it is conductive on one side and insulating on the other. The problem is that it's small, you can make only 2 round "sandwiches" from one juice box.
- A foil used to store food on delovery. Generally "plastic foil - aluminum foil - paper" composite. Works beautifully and may be present in large quantities in a pizza on delivery. That's where I found my foil.
On the right you can see testing method for capacitive layer with PC XT keyboard and TTL probe. The probe is set on Clock line, and the pads are put to contacts. Change in counter of probe (or generally blinking of LEDs) means that the contacts has been detected. This will not always work as some keyboards may need initialization, so computer or some interface has to be used for checking.
||Now reducing sponge's thickness. Do not reduce e.g.
20mm to 5mm this way because you will produce too much toxic substances,
but I have successfully reduced 8mm to 5mm in a small, 4x10cm square. My
problem was that the commonly accessible sponge used for packing fruits
(the advantage: it was free) was about 8mm thick, so I cut a small part
of it and removed layer by placing it on a hot, flat surface. The sponge
is then removed from plate right when decay is visible (it will mostly
progress for some time a bit more) in form of dark-brown powder. This
MUST be done outside, in well ventilated environment as with high
temperature the sponge will (a) transform to brown powder and (b) emit
toxic fumes. Use something not extremely hot (I used an old cloth iron
mounted with its flat surface upwards) as above 300-350 degrees Celsius
this reaction may make World War I - grade poison gases (sometimes
called "yellow smoke").
Now it is needed to glue it together. What I