What exactly constitutes an iconic watch design? Is it something that derives from design for its own sake and somehow captures the imagination? Is it simply a period style that sustains its appeal over the long term, occupying an affectionate place in our consciousness, there to root us in the comfort of familiarity? Or is it something born from a different approach to a practical brief? I tend to think it is all of these things. Examples of the first might be a Gérald Genta design from the 1970’s – a Patek Philippe Nautilus for example. Of the second, perhaps a Rolex Oyster cased watch such as the Explorer or arguably the most widely recognizable watch icon of the 20^th century, the Omega Speedmaster.  And of the third? Well, perhaps there are rather fewer of those, one obvious example being the Omega PloProf, another the subject of today’s post.

In 1968, a year or so after Seiko had proudly announced the release of its first 300m professional diver’s watch (the 6215-7000) it received a letter from a professional saturation diver from Kure City, Hiroshima Prefecture.  In his letter, he described the detrimental effects that the harsh conditions in which he worked had on the Seiko diver’s watches he had been using, including the latest 300m model.  His work required him to work at depths of 350m in diving capsules in which he would be breathing a helium/oxygen mixture. The issues he had been having were two fold: on the one hand, he needed a watch capable of absorbing the punishment of knocks against the sea floor but more importantly, a watch that could resist penetration from the helium gas in the He/O₂ mixture he was breathing in the diving chamber. The problem with helium is that not only is it an inert gas and therefore exists in its natural state as a stable atom, rather than bound up in a molecule, but it is also the second smallest and lightest element in the universe. Its exceptionally compact dimensions means that it can wriggle through the smallest of gaps and at the pressures operating in a diving vessel, can penetrate and pressurize the interior of a watch. When the diver ascends from the depths and the external pressure reduces, a much larger pressure differential is created than the watch crystal is designed to cope with. The result can then be that the crystal breaks its seal and pops off or in some cases fractures resulting in an explosive release of the pressurized gas during the ascent.

There are essentially two approaches to dealing with this problem: some manufacturers such as Rolex introduced a helium pressure relief valve that would relieve the interior pressure during the ascent.

Other manufacturers, such as Seiko, took a different view.   Their approach was to start with a clean sheet of paper and design a watch that resisted helium ingress in the first place and did so using a case construction that was also capable of dealing with the rougher conditions experienced by the commercial saturation divers.

So in 1968, Seiko established a new project team who were tasked with producing a watch that was to be designed from the outset for saturation diving rather than simply an adaptation of an existing design (such as the approach adopted by Rolex in creating the Sea Dweller from the Submariner).  The design brief was to produce a shock-resistant, corrosion proof, anti-magnetic watch with a case that was impermeable to helium gas.  In 1975, after seven years of research and development, the first Seiko Professional 600 m diver’s watch emerged and the Tuna dynasty was born.

That first watch was fitted with the high beat mechanical 6159 movement, essentially the same movement used in the 61 series Grand Seiko watches.  The case was of a monocoque design, constructed from titanium, and featured an L-shaped crystal gasket and a screw-down crystal retaining ring. Its distinctive appearance owed much to its separate shroud that screwed onto the case, providing additional protection to the bezel and crown.

The watch was a bit of a beast though and at 50mm diameter, not one designed to slip discreetly under a tailored shirt cuff.  This was no desk diver, but a purpose build tool watch.  The appearance of its stubby cylindrical case shape earned it the nickname Tuna can, shortened subsequently to Tuna and spawning a long line of models that sustains to the present day.

The exotic high beat GS movement fitted to those first watches was subsequently ‘upgraded’ in 1978 to the five jewel 7549A quartz movement in the celebrated 7549-7009 Golden Tuna, named for its distinctive gold-coloured titanium nitride coated titanium case. The Tuna model line continued to evolve through the 1970’s, 80’s and beyond spawning in the late 1970’s the slightly more modestly proportioned 300m-rated 7549-7010, forgoing monocoque construction for a more conventional two piece 45mm steel case and from the mid-1980’s, a beefier 1000m-rated version of the Golden Tuna featuring a ceramic shroud and the 7C46 quartz movement. All three model lines (300m, 600m and 1000m) continued to evolve through to the present day with the top of the line models now featuring spring drive movements.

I see no reasonable prospect of my ever purchasing one of the 600m or 1000m watches, not least because my wrist is simply too puny to accommodate a 50mm watch, no matter how celebrated or iconic. And so it should come as no surprise that the subject of today’s post is an example of the relatively svelte 300m rated 7549-7010 professional series Tuna.

Now, it is worth remarking at the outset that the large majority of the more recently acquired watches featured on this blog hark from the Japanese domestic market via Yahoo Japan auctions. There is always an element of hazard tied up in bidding on Yahoo auctions because many of them are illustrated with frankly piss poor photographic documentation. I’ve ridden the wave quite successfully however, for the most part buying decent, honest watches for prices I can live with. In this particular instance, I was attracted by what appeared to be a very presentable, original Seiko 7549-7010 and the price I paid seemed reasonable, given the photographic evidence I was working from. Indeed, with the watch having landed, I was satisfied that I’d secured a rather good example of the breed, not withstanding the odd nibble around the inner diameter of the bezel insert.

It looks good, doesn’t it? The only slight snag is that it doesn’t actually work. Turning it over and opening it up reveals at least one and probably two likely causes.

Obviously there is no battery, but in appraising the crime scene for clues, in the manner of your favourite detective – Hercule Poirot perhaps or even the celebrated Inspector Clouseau – we see two incriminating pieces of evidence. Or rather, one piece present and the other absent. Starting with the latter, we note that the screw supposed to be securing the battery holding spring adjacent to the coil is missing. This is a common feature of 7548 and 7549 movements and usually the result of a clumsy battery exchange in which the sprung holding spring launches the screw into oblivion at the first opportunity. The surprised watchmaker/bungling amateur then reacts with a start by plunging their screwdriver into the coil, thereby rendering it permanently and fatally injured. This would appear to be the scenario presented to us having noted the obvious imprint in the coil adjacent to the missing screw hole.

Before investigating further though, I refitted the caseback and removed the shroud from the mid case.

The corrosion visible on the side of the case adjacent to the screw holes is reciprocated on the inner surface of the shroud.

Clearly, it is looking like we do not quite have what I were hoping for at the outset but nevertheless, I remain optimistic that there is plenty of potential here, not least because the outward facing parts of the watch – the dial and handset – are in super condition.

Removing the hands requires a little more effort than I am accustomed to, in part because of the larger hand hole sizes used in this caliber, but also because I suspect this is the first time the hands have been removed since the watch left the factory in 1983.

You can see the impressive diameter of the hour wheel pinion in the photo above and again below, set against the cannon pinion.

These parts are unique to the 7549 and consequently, the hands are not interchangeable between the 7549 and 7548 calibres unless you also exchange the cannon pinion and hour wheel. For reference, the hour hand hole size is about 2.2 mm compared with 1.5mm for the 7548 whilst the minute hand is 1.4mm to the 7548’s 0.9mm. Quite a difference.

Time I think to take a closer look at the business side of the movement, and ascertain whether the problems extend beyond those due to careless handling. Looking closer in the region of the missing battery holding spring screw and things take a turn for the worse.

It looks like we don’t actually have a missing screw – just a missing screw head. The thread itself looks still to be in position and its blackened state suggests that something else might be afoot. The close-up shot above also gives us a better view of the damage to the coil. Further insights into the sequence of events that appear to have befallen this movement require removal of the circuit block.

All becomes clear. What appears to have happened here is that battery electrolyte has leaked, pooled at the base of the battery compartment and then flowed along the path of least resistance, following the lines of the copper circuit connecting the battery negative terminal to the integrated circuit. The electrolyte is a conductor and having bridged the two sides of the circuit, we have a short. The integrated circuit burns out, further evidence of which can be seen in the state of end of the stop lever that will have been in the firing line directly beneath the IC.

The short circuit was so complete that the screw securing the battery retaining spring has become scorched to the point that its integrity is compromised. With most of the remaining fixtures and fittings removed, we can see both the scarring to the rotor stator as well as an impression of the blackened screw thread remnant.

The casualty list is as follows:

1. Main plate effectively a write-off, the screw remnant permanently embedded and resisting all attempts to extricate it.
2. Coil damaged and inoperative.
3. Circuit burned out and non-functioning.
4. Stop lever burned to a crisp.
5. Rotor stator damaged (but functional).
6. Battery retaining spring screw (effectively) missing.
7. The train wheel bridge also appears damaged adjacent to the battery compartment but is otherwise functional.

Many of the parts required are long obsolete and very difficult to find at reasonable cost and so what we need therefore is some sort of donor, secured at modest expense. After a little bit of detective work and some patience, I eventually settle upon a Seiko 7550 railway pocket watch. I suspect that its sacrifice might cause some upset or even outrage from railway enthusiasts, but needs must and all that.  In any case, let’s think of it as a temporary organ donation, rather than a mortal sacrifice.

The reason I chose this option was because, like the 7548 and 7549, it is a 5 jewel movement, with a correctly marked circuit block, but can be had for a good deal less than a fully functioning 7548.  The repair of my Tuna can now proceed with parts coming not only from the 7550 but also a spare 7546.

With examples of the 7550, 7549 and 7546 main plates in front of me (left to right below), it is completely clear that the three are essentially the same and that the complete movements differ only in the markings on the circuit block, the presence or not of an additional jeweled bearing on the train wheel bridge and differences in the dimensions of the cannon pinion and hour wheel. The coil on the 7549 is additionally coated in plastic but I am fairly confident is otherwise the same as the coils used in the lesser movements. The only difference I can see in the 7549 main plate compared to the other two is a minor difference in the machining of the plate adjacent to the date jumper.

So, the reconstruction of our 7549A begins with the main plate from the 7550, with assorted other parts coming either from the original 7549 movement or the 7550 and 7546 donors.

As we’ve covered the essentials of the 75 series quartz movement recently in the tale of the 7548-700B, I propose to skip most of the details in putting the movement back together, pausing only to note details such as the correct reporting of the jewel count in the marking on the circuit block.

The movement, by the way, ticks along nicely with a battery pressed into place and so we can move on and complete the calendar, pausing once more to admire the impressive presence of that hour wheel poking though the day disk.

Let’s take a break from the movement and turn our attention to the case, which, now shed of most of its fixtures, more fully reveals its imperfections.

The corrosion looks somewhat unsightly, but should clean up nicely, with the actual damage to the case likely to be superficial. The 300m rating of the case owes much to the screw down crystal retaining ring, the removal of which is greatly facilitated by my case back opening tool.

The crystal can now just be pushed out from the rear with the remaining case parts following.

Most of the baked on grime is removed with the aid of elbow grease and peg wood before an extended ultrasonic bath in a detergent solution at about 50 °C. The result is somewhat pleasing and so with the crystal gasket cleaned too, a brand new crystal can be fitted. The original crystal is no longer available as a 1:1 OEM part (325W16GA) but fortunately, the current 300m Tuna crystal is compatible and makes for a perfect, if slightly pricey, alternate.

The final fiddle is to dismantle and clean the crown and replace the gasket.

The movement is running sweetly and so it is time to refit the dial and hands.

The rest is down hill, everything coming together, the sum of the parts now most definitely whole.

The watch came to me fitted with an aftermarket facsimile of the original Seiko flat vent strap and as opportunity would have it, a recent work trip to Japan offered an easy route to securing a modern DAL1BP at very modest expense to complete the job.

These days my tastes tend to lean more towards more modestly proportioned watches, with diver’s watches assuming a rarer presence on my wrist, but I have to say I am pleased finally to have sampled the Tuna and to have had reinforced just what a comfortable watch it is to wear despite its considerable footprint (47mm across the beam with the shroud fitted). The secret to that comfort is that it is also 47mm top to toe which makes it quite modestly proportioned in that dimension compared to many other watches of much smaller lateral size.

The Seiko Tuna has a style, or rather design, that takes a little getting used to, but its appeal lies in the combination of its four-square proportions, that soft domed crystal and the pleasing serration of its bezel. At first acquaintance, it can seem a bit of an oddball, but still it manages somehow to be perfectly coherent in its proportions and in the arrangement of its features. I think that it deserves its iconic status. To those in the know, it is instantly recognizable, a classic and a unique and welcome piece in a sea of identikit diver’s watches.

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