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Bulova Accutron 214 Part 2
Hello fellow electronics enthusiasts! In Part 1, I showed you some photos of the amazing Accutron 214 components and gave you a basic description of them and how the movement operates. In this article we will take a deeper look at the electronic circuit and the indexing mechanism. In Part 3, we will have the movement cleaned and reassembled to test and see if it works. If it doesn’t work, we will repair it – which may need a Part 4: we shall see.
Looking at the schematic of the electronic circuit of the Accutron, it seems to be very simple. It is simple – but it is also very elegant. Not only does the circuit control the pulsed current with its fixed coils and tuning fork cups/conical magnets, it also has an inherent built-in amplitude control system.
As I mentioned in Part 1, about 25% of the turns of Drive Coil 1 (DC1) make up the phase sensing coil. It is the phase sensing coil that controls the instant the pulses of current are applied to the drive coils to maintain the tuning fork oscillations. The coils are fixed to the pillar plate and the tuning fork cups with their associated magnets are attracted or repelled depending on the polarity (direction of the current flow). So the coils with their associated magnets serve three functions; they drive the tuning fork by converting pulses of electrical current into mechanical impulses, they control the ability in which the electronic circuit may sense the tuning fork amplitude, and they control the instant in the tuning fork cycle during which the driving current pulse is delivered to the coils.
Here is the schematic and the photo of the movement showing the components from Part 1 for you to refer to.
The 220nF cap in parallel with the 3M9 resistor keeps the transistor in the non-conducting state throughout most of the cycle of the tuning fork. As previously explained, the alternating pulses of voltage are induced in the phase sensing coil by the oscillations of the magnet associated with it. Through the base of the transistor this voltage is added to the battery cell voltage to charge the cap once each cycle by the peaks of the alternating voltage induced in the phase sensing coil. These recharging pulses of current cause the transistor to conduct momentarily and allow current to flow in the drive coils to pulse the tuning fork and maintain its oscillations.
The transistor is caused to conduct at the point where the voltages induced in the phase sensing coil and drive coils are near their maximum instantaneous values, and when the drive coil induced voltage is opposite in polarity to the battery cell voltage. Therefore, if the amplitude of the tuning fork at the instant the transistor conducts current and the induced voltage in the drive coils exactly equals the battery cell voltage, no current would flow since the two voltages are opposite in polarity which would cause them to cancel each other out.
So, the design of the magnet and coil system is the key to the operation of the amplitude control system so that the proper amplitude of oscillation for the tuning fork, the voltage induced in the drive coils has a peak value about 10% less than the battery cell voltage. It is because of this that a 10% increase in amplitude resulting from a disturbance, such as the movement being bumped or jarred, would cause the driving current pulses to be reduced to zero and the tuning fork would quickly return to its proper amplitude. Also, a 10% decrease in amplitude of the tuning fork would cause the driving current pulses to double and again return the tuning fork very quickly to the proper amplitude.
As I previously mentioned, one of the tines of the tuning fork has an index finger and jewel which moves the index wheel to turn the watch wheels and hands as the tuning fork vibrates. It is called the ratchet system. Even though the ratchet system is very tiny it does not alter the physical principles of its operation in any way. Below is a photo that gives a basic view of the index wheel ratchet system.
When the tuning fork tine oscillates it moves the index jewel to advance the index wheel in the forward direction as seen in the photo above. The pawl jewel drops into a tooth preventing the index wheel from reversing direction past the advanced tooth when the index jewel pulls back in the reverse direction by the tuning fork. The jewel fingers keep a little downward pressure (torque) against the index wheel which keeps them from jumping away from the index wheel and also mimics the draw in a mechanical watch escapement by pulling the index wheel back a little so that the pawl jewel rests at the step in the index wheel. To learn about mechanical watch lever escapements you can look here. The lever escapement is the most common that you will probably be acquainted with. You can explore other types of escapements from links on that page if you wish.
In the simplified photo above of the index wheel teeth and jewels, we see how the jewels advance the index wheel under different amplitudes. Figure (A) shows the movement stopped at the rest position. Figures (B) & (C) show the end of the forward/return strokes of the one-tooth amplitude. Figures (D) & (E) show a two-tooth amplitude, with figure (F) showing the middle of the two-tooth return stroke. Figures (G) & (H) show the three-tooth amplitude.
Using the distance between the teeth as a measure of the amplitude, we can see what happens when the tuning fork vibrates at various amplitudes. Figures B & C show a complete cycle of oscillation at an amplitude of one-tooth (from ½ tooth right to ½ tooth left of the rest position). You can see that when the index jewel moves ½ tooth to the right, the index jewel picks up another tooth and on its return stroke to the left it pushes the index wheel far enough for the pawl jewel to drop off of the end of tooth # 2 so that we have advanced a movement of one tooth. From this we can deduce that further oscillations at the amplitude of one tooth will advance the index wheel one tooth per cycle.
In figures D, E, and F we can see what happens when we increase the amplitude to two teeth (one tooth right to one tooth left of the rest position). Notice that when the index jewel moves one tooth to the right, it drops off of tooth #7 and moves halfway along tooth #8. So on the return stroke to the left the first half tooth of travel causes no movement of the index wheel since the index jewel does not start to push the index wheel until the index jewel reaches tooth #7. Also notice that in figure E, tooth #2 passes beyond the pawl jewel; but after the start of the return stroke back to the right the “draw” effect exerts force on the index wheel to pull it back ½ a tooth against tooth #2, as shown in figure F.
Figures G & H show the effect of a three-tooth amplitude. Note that when the index jewel moves 1 ½ teeth to the right the index jewel will pick up tooth #8. At the stroke to the left the index jewel has moved tooth #8 into the position where tooth #5 was.
The pawl jewel dropped off the end of tooth #4, and we have achieved a three-tooth advance of the index wheel.
So we can see that for any amplitude from just over one tooth to just under three teeth the index wheel will only advance one tooth for each oscillation of the tuning fork. We have demonstrated that the Accutron index mechanism permits wide variations in tuning fork amplitude before the movement hands fail to advance in exact synchronism with the oscillations of the tuning fork.
Notice also that the jewels rest flat against the index wheel teeth. This helps distribute the already minute forces holding the jewels against the index wheel to help keep them from wearing out. According to the Accutron manual, they examined movements that had been running for years, and after examination could find no signs of wear. I can believe it. My own examinations in the coming years will tell if that is true but I doubt that Bulova would make that claim if it were not true. And how old an Accutron is that I buy is no guarantee because I have no way of knowing exactly how long it actually ran during its life. I can say that I have not seen any evidence of wear in my 214’s index wheel and jewels.
The next few photos are of the Accutron 214 movement being taken apart.
In Part 3 we will get the cleaned movement back together and run some checks.
Robert Calk is a hobbyist from the USA who loves Electronics, Leatherworking, and Watchmaking.
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