In E&MM October '83 we featured a DIY mains plugboard with filtering against interference and 'spikes' on the mains supply. In case you didn't know, 240V mains is randomly afflicted by short-term spikes (excess voltages) of 1000 volts or more. Higher voltages typically occur if a local thunderstorm involves a direct lightning strike on power lines. In both instances, sophisticated digital gear can be well and truly zapped, and if your DX7, Drumulator (or whatever) dies, it goes without saying that the event could prove a nasty blow to your bank balance.

So, for musicians who are not of great practical bent or haven't got the time/space to get into constructing a complete DIY mains filter system, here's a cheap and simple way of protecting your gear. You simply wire a small, wire-ended component known as a voltage-dependent resistor across the live and neutral pins inside every mains plug belonging to equipment which needs protection. The VDR exhibits a very high resistance to 240V mains, and effectively 'isn't there' under normal conditions. When a 1000 volt spike arises, however, the VDR's resistance drops to a few ohms, effectively shorting-out or 'clamping' the excess energy.

When wiring, sleeve the leads, and take care not to bend them too close to the seal - otherwise they may break off. If you don't find a VDR suitable for 240V mains in your usual electronic supplier's catalogue, ask your dealer to obtain one for you, from RS say, stock no. 238-609. The cost should be around 70p, which is very cheap insurance if it saves a £250 repair job, not to mention the inconvenience of being out of commission for a few weeks.

* Use one VDR per item of equipment.
* Sleeve exposed wire.
* The closer to the equipment, the more effective the VDR. It is therefore not worthwhile attempting to protect all the equipment in one go with a VDR connected at the far (source) end of the master mains extension lead.