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An idle quest for a birth year watch earlier this year landed two, each one interesting for different reasons and both turning out to provide a technical challenge or two.  This post concerns the first, a Seiko Sportsmatic 7606-7991 from September 1964 (see, I told you I was middle-aged), sourced from a Japanese Ebayer, and fitted with the 7606A, not a calibre you see that often, although the sister 25 jewel 7619 and date only 7625 base calibre are somewhat thicker on the ground.

Photocredit: Kerione (Ebay)

The 7606A is a 23 jewel automatic, with day/date complications and a quickset date feature, operated using a push botton located at 3 o’clock position to the crown’s 4 o’clock.  It is this feature that marks this one as out of the ordinary and makes the watch just that bit more distinctive than many of the other Seiko dress watches from this period.  The case has considerable presence, with square, bold lugs, a very attractive dial/handset, a wide indexed chapter ring and a very large acrylic crystal.  The one I found looked presentable but with an inoperable day complication, stuck half-way between Wednesday and Thursday and a sticky half-functioning date quickset.  The case back on this one is a press-fit, snap-on type, requiring a proper tool with a thin but blunt blade to prise it open without risk of damage.  Once open, we can see what lurks beneath:

Clearly, pretty grubby in there, some rust, and a foreign body sitting bold as you like on the rotor, which itself appears to be coming apart at the seams.  A better view of the stray part suggests a badly rusted screw, its origin not yet clear but it looks like one of the dial retaining screws.

So, with the movement extracted from the case, which required destruction of the case back gasket, and the rotor removed, we can take in its general state:

All we get from this view is confirmation that its been a while since its last service, a suggestion of rust in the autowinding bearings (centre) but other than that it all looks pretty standard stuff.  With the autowinding mechanism off we see that the underlying architecture features not the single train bridge of many of the 6 series movements I’ve worked on before but separate train and barrel bridges:

This seems like a good point to flip it over and take a look at the calendar side:

On this movement, the day and day change over more or less in tandem around midnight (day first, then date) in contrast to many of the later Seiko movements I’ve worked on which seem to complete the date changeover before the day gets its act together.  We’ll see shortly how that works.  Off with the day disk:

taking the hour wheel and film washer with it.  Note the angry looking star of teeth  sitting on its underside.  With the hour wheel back in place we can take a proper look under the hood at the calendar mechanism and see how that simultaneous day/date change works.

The key to the day/date changeover is the date driving wheel on which sits the date finger and day finger (indicated above).  The date driving wheel is itself driven by the hour wheel (centre), two revolutions of which turns the day driving wheel through 360 degrees.  As it turns, the day finger engages with the teeth on the underside of the day wheel (see preceding photo) slowly turning it clockwise whilst the day jumper slots into position between two of the teeth, ensuring that the day aligns correctly with the date, once that has changed too, and holding it in position until the next changeover.   Meanwhile, the date finger, which points in the opposite direction, starts to engage with the teeth on inner diameter of the date disk, turning it anticlockwise, with the sprung date jumper then ensuring it moves exactly one date forward.    Note that in the photo above the day and date fingers in the photo above are aligned more or less at mid-day rather than midnight and so would be half way through their journey to the next changeover.

Continuing to dismantle the movement, but still focussing for the moment on the day driving wheel, we get a good idea just how complex and nicely engineered these older movements are.  Removing the solid day finger, we get a look at the separate date finger which is sprung and can move about a pivot sitting at its elbow, allowing a more progressive day changeover with the force gradually building as the wheel advances until the resistance provided by the date jumper is overcome.  In operation the day starts to change a little before any action is perceptable from the date.

The finger is easily removeable for cleaning:

and we see the natural curl of the spring.  With pretty much everything else removed, we pause for a moment to look at the date quickset mechanism, operated by that cool little button at 3 on the case.

The key parts here are the date corrector lever, which moves inboard when the 3 o’clock button is depressed.  This lever in turn exerts a force on a lip at the edge of the date corrector itself which moves against one of the teeth on the inner circumference of the date disk.  You can probably see better how this works from this figure taken from the 7606A technical manual:

A view of the now near naked calendar side of the mainplate shows the date corrector and the spring which provides the resistance to the button and returns it to its rest position once the date has been changed (or corrected).

One parting shot from this side shows two of the diafix jewel holes with springs and cap jewels removed (an operation considerably fiddlier than for diashock jewels, which themselves take some practice dissembling and reassembling).  The two diafixes are used as bearings for the third wheel and escape wheel.

Stripping the other side of the movement was unevenful and after a thorough clean we can start to put it all back together again, paying attention to proper application of the correct amount (by my reckoning anyway*) of the correct lubricants in the proper places.  Here’s the calendar side coming back together, diafixes refitted

and back to the other side, with most of the fixtures and fittings back in place:

Note that only the escape wheel merits a diafix on the third wheel bridge, the third wheel pivot making do with an ordinary jewelled bearing.  Next, as usual at this point, we turn our attention to the case, which pre-makeover looks grim:

although most of the rust is just on the surface and cleans up nicely, leaving just a few unsightly marks that will be hidden by the bezel in any case.  The crystal is huge (34mm diameter) and sits, with the chapter ring refitted, astride the lip just so:

and is held securely in place by the press-fit bezel

I like the sharp corners of the acrylic crystal and the gentle dome, which catches the light nicely:

With all of the calendar parts refitted and the day wheel cleaned up as best I can

we can refit the dial, hands and movement ring

before fitting to the case.

having replaced the broken rotor with a new service replacement from a sister 7605A.  Close it up

flip it over and admire

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