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The first article in the horology section of the ESP site was about the repair of a Kundo.The material was updated in April of last year.

There is a circuit diagram to the left and a Kundo electric clock on the right.The clock uses a pendulum and a very simple circuit.There are many variations of the basic schematic.I was unable to use the original version of the circuit because the transistor was faulty.Most germanium transistors are PNP.Many people think that Silicon transistors won't work in these clocks because they always used a germanium transistor.

This isn't necessarily true, as I have replaced the germanium transistor with Silicon in the Kundo movement, and it works just fine.The clock needed a complete rewind of the original coil because it was open circuit.The 0.0635mm wire is so fine that it is difficult to avoid breakage, especially with several thousand turns.The movement works well and only requires a case, face and hands, which are only temporary.).

The original did use a germanium transistor, but I replaced it with a BC549 Silicon device to see if it would work.Although fitting the circuitry is difficult, I tried an AC128 germanium transistor.After reassembling the motor, I discovered that the transistors on the AC128 are made of steel and magnetic.This causes the pendulum to swing wildly.The black painted glass case was common in the early days of English and European transistors.

There is no doubt that the circuit works better with a germanium transistor, but I don't have any of the old glass case types, so I reverted to the Silicon device.The drive is external due to the fact that a Silicon transistor would not work in a simplified circuit.

It is worth restoring since the movement is a 6 jewel type and is of good quality.

I measured when I first used the transistor.These are real power misers, at least with a transistor.With a peak current of 1.7mA, the average current drain is less than 200A.The power is small.3 beats/second is the pendulum rate.Every 667ms, the pendulum is pulsed.Depending on the direction of the magnets and coil, the pendulum can be impulsed either left to right or right to left.

A tiny current is sent to the base of the transistor via the outer coil when a magnet is buried inside the pendulum.The transistor draws collector current and the current in the inner coil causes the transistor to turn on more.The transistor turns off completely when the current can't increase any further, and the 5.1kresistor is fitted to limit the maximum back EMF swing which can easily reach 20 or 30 Volts.The magnet is pushed away by the impulse.

The components of the clock's motor system can be seen above.There is a magnet in the curved section of the pendulum.The coil is held by the coil bobbin.The center image shows the coil.I colour the wires when I rewound the coil.The coloured wire I had was too thick, but it works.

There is a transistor mounting on the right.The germanium device is much longer than the Silicon transistor used as a test, so the hole is designed to take it.There is plenty of room for wiring in the bobbin.The 5.1k resistor is tucked into the bobbin and isn't shown.I haven't progressed very far, and forgot the exact wiring, almost a year after I repaired the motor.

The power is transferred to the movement by means of a tiny pawl that is connected to a pendulum, and this drives a wheel with a detent to prevent it from turning backwards.There is a photo on the left of a typical Kundo drive system.The motion is geared down by the motion train, with each wheel turning slower than the one driving it.Only a small amount of the pendulum's energy is lost at each right-to- left swing which advances the wheel, so the electric motor unit only needs to be replaced to keep the clock running.There is no need for a power switch.Simply locking the pendulum in place prevents the battery from being discharged since the circuit needs the magnetic impulse to do anything at all.

A common mistake is to assume that a battery will last a long time without a current drain, but they will eventually leak and damage the battery housing.Standard zinc-carbon cells are marginally better than alkaline cells in that they will leak if left in place, and the damage can be considerable.Remove the battery if the clock isn't being used.

Too much pendulum over-swing can be prevented with a small amount of damping.This is provided by a brass ring which forms a short turn for the magnet as it passes through the coil bobbin.The system has a bit more power than is needed due to the fact that the repaired coil assembly is incomplete.

The Kundo style of motor does a good job of driving the pendulum at the centre of its swing for best timekeeping.It is close to the centre of the curved bar that the drive is delivered.Timekeeping is quite good once the clock is regulated.

I had to do another coil rewind on another Kundo clock using the same motor unit.I used the counter to count the number of turns.The base has 5,000 turns and the collector has 3,000.The second unit used the original germanium transistor and refused to work with Silicon.The only difference is the strength of the magnet, although in theory the second one should have worked with a Silicon device.

I was able to determine the likely cause of the coil failure.With a small amount of power and very low voltage, these coils should last a long time, but this is not the case.It looks like the insulation for the wire is attacking the wiring.There were patches of green gunge on the windings next to the insulation.It's necessary to remove the old coil completely and start over if you want to get to it.I tried to remove only the outer winding of the motor, but the damage to the inner winding seems inevitable.It might be possible for someone with close to infinite patience, but that's not me.

If anyone has more exact info, please let me know.

The wire size is approximately 0.0635mm.It's very easy to break such fine wire if you don't have a steady speed.With so many turns, it's tempting to increase the speed of the winder, but that increases the chance that the wire will break.It will take 1 hour and 23 minutes to wind 5,000 turns.If you are very patient, this is a good speed, but I expect that most people will get bored rather quickly, and will run the winder faster that that.

The problem with the Silicon transistor is that it has a 'barrier' voltage of 0.65V, which is less than germanium.When the weather gets cold, the single Silicon transistor drive simply stops.The negative tempco means that when the temperature falls, the barrier voltage increases.The same thing happens with germanium, but it is less of a problem because of the lower barrier voltage.Germanium has a high leakage current that is highly temperature dependent.

There are two alternative motor designs.They have the advantage of using transistors that are readily available, and the electronics are mounted outside the coil housing.It's easy to experiment because the coil assembly doesn't have to be dismantled.Since everything is easily hidden, electric clocks using similar systems already use just a single coil and an external board with the electronics, and it doesn't affect the appearance at all.

Only two wires are needed from the coil housing.A circuit that is very similar to the one I'm using now is based on one described in the Free Pendulum Clock article, but there are a few minor changes needed as the first motor drive I built is much too powerful for the Kundo clock, and must be tamed.This must be done in a way that gives consistent results, yet uses a very simple design so it's easy to build without a printed circuit board.This is a challenge, but I know it can be done.

I decided to use a two transistor driver since I have no glass or germanium cased transistors for the original circuit.I settled on the motor drive circuit shown below after many attempts to get it to work.It needs 3V to run.The drive seems to be a good solution.There is no need to use two coils and rewire to the internal circuit if you use an external switch circuit.

This circuit only requires a single coil, and my resistance is about 1k.I don't know how many turns are used, since the coil was re-wound a long time ago, but I think there's around 5,000 turns.The circuit can be built on a piece of Veroboard or similar prototyping board with no more than 25mm square.

There is about 130mV at the base of Q1, and the circuit provides a small bias voltage to the first transistor via R4.Enough to ensure that the small pulse from the coil will turn on Q1, thence Q2, and send a drive pulse to the motor coil via C1, but not much.The average current drain is just over 200A, and it will run for a long time on a pair of alkaline 'C' cells.The next best thing was 3V because it refused to run with 1.5V.I don't like using germanium transistors if it can be avoided, and that would probably improve matters, but I think it's a bad idea.

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