I recently had a few problems with conductive plastic collapsible domes. No, not an obscure medical condition... The first thing that happened was that a couple of keys on one of my keyboards occasionally stopped working. I would press them as usual, and sometimes they would play whilst at other times they wouldn't. Opening up the keyboard I discovered that underneath each key was a small rubber moulding rather than the busbars and springy gold contacts I might have expected. Little bits of soft plastic mean only one thing where switches are concerned: cheap, fragile and a somewhat limited life.

Collapsible dome conductive plastic switches (lets's call them dome switches for brevity) come in two parts: a PCB forms the base, and has a pair of intertwining tracks with gold plating forming the electrical contacts; above the PCB is a dome of soft plastic with a piece of conductive plastic stuck to the ceiling of the dome. When you press the dome, it collapses and the conductive plastic is squashed against the tracks on the PCB, thus connecting them together. When you release the dome, it springs back into its normal shape and lifts up the conductive plastic, thus breaking the contact. Dome switches have all sorts of advantages, since lots of switches can be moulded into a sheet and then placed on top of a PCB for an almost instant low-cost product — most calculators, phones, and TV or Video remote controls are made in this way. Conventional switches are much more expensive, often much larger, and need to be fitted individually. The problems with dome switches arise when the rubber ages and becomes less flexible, or if the conductive plastic begins to fall apart with wear and tear from being squashed onto the PCB, or if dust or dirt gets between the PCB tracks and the conductive plastic.

My problem was now made more difficult by the nature of the switches. Instead of individual contacts under the keys, I had a strip of soft plastic on top of a PCB, across the whole of the keyboard. Removing this piece of delicate plastic involved pulling the keyboard assembly apart, since the PCB was held in by lots of twisted metalwork — easy to assemble at the factory, but a pain to try and repair. The problem eventually resolved itself into what I expected — some dust or dirt had got inside a dome and was preventing a reliable connection. After cleaning the tracks with isopropanol on a cotton bud, and blowing the dome clean with a camera lens blower, I re-assembled the keyboard and tested it — success at last. Moving back to my trusty ST, I went back to typing articles for SOS, and immediately noticed that the 't' key wasn't always working. You guessed it — more dome switches and more work.

Secure in the thought that lightning can surely only ever strike twice, the phonecall came as a surprise. Did I have any thoughts on the reliability of electronic pianos? "It's funny you should say that..." I replied. I must confess that I learned about synthesizers when they were new and exciting, and when the contact switches were made of gold, silver, platinum and other rare metals. 70s Monosynths often used single whisker contacts onto bus-bars, and these could corrode with time, and pick up dust and grease which would eventually cause problems. Matters would hardly be improved by servicing technicians who sprayed 'contact cleaner' aerosol over them, since this gradually goes sticky and... well, you can guess the rest. The problems I found when I serviced keyboards were probably more to do with the ravages of cigarette ash, coca-cola and coffee than gradually stiffening rubber. You got the occasional sticking contact through mechanical failure, but most of the time a thorough clean with isopropanol would do the trick.

I can still remember opening up a Yamaha monosynth in the late 70s and being amazed at the unusual contact system — bus bars covered with conductive plastic. Innovations like this didn't seem to catch on, and the real solution lay in adapting the sort of contacts you get in a microswitch or a relay, where the contacts sort of wipe across each other slightly. Such neat and simple contacts may not look hi-tech, but they certainly last, mainly because the action of one metal surface wiping against another tends to keep things clean. The reliability of such wiping contacts can be quite astonishing. I played a DX7 solidly every day for about six years and had no problems in that time. You still find such contacts on expensive 'top-of-the-range' models.

I have sometimes thought of mentioning the contact type in my reviews of keyboard instruments, but this is often difficult to determine and manufacturers do not always want to reveal such details, especially in the context of criticism. Interestingly enough several variables, like the price, weight and prestige of an instrument, all seem directly related to the quality of its contacts. The problem is that the contacts are hidden deep inside the instrument, and only really affect the long-term playability. If you were to buy a piano that needed its dome switches changing after a year or two of nightly gigging, would you be disappointed? Asking about the contacts may earn you a blank stare, but if enough people ask, we may influence the manufacturers to put decent long-lasting contacts into instruments. Conversely, if no-one ever mentions it, things may get worse.

Putting this sort of thing into context: all electrical contacts are designed for a specific lifetime. The commonly encountered DIN plugs and sockets are probably only designed for hundreds of insertion/removal cycles, whilst some low-cost D-type connectors for computer applications are only intended for use in the tens of insertion/removal cycles. Military or telecomms specification D-type connectors, on the other hand, may cost 10 times more but can have insertion/removal cycle lifetimes of the order of tens or hundreds of thousands. The art of the designer is to get the lifetime of all the component parts of his design just right, so they all fail just after the intended lifetime of the piece of equipment. If the designer gets it wrong, then the equipment either costs too much to be competitive (and lasts too long), or else it fails too early because of just one part.

Whilst the wiping contacts used in pro synthesizers may be long-lived, some other components, particularly the aftertouch sensors in some designs, have less staying power. Channel aftertouch can be achieved in a number of ways. There was the mechanical ingenuity of the EMS Polysynthi (apparently the entire keyboard moved, and this, moved a lever connected to a pot), or the opto-electronic method employed by the cheaper models in the Yamaha CS series where the keys pressed down on a bar which rotated a piece of plastic which was shaped to progressively let more light fall onto a light sensor.

Conductive plastic (as in collapsible domes) can also be applied to aftertouch mechanisms — the Yamaha CS80 used individual lumps of conductive plastic for each key, and this design is still used in instruments like the GEM S2 workstation. Conductive plastic with sufficient toughness to withstand serious use has the disadvantage that considerable force is often required to produce the desired effect, and increasing the electrical gain only produces spurious and unwanted aftertouch responses with normal key-presses. Softening the plastic may make performance easier but it also makes the conductive plastic wear out faster... Serious systems may use more sophisticated opto-electronic systems in which mechanical force bends glass fibre sensors, and there is some means of detecting the variable light transmission. Other systems use variable inductors where pressing the key moves a piece of metal in and out of a coil. And you thought that synthesizers were full of sophisticated technology, didn't you?