The Derby Swissonic speedometer watch despite being a brilliant invention has a set of disadvantages that tend to stop the movement. Here are a few steps for disassembling shown for the first time ever in the WWW.
Some Derbys might have a broken battery contact that can be cut from any beer can and shaped to fit. However if the battery connection is correct and the balance does not rotate it will usually be caused by a non-repairable fault on the electromagnetic coil. A last try might be taken to check and replace the electronic components on the battery spring that forwards the energy to the coil.
The Derby in most cases stops due to mechanical errors that can be noticed on the balance wheel and plate. Note that the balance on electric watches does not stop after the oil dries out on the jewels or when they get dirty. Continuous supply of power from the battery operates a dry movement and will cause the balance pinion and jewels to wear out due to friction and results in permanent damage. This causes disallignment of the pinion (left-right) and in result the magnets on the balance will not fit properly in between the electric coil resulting in erratic rotations. Repairing the (worn out and shortened) balance pinion on a lathe is the best solution however a quick fix might also help by moving the jewels up or down on the plates thus eliminating the spare space in which the balance would be moving horizontally. Below: notice the fourth picture where two plates are compared, the jewel was removed and inserted again slightly upwards.
Correcting the brass movement is usually the first stage and its functioning can be easily checked by connecting it alone to the coil and battery.
Very often the brass movement does work alone but when assembled to the plastic parts it does no longer forward the motion to the transfer rod filled with small gears. This is usually caused by friction on the plastic gears and requires oiling with special care applied to the setting crown. Even when it is pressed in, a metal transfer gear is still connected to the plastic train (as shown below). Slight tension from that part and the movement will not work.
One last but most frequent error is the broken spring situated at the seconds drum and mounted to the light-green wheel. When the spring is OK it gathers power in between every rotation as the green wheel rotates slightly quicker then the seconds drum. The spring releases the power when a pin on the transfer rod falls into the indentation on the seconds drum and in result advances the time by one minute.
A lot of work must usually be applied to a non-working Derby Swissonic due to the elastic plastic parts. Sometimes screwing the back to the case with to much pressure may even cause a fault.
Even if you find a working Derby at an auction you should ask whether the watch works in all positions. Even if it does work when placed flat on a desk it might stop when turned to the left or right side – due to the damaged balance pinion moving from one side to the other. Some sellers only mention that the watch works although it will not show elapsing time. Bear this in mind as this will mean that only the brass balance part is in working condition but does not transfer the motion to the plastic train. This might mean that the train is stuck for some reason but in most cases it will mean that the spring is broken.
New springs are not available and creating a similar one with identical properties is almost impossible. However there is one workaround that will make your Derby working again and requires a small modification of the seconds drum. Please check the picture below and compare.
Removing the gears on the seconds drum is the solution. After doing this, the green wheel MUST be fastened securely (glued) in the seconds drum. Note that it was rotating individually before. As the broken spring has been removed it is the green wheel which is responsible for rotating the seconds drum. One disadvantage of this modification is that the time will no longer jump but it will only rotate to a following minute. This way or another you will have a working Derby Swissonic by following these instructions if no more errors will be found. And believe me that once one is solved another one can appear.
As a summary I can admit that the Derby is a wonderful yet stupid creation. However if you find a NOS perfect condition piece it will probably be working fine for years.
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