Seiko Gen 1

This Seiko 7A28-7120 chronograph came in for a full service, a new crystal and a seal change recently, it also had a problem with the chronograph minute counter which wasn’t working. More commonly referred to as the Seiko Gen 1 (generation 1) these were first issued by the MOD to British military pilots in October 1984 and weren’t replaced until November 1990. Seiko supplied a Gen 2 chronograph later on in the 1990’s. The Gen 1’s were also the first quartz powered chronographs issued by the MOD replacing the venerable mechanical Valjoux 7733 powered ones that were issued for the preceding decade or so. The MoD bought and issued a total of 11,307 Gen 1’s, which makes it one of the most popular issued military chronographs to date. This particular watch had been sent by the owner to Seiko UK for the work to be carried out but it was returned as they don’t carry the spare parts anymore, this was when it made its way to me. The movement has a crown at 8 and the start, stop and reset at 2, 4 and 10. The large seconds hand is the chronograph running seconds hand, the sub dial at three is the 1/10th second counter, the one at 9 is the minute counter and the one at 6 is the running seconds.


When stripping any chrono always identify which hand comes from which sub dial so that they go back on the same pinion, with chronographs it’s a must as the pinions are generally different sizes. The easiest way I find to do this is to turn them over, place them in the correct order (left to right) and photograph the back, they always seem to differ and makes selecting the correct hand a breeze.


These 7A28’s are very well designed and engineered movements, made before the practice of using plastic mainplates was developed.


With the chrono and main bridge removed you can see the running train is remarkably compact. There is a separate coil and step rotor for the 1/10th second counter, the minute counter, the chrono seconds counter and the hour, minute and seconds train.


The substantial (for a quartz) mainplate after stripping.


The parts stripped and ready for cleaning.



This was the minute counters coil, you can see where it had been poked with something (usually a screwdriver at a battery change), it checked out as having an open circuit so I made a note to source a new one. When I examined it again I noticed one of the fine wires wasn’t actually making contact with the brass terminal so the soldering iron was fired up and the joint was remade which did the trick.


The movement was re-assembled after cleaning and inspection.


When I put the movement on test the chronograph function worked perfectly including the minute counter which was great, however, the running seconds and time indication (hour and minute hands) didn’t want to run! After much head scratching I eventually traced the problem down to a worn bush on an intermediate wheels pivot. It looked like the bush had a figure of eight hole and when I reassembled it the pivot had gone in the wrong hole of the eight. It was a strange problem that I’ve never seen before, I think it must have been a manufacturing glitch as there was no wear what so ever on the wheels pivot and when I reassembled the pivot into the wrong half of the hole it must have been enough to open out the slot and allow it to move about. A new mainplate would have to be sourced.


The replacement mainplate arrived complete with the motion work and bridges, here it is next to the original.


It was fully stripped and cleaned.


And the original movement had to be stripped once more to swap all the components over.


One problem with the replacement plate that I didn’t spot until I came to fit the top cover was one of the posts had been cut down.


A replacement was pressed out of the original mainplate and transferred across to the new one.


Once it was fitted the movement could be completed.



The dial and hands were installed and my attention turned to the case.


The case was completely stripped and all the components were ultrasonically cleaned.


The pushers were re-sealed and lubricated. These are held in place by tiny clips that take a bit of a knack to fit, I’ve lost many of these into the ether in the past!


The replacement NOS crystal was fitted.


Which meant the case was now complete with lovely smooth functioning pushers.


The movement was re-installed into the case and fitted with a new battery, the caseback was then fitted along with a new seal.


The finished article, a lovely example of an issued Gen 1 chronograph.





  1. Hi
    I have one of these Seiko watches and although the watch works fine I have been told that the chronograph element is faulty and Seiko will not repair it. Some of the dials do not work and the second hand does not return to 12. Is this something that you could repair for me.
    Regards Alan


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