Arguably, the two most important innovations in watchmaking are the mechanical lever escapement and the quartz oscillator.  The second of these almost did for the first but as we edge our way closer and closer to midnight on the doomsday clock, both technologies sit quite happily adjacent to one another (accepting, of course, that the traditional wristwatch is a dying breed in the face of the emergence of the smart watch). 

In getting to this point, we need to acknowledge two fascinating horological cul-de-sacs that shaped to some extent the emergence of the quartz oscillator movement as the way forward in electrically-powered watch movements.  The first of these was the electronic balance-driven movement, pioneered by Hamilton in the late 1950s and refined subsequently by Citizen and ESA with the introduction of transistor-switching.  I have described the history behind this technology in some detail previously in articles on the Citizen Cosmostron and Seiko EL-370 and Elnix.  Rather than revisiting that again here, I’ll note simply that the birth of the Japanese electronic watch dates from 1966 with the release of the Citizen X8 Electric watch, powered by a transistor-switched moving magnet balance-type motor.  The development of the quartz-oscillator movement by Seiko in 1969 seriously undermined the case for the electronic balance-type movement but both types of watch featured in Seiko and Citizen catalogues until the mid-1970s by which point the quartz tsunami had all but snuffed the life out of the mechanical movement and rendered all other types of electronic watch entirely redundant.

However, this narrative has thus far failed to acknowledge the third strand in the electronic watch story, one that dates back to 1952 and the start of an 8-year period of development that would lead in 1960 to the release of the Bulova Accutron Tuning Fork electronic watch.  The brilliant concept behind the Accutron was to replace the low-frequency balance oscillator of the traditional mechanical movement (as well as of the emerging electronic balance-driven watches), with a much higher-frequency oscillator – an idea extended a further 100-fold in the quartz oscillator a decade and a half later.  A Swiss engineer, Max Hetzel, was tasked by the Bulova Watch Company to take up this challenge and his solution was to replace the balance with a transistor-driven tuning fork oscillating at 360 Hz – nearly 150 times higher than the low-beat mechanical standard at the time.   By 1953, Hetzel had produced his first working prototype in Bienne and by 1955, the concept had been miniaturised sufficiently to produce a number of watch-sized concept models.  Hetzel subsequently moved to New York to work on the production engineering phase and in October 1960, the Accutron 214 was announced with sales starting towards the end of 1960.

The much higher operating frequency of an oscillator devoid of bearings and pivots, combined with the much lower number of moving parts, meant that the Accutron was able to lay claim to being inherently much more accurate than even the best mechanical chronometers, both in a static sense and in terms of its resistance to positional variation.  The advertising campaigns at the time boasted of 99.9977% accuracy which amounted to an effective daily rate of plus/minus 2 seconds.  The advantages appeared overwhelming.

By 1965, Bulova had sold 350,000 Accutrons and the technology was being touted as an American success story.  NASA used Accutron instrument watches and clocks in the Gemini and Apollo programs, they were used as the first certified watches on the American railroad and Accutron clocks were used in Air Force One and by the American army in their aircrafts and ships.

Cal 218, the second generation Accutron movement appeared in 1967 and improved in a number of ways over the 214 including the addition for the first time of a calendar complication in the 218D.

In 1968, Bulova licensed the technology to Ebauches SA, and in 1970 to Citizen who entered into a joint venture as the Bulova-Citizen Watch Co. to produce the 218G in Japan under licence.  This movement would form the basis of the Citizen Hisonic line of watches, an example of which is the subject of the present entry. 

The watch pictured above was one of two Hisonics that I bought during a period a couple of years ago when my interest became particularly piqued by the perplexing vintage Citizen universe.  The Hisonics were on my radar because a reader of this blog (thanks Simon S!) had, in 2018, very kindly donated an old Bulova Accutron in the hope that at some point I may get around to dipping my toe into the world of the hummer.  Nearly seven years later, and it appears that I have done just that.

This particular Hisonic is a 3711-373111 Y dating from January 1973, if my interpretation of the serial number on the case back is correct.

The marker pen-inscribed X on the case back is my note-to-self that this is a non-functioning watch, there being no tuning fork hum with an electrical potential applied.  There are almost no vintage Citizen catalogues from this era online other than one from 1974 but happily that one does feature my watch on p.18.

You may have noticed from the case back shot that this watch features a two-part case back in which the back itself is compressed onto a flat rubber gasket by a separate threaded ring.  With the ring removed, the case back can be lifted up with the aid of a tab located at the twelve o’clock position.

The first clue to the non-responsive state of the movement arrives with the removal of the case back.

It is clear from the staining to the right-hand side of the movement that this watch has suffered a catastrophic battery electrolyte leakage.  The second set-back comes when I remove the crown and stem.  This is achieved by turning the setting screw (indicated in the right-hand photo above) about two turns anti-clockwise with the crown pulled one click out to the time-setting position.  I successfully withdrew the crown but in loosening the setting screw, I felt a slight jolt, suggesting that the screw thread may have lost purchase with the setting lever.  My attempt to tighten it back down subsequently resulted only in the suspicion that the screw head may have parted company with its thread.

Setting that concern aside for the moment, with the stem removed, we are free to release the movement from the case and survey the dial and hands.

Both could do with a clean but are otherwise in very good shape.  The dial is secured to the movement using two dial feet screws.

With the hands removed, the dial is released by loosening each dial foot screw, and we can start to strip back the calendar components. 

This movement features both an instant date change mechanism, whose operation relies on the concerted actions of the date trip wheel, arm and spring, visible in the lower two photos above and, highlighted in the photo below.

In addition, the movement also features a quickset mechanism that is operated, unusually, with the crown at its innermost position.  We will get a clearer look at that a little later on.  In the meantime, let’s take a look at the almost denuded dial-side of the movement, the pair of coils acting as the most prominent landmark.

Before moving further with the account of the deconstruction, I think we should pause to review how this type of movement actually works because that will help us to understand the roles played by the various components that we will meet along the way.

In a mechanical watch the power is supplied by a mainspring and the regulating mechanism is the balance and escapement.  The periodic rotation of the balance wheel is converted to incremental motion of the gear train by the escapement.  In an Accutron movement, the power is provided by a battery, and the regulating mechanism is a vibrating tuning fork.   The vibrational motion of the tuning fork is converted to motion in the gear train via an index mechanism employing two very fine linear sprung fingers, each topped by a ruby jewel.

The vibration of the tuning fork is achieved using resonant feedback governed by a simple electronic circuit connected to a pair of coils, each of which is contained within its own iron cup containing a small magnet.  Electrical current flows through both of the coils via a transistor, with the coil closest to the circuit having a secondary coil which provides a feedback voltage.  The initial current flow causes the fork to flex in one direction as a result of the play between the electrically induced magnetic field provided by the coils and the magnets contained within each of the iron cups.

The secondary coil provides a feedback voltage in response to this movement of the fork causing the transistor to stop the flow of current which stops the force applied to the fork.  The fork then springs back and this motion induces a reverse voltage which prompts the transistor to allow current to flow once more.  This fluctuating current in the coils produces a varying magnetic field whose period is aligned to the natural resonant frequency of the tuning fork.

The index mechanism consists of an extremely finely-engineered 320-tooth index wheel driven by the two operating fingers, one of which, the index finger, is attached to one arm of the tuning fork and the other, the pawl finger, is attached to the main plate on a small movable bridge. The jewels of each finger rest on the teeth of the index wheel and the vibration of the tuning fork is converted into rotational motion of the index wheel by the coordinated action of the two: as the arm of the tuning fork moves inwards, the index finger pushes the wheel forward by one and a half teeth; the reverse movement causes it to slip back half a tooth at which point the pawl jewel drops down one tooth, locking the index wheel at its new position.  This ingenious system is inherently tolerant of a degree of latitude in the amplitude of vibration but if too high an amplitude is generated, such as from a battery whose voltage is too high, then the index wheel will advance more than one tooth per cycle and the movement will run fast.  In order to run correctly, the mechanism needs to be phased correctly to the potential provided by the battery as well as to cope with its gradual reduction as the battery slowly depletes.

Now where were we?  Oh yes, we were about to remove the train wheel bridge but first we have to move the index and pawl fingers away from the index wheel to avoid damaging them as we lift the bridge away.

Before removing the four screws that secure the train bridge, let’s take a quick look at the cell coil assembly post where it connects to the cell strap, this providing a plot twist later on in the story.

It has clearly suffered from the corrosive effects of the electrolyte leakage.  I did not make too much of this at the time but we will need to revisit this in due course when reassembling the movement.  With the four bridge screws removed, we can lift away the bridge and observe the four-wheel gear train.

These four wheels are identified, from outboard to inboard, as the fourth, third, and second wheels, all driven by the tiny and oh-so-fragile index wheel.  The third wheel carries the seconds hand pinion and the fourth wheel drives the hour and minute wheels via the centre wheel assembly on the dial side.

I remove the four wheels, having first made sure to push the crown to its inner-most position, freeing the hacking lever from the third wheel on which it acts.  The next step requires that the pawl finger be rotated by 180 degrees clockwise to allow removal of the tuning fork.  This is the point at which minor calamity number two presents itself.

Evidently, a hairline stress fracture has been lurking in the pawl finger collet and when I started to rotate it out of the way, the collet started to give way, the lever drooped slightly and you can see that the jewel has caught on one of the tuning fork screws.  I levered the collet back up to allow the jewel to clear the screw but the fracture is now fully resolved and the part is no longer fit for purpose, its collet in two separate pieces.

Regardless, we can press on with the removal of the tuning fork.  This is achieved first by removing the two securing screws at the base of the fork and then pushing the base of the fork through an access hole on the dial side using the unsharpened end of a spring bar tool.

The pawl finger plate, hacking components and setting parts follow, resulting in an almost completely stripped main plate.  Two things we should note before completing the deconstruction are the spacers of different colours that sit beneath the tuning fork.

These spacers are of different thicknesses and will need to be refitted in the same positions after cleaning.  The second point of interest is the contact spring fitted to the train side of the main plate.

In normal operation and at the time setting (hacked) position, this sprung contact connects with its  partner contact on the underside of the electronic circuit.  In both states, the current continues to flow and the tuning fork hums.  However, pulling the stem out to its third position (second indent on the setting lever) breaks this contact and the current stops, preserving the battery should the watch need to be stored for a period unused.

While we are on the subject of the setting parts, we’d better take a look at the setting lever and its screw.

As suspected, the thread has sheared and remains embedded in the setting lever.  I use my screw thread extractor to perform the task for which it was designed.

By my reckoning, in order to return this watch to a functional state, we are going to need to source at least two components: a setting lever screw and a pawl finger with possibly more, given the absence of a 360 Hz hum with a voltage applied.  To that end, I am calling on our donated Bulova, whose movement, a 2182F, shares a great many components in common with the Citizen-manufactured 3711 Hisonic.

Here are the assembled parts, including an additional setting screw and pawl finger donated by the Bulova, ready for cleaning.

In spite of the somewhat daunting prospect that this watch presented at the outset, it is interesting to see how few components there are compared to a similarly specified mechanical watch.  The service manual urges extreme caution in handling and cleaning the index wheel, pawl finger and index finger and so I do just that, treating them only to an ultrasonic clean, reserving the agitations inflicted by the watch cleaning machine for the remaining parts (electronic components excluded).

With cleaning complete, we begin working our way methodically through a reconstruction, with the objective of ending up with a complete and fully functioning movement.

The first step is to restore the formerly broken setting lever, its replacement setting lever screw sourced from the donor movement.  The sequence of steps shown below follow the service manual instructions, beginning with: the stem, clutch, date corrector and setting lever (top left); the centre second spring and ground plate (top right); the centre wheel assembly (lubricated first) and the fourth wheel bridge (bottom right); and finally, the clutch lever, its spring, and the setting and minute wheels (bottom left).

The collection of setting parts (bottom left, above) is capped off with the yoke.

I’ve separated this step from the previous collage in order to highlight the date corrector that sits behind the clutch wheel.  You will notice also the spring to its left, whose job it is to resist accidental quicksettery by providing an emphatic resistance to clockwise rotation of the crown.  It is there because, as I mentioned earlier, the quickset is operated with the crown at its innermost position (at rest) and advancing the date manually needs to be a deliberate action, not an accidental one.

Next, we fit the hack lever and its spring, the pawl bridge sub-assembly (harvested from our donor), with the jewel and its finger rotated well out of the way for the moment (below, left), and the contact spring, which you can see resting against the top edge of the setting lever.

We are ready (no we’re not) to refit the tuning fork, complete with its pair (or rather triplet, see above) of coils, having first re-seated the second of the two spacer washers.

Making sure first to push the crown into its innermost position to release the obstruction that the hacking lever would present, we can fit all four wheels of the train, the third (seconds) and index wheels first, followed by the second and fourth wheels.

All four wheels stand reasonably proud in anticipation of aligning their respective pivots with the jewelled holes in the train wheel bridge.  That follows, aided by a little judicious jiggling to get all four wheels properly seated.

Incidentally, the staining inflicted by the electrolyte on the movement and train bridge is permanent but I rather like the patination that it has left.  Nevertheless, I did spend some time cleaning the plastic cell coil (right) and component coil (left) contact supports, paying particular attention to the cell coil assembly which had suffered from the corrosive affects of the leaked electrolyte.  Alas, my efforts appeared, when I applied a suitable voltage to test the electronics, to be in vain.  The tuning fork failed to shake itself into action upon application of 1.4V from my Horotec FlashTest quartz timing machine, nor 1.6V nor even 1.8V.  

Perhaps predictably, the problem lay with the cell coil assembly.  When I tested its resistance (yes, I know, I should have tested it earlier), the circuit was open.  The coil was dead.  And of course, you can’t replace the coil without removing the bridge, the train wheels, the pawl bridge and its cam and the train wheels and then releasing and levering up the tuning fork.  This I did, having established that the donor coil from the Bulova watch was healthy and, retracing my steps, returned to a point at which I could establish that, yes, the tuning fork vibrated in response to electrical stimulation.

The tuning fork may well be vibrating, but the watch is not yet running because the index wheel and its two fingers need phasing to the voltage supplied by a modern 1.55V silver oxide batter rather than the 1.35V with which these movements were originally designed to operate.  The first part of this process is to ensure that both index and pawl jewels are sitting perpendicular to the index wheel when engaged and that they touch the wheel at the mid-point of each jewel (Fig. 16 below).  This may, and did, require adjustment by bending the relevant posts either towards or away from the index wheel to, respectively, lower or raise the jewel relative to the wheel (Fig. 17).

With that done, we are in a position to adjust the tension of the index finger.  We start by rotating the index finger collet until the index jewel completely disengages from the index wheel.  Beneath the index finger is an elbowed gauge which provides the means to set the correct force of the jewel on the index wheel.  In its detached state, the gauge tip must not touch the finger but should be within one index finger thickness of the finger (top and lower left, below).  The gap is adjusted by bending the index gauge at its attached end.

With that done, the collet is rotated until the index jewel comes into contact with the index wheel at which point the gap between the tip of the gauge and the finger will start to increase with increasing force.  The finger needs to be adjusted such that the gap increases by between 1 and 1½ times its original gap (top and lower right, above). In case you haven’t realised already, clicking on each of the images will open the image full-sized in a separate window. You may want to do this to get a clearer view of some of the details.

Next, we turn our attention to the pawl finger and begin by loosening the pawl bridge screw slightly but leaving the pawl bridge pivot screw tightened.  We then turn the pawl bridge cam clockwise until the pawl finger reaches its maximum distance from the index wheel.  At this point it should be no more than one half of the jewel thickness away from the index wheel.

We are now ready to begin phasing the index mechanism.  With the movement held in a metal movement holder (not having the correct Accutron movement holder) I attach the negative lead from my Horotec Flashtest to one of the feet of the holder and touch the positive lead to the cell coil terminal, having first set the Horotec to supply 1.1V.

The cam is then turned very slowly until the pawl jewel comes into contact with the index wheel and the wheel starts to turn.  We keep turning the cam until the index wheel stops, at which point the index jewel is sitting at the base of one of the index wheel teeth.  We continue to turn the cam until the index wheel starts again at which point the index mechanism will be in phase.  We now raise the voltage of the supply to 1.8 V and observe whether the index wheel is running at its correct rate or too quickly.  In my case, the wheel is turning at its normal speed and there would appear to be no need to make further adjustments.

Satisfied that we have a working movement, I move on to reconstructing the calendar mechanism.  In spite of the sophistication of the instant change date mechanism, the process of piecing it altogether is relatively straightforward.  The date trip wheel, arm and spring are fitted first (top left, below), followed by the date disk and its detent – in Seiko lingo, the detent is referred to as the jumper – (top right, below).  I puzzled momentarily about how the detent spring should be fitted before realising that you fit this after you fit the date bridge (bottom right, below) – date dial guard in Seiko speak.  Once the date bridge is fitted, the detent spring can be slid through the hole into its operational position (bottom left, below).

The photo shown bottom left above was taken approximately at the point at which the date disk ticks over.  If you look into the bottom left hand corner, you will be able to see the tip of the elbow of the date trip wheel arm between two of the teeth of the date disk.  At this point, and for a few hours beyond this point, the quickset mechanism cannot be operated because the elbow will block the advance of the date disk.  However, by about 7am (or maybe a little sooner), the elbow has retreated and the date corrector lever on the stem is free to advance the date, as required.

We can now refit the dial and hands, at which point we are in a position to make some judgements about timekeeping (in the absence of any sort of timing machine that works with a tuning fork movement).

I left the movement to run for a day, having synchronised it to a time reference and got on with cleaning the casing parts and fitting a new crystal.

I had sourced a crystal at the time that I bought the watch and so was all set to fit that once I’d got the lay of the land around the case construction.  I’d assumed that the bezel would be removable and perhaps play some sort of role in securing the crystal but in the absence of a cutout or any sense at all that it was up for being separated from the case, I concluded that the bezel had been welded or glued to the case in the factory and was not intended to be a service item.

The crystal is supplied with a new nylon gasket and with that fitted into the recess in the case, the crystal is pressed into position using a crystal press.

The original crown gasket had become hardened and brittle with age but this being one of those crowns with a captured gasket sitting behind a retaining washer, some abuse was required to extract the old gasket, this crown being of a dimension that resisted my usual approach to washer removal.  With a suitable replacement identified, I fitted the new gasket, staked the washer back into position as neatly as I could and lubricated the gasket with silicone grease.

We are very close at this point to wrapping things up but first we need to make a judgement about the timekeeping and make any regulatory adjustments, as required.  Over the course of the previous 24 hours, the watch had lost about 2 seconds, which is extremely impressive out of the box.  But as I like my watches to run flat or even a little fast, I thought that I would perform a regulation according to the instructions given in the Accutron 218 service manual.

Each of the coil cups on the tuning fork is fitted with a serrated regulator arm of seven divisions (4 projections and 3 indentations).  Regulation is achieved by moving one or other or both arms, one or more divisions in either direction.  Rotating one arm one division towards the centre of the movement advances the movement by 2 seconds per day and away from the movement centre will retard it by the same amount.  In my case, I moved the arm attached to the cell coil cup one division towards the movement centre.

Subsequent monitoring of the timekeeping confirmed that the movement was now running ever so slightly fast of flat – i.e. it was gaining maybe half a second per day.

And with that, all that remains is to fit and secure the case back (having first identified a suitable gasket), attach the other end of the original bracelet, and run it through its paces as its designers intended.

I’ve been itching to have a crack at a tuning fork movement for some considerable time but the intimidation factor has kept me at bay and I’ve bided my time.  However, this new period in my life has provided the motivation and room to cultivate some intent to get on with it and with the arrival of a window of opportunity, I am so pleased to have done so. 

I have emerged from the experience so much better educated on the history and technology and awed by the beautiful, ingenious but also relative simplicity of a design philosophy that fully reached the original objectives.  The tuning fork is a much, much better solution to the electronic watch concept than the electronic balance-driven movements and yet both were victims of the quartz revolution, falling by the wayside within a decade and a half of their invention.

I should also observe that this Citizen Hisonic is a very handsome, smart and enjoyable watch.  My appetite has been whetted to sample more from this period of Citizen’s output and fortunately, I seem to have plenty of options waiting in the wings.  Until next time.

References

1.  https://accutronwatchpage.com

2. https://www.watch-wiki.net/doku.php?id=bulova_accutron

3.  https://oldfathertime.com/accutron_history1.htm

4.  https://watchdoctor.biz/accutron/

5.  https://reference.grail-watch.com/movement/accutron-218

6.  https://www.watchonista.com/articles/bulova-accutron-tuning-fork-revolution