A year or so after the release of their first automatic chronograph (the 6139), Seiko followed up with the 6138A, based closely on the 6139 but with the addition of an hour register as well as the facility to hand wind.
I’ve owned just one 6138 before, the 6138-0011, which I sold some time ago and had not really planned on getting another. But my experience rebuilding a 6139 recently tweaked my interest again in these old chronographs and all it took was a dodgy looking eBay auction from a UK seller that was cheap enough not to cause me to hesitate. This one is a 6138-8020, sometimes referred to as the Panda. Here’s the seller’s photo
It would have been tempting to conclude, upon receipt, that this one was beyond help (at least mine at any rate). The seller claimed it had been in a drawer untouched since conking out a decade or two earlier, but it was clear to me that someone had been ‘at’ it. The sweep seconds hand was sitting at a tilt and colliding with the minute hand. The watch would not run. Nor hand wind. And the paint on the minute and hour sub-register hands was flaking off from cack-handed attempts to remove them in the past. On the plus side, the dial was really rather nice, aside from some minor scratches on the panda eyes. So, in I dived. I won’t go into the same detail as I did with the 6139B (here) because the architecture on the winder side is largely the same and if you are interested in how the chrono works, you can look at the earlier post. But the strip down did reveal one or two other nasty surprises.
With the automatic winding mechanism and chronograph bridge removed we get a view of the minute recording wheel, the intermediate minute recording wheel, the centre chronograph wheel and the reset hammer.
The shaft and heart had become detached from the wheel. This is something that I had seen before on Rich Askham’s blog (here) and is apparently quite a common problem with 6138’s although not on the 6139. It may be that when resetting the chronograph, the additional length of the shaft required to reach through to the dial side of the thicker movement increases the torque on the wheel beyond the point where it can survive the forces transmitted, and over time it fails. This would have been a show-stopper but I managed to find a new minute recording wheel on Cousin’s site, by a rather circuitous route, but it saved the day.
The next photo shows, I think, why the sweep seconds hand was askew. The photo is not so good but you can just see that the lip on the coupling lever on the balance side of the movement is sitting above the clutch ring on the chrono wheel rather than below it. The only way I can think that this might have happened was perhaps a previous aborted attempt at disassembling the movement although I believe the chrono wheels can list to one side if the spring weakens unevenly. We will see in due course whether that turns out to be a problem with this one.
With most of the rest of the parts on this side of the movement now removed, it is worth pausing to look at the barrel arbor hole in the main plate. On the 6138 this is jeweled to accommodate the intermediate hour recording wheel that is mounted on the bottom of the barrel and which emerges onto the dial side of the movement to mesh with the hour recording wheel.
Fortunately I was able to bend it more or less back into shape without breaking the spring. With the calendar parts removed we can now see the additional calendar plate that sits on top of the main plate:
To the left of the minute wheel and cannon pinion, we have the hour register wheel. Circling round from bottom left to top left of that wheel is the intermediate hour recording wheel stop lever (phew) whose job it is either to move the stop lever itself against the hour wheel, stopping it, or to release its grip. These two actions are facilitated by the stop lever spring which you can see due south of the hour recording wheel. The strength exerted by this spring is controlled by an eccentric screw in which the spring is mounted. When the chronograph is started, the coupling levers on the other side of the movement release the cluch ring by moving apart and the outward movement of the second coupling lever pushes a pin connected to the reverse side of the intermediate hour recording wheel stop lever, pushing it against the spring and releasing the stop lever’s grip on the hour recording wheel.
All of the gif files above taken from the 6138 service manual found here: http://people.timezone.com/kohei/6138manualPDF.PDF
So, removing this lot completes the dismantling of the movement:
Or so I thought. The eagle eyed among you might have spotted something amiss in the photo above. The hour register is not functioning. Back off with the hands, dial, calendar mechanism, and plate and some fiddling and cursing. Eventually, it turned out that releasing some of the tension on the stop lever spring by turning the eccentric adjuster screw released enough force from the stop lever to allow the hour register wheel to function correctly: