It is an unfortunate fact that it just doesn’t seem possible to manufacture filament bulbs with very low power, (typically about 1 Watt), that work off 230 Volts, so what is usually done is that low voltage bulbs are used and a number of them are connected in series across the mains supply. However, to make fault finding slightly easier, each bulb is rather special. The filament of each bulb, (except the “flashing” variety), is held in tension by two spring wires. If and when the filament breaks, the supporting wires spring apart and one of them falls against another wire connected to the opposite spring wire, resulting in the bulb becoming a short circuit rather than an open circuit. The net result is that although the failed bulb does not light, all the rest do, and with slightly increased brightness. It should then be easy to identify the failed bulb and replace it.
The problem arises when a bulb does not fail in the intended manner or when a bulb is loose in its holder, resulting in an open circuit. The usual course is then to check each bulb individually for tightness in its holder, a slightly tedious task. It is particularly irksome if the bulbs are of the cheap wire-ended type which will not stand repeated withdawal and re-insertion before the fragile copper wire ends break off. If however all bulbs appear to be tight, you have a real problem in finding the culprit bulb.
A method which I have used with some success involves detecting the electric field from the mains wires feeding each bulb, using an ordinary, low performance oscilloscope. The procedure is as follows: connect one end of two or three yards of coax to the input and earth terminals of the oscilloscope. Cut back the other end of the coax to expose an inch or two of centre conductor. Curl this into a “hair pin” for compactness. This will be the “field probe”. Lay the array of Christmas Tree lights on the floor, connected to the mains. If for space reasons you need to fold the array, make sure the rows are at least a few inches, (preferably more than a foot), apart. With the oscilloscope time base running at a suitable rate, (say 10 ms per cm), bring the “field probe” up close to the bulb nearest to one end of the array and note the magnitude of the pick-up as shown on the ’scope. Move the field probe on to the next bulb and repeat the procedure. The pick-up will be very similar until one bulb past the defective bulb is reached. At this point the pick-up will either greatly increase or decrease depending on which end of the array is connected to the live side of the mains. This procedure even worked for a complicated “three circuits in one loom” array.
Two points are worth noting: Firstly, if you can identify “flashing bulbs” by their external appearance, it makes life simpler to replace these with ordinary bulbs before you begin. Secondly, beware of laying out the array on the floor of an upstairs room; there is often mains wiring under the floor boards for the ceiling lights in the room below. This can produce its own electric field which can confuse the ’scope observations.
Happy Christmas, John, G0NVZ (Xmas 2002)