Seiko 7A38 - by the numbers



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Forum Home > Bend it, Mend it - Mods & Wreckers > We're going in ........ deep inside the 7Axx

Sir Alan
Posts: 457

I did a physics degree many, many years ago - which I think makes me the type of person who asks a lot of how and why questions :D


I also like taking things apart ;)

As I have been learning more and more about the insides of the 7Axx , I wanted to find out how the movement actually works.

Right at the beginning of this adventure, I took a broken 7A38 movement apart, but I didn't really pay much attention to what I was doing, and I certainly knew that I wasn't going to be putting it back together!!

I also didn't take any photos, so, whilst at the time it might have looked like it made sense, only a few hours later I was left with a collection of tiny parts :roll:

Recently I acquired two watches, both listed as not working. Taking each apart, it was evident that one had suffered major water ingress (and resultant rust) and the other lesser such damage.

It was my intention to dissasemble the badly rusted movement, taking pictures along the way. Tonight when I went to fix the other movement (something I thought would be quick and easy) I found that whilst the extent of the rust was much reduced, it had attacked the tiny post that one of the pusher clips sits on (the 4pm pusher), so much so that trying to remove the rusted pusher clip I also removed the post :mad:

Doh - so now I have two movements, both with the base plate badly damaged, but with all other parts (including the precious circuit) intact.

So, what's really inside the 7A38 movement?

I'm going to take pictures in natural light as I dissasemble each of the bridges. Tonite I had a 'practice' run on one of the movements, removing the smaller bridges completely (they will be easy to put back together) and getting this far with the main bridge:

I've got a really good idea now what's in there and how all the parts work together and why.  I also know now why some of the parts are so heavily magnetised.

More to follow, lots more ;)

May 20, 2012 at 5:35 PM Flag Quote & Reply

Sir Alan
Posts: 457

We had a very sunny day today, so I thought this was a good opportunity for some clear photos. 

Here is the movement with the three bridges in place:

So, let's start taking things apart ;)

We'll start with this one:

One screw and it's off:


let's drop the coil back in place, and remove the two gear wheels

Now, the interesting piece is the tiny gear in the circular well above the coil:

this is a very strong magnet, and is driven (to rotate) by the coil, which in turn is activated by the circuit block. 

This little guy then drives the gears which ultimately drive the sub-dial 1/10s hand.

So simple.

And there was me thinking that these parts shouldn't be magnetised!!!!  (I nearly bought a de-magnetiser - doh!!)

Here is the set of parts:

six parts for this bridge.

Now a quick look at the other bridge:

and inside:

It looks just like the other bridge. Here are all the parts:

And this leaves us with this:

and now the fun starts :)

Two screws and the bridge comes off, but the first 'complication' begins:

two of the gears lift off with the bridge :roll:

Luckily I've been under here before, so putting them back in place isn't too tricky:

Now some reference photos for later (re-assembly)

there's a lot going on in here, and getting it all back together is going to be tricky :roll:

First let's remove the gears:

leaving only the two magnet gears:

more to follow .......

May 22, 2012 at 11:32 AM Flag Quote & Reply

Sir Alan
Posts: 457

OK, so now lets complete this strip down.

First we remove the crown stem and clutch assembly:

that's this little lot:

which leaves us with:

the black plastic block simply lifts out, revealing:

then this

and finally this:

and all looking pretty bare.

This is what was in there:

One thing I have learned in taking this, and several other movements apart is that there are lots of tiny bits of dirt, rust particles, etc. which stick to the gears, and if they are metallic to the magnets - you can see lots of examples in the photos above. These can't be good, maybe they create some resistance, maybe they even prevent movement.

When I put this all back together (and I am going to do this!!) I'm going to give every part a thorough clean first, and then I'm going to oil things.

If I get it right, I can't see any reason why I can't end up with a movement as good as when it left the factory a quarter of a century ago.

I'm really excited at the prospect of doing this. :D

May 23, 2012 at 5:36 PM Flag Quote & Reply

Posts: 17

You've already said it but, damn!...those close up pic's really do show all the rust, dust and dirt!!...

May 24, 2012 at 3:34 PM Flag Quote & Reply

Sir Alan
Posts: 457

A brief update.

I've cleaned one full set of parts and started a rebuild from scratch. I'm taking photos as I go ready for the full story.

But, I'm currently struggling re-assembling the main bridge. This is (I'm certain) the only really difficult part of a full rebuild. At the moment I've had 2 attempts at about 90 minutes each.

As with all things, there will be a technique to doing this - I just haven't found it yet.

The challenge is to get this:

to sit down on these:

which involves getting all seven (7) gears to sit in their respective bearing point. Grrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr. 

June 2, 2012 at 12:35 PM Flag Quote & Reply

Sir Alan
Posts: 457

I haven't forgotten or given up on this little challenge. I'm in contemplate mode at the moment. :roll: :| :)

This is definitely not a job to rush. There will be a technique to this, and I'm sure I'll suss it out...........

July 8, 2012 at 2:50 PM Flag Quote & Reply

Posts: 508

Hi Sir Alan, have you got any progress since last June?

This is quite interesting, since I am learning how the 7A38 - and my 7A34 are really made inside, and how they work. This "watch world" is for me completely new, so the more I read - and watch pitrures - the more I learn.

January 8, 2013 at 3:37 AM Flag Quote & Reply

Sir Alan
Posts: 457

In a word - no :roll:

I've hit a bit of a block getting the main bridge back on. I've had 3 attempts all with no success.

I've got another 'plan' which might work, but I just need a couple of hours quiet time to focus. I will update this thread, promise.

January 8, 2013 at 4:09 AM Flag Quote & Reply

Posts: 508

Any progress? :P

February 14, 2013 at 4:09 PM Flag Quote & Reply

Site Owner
Posts: 12250


Being Italian, you may find this thread by 'Riddle?' about the restoration of his 7A28-7040 on Orologi & Passioni of interest.

For the rest of us non-italians (google translate doesn't work on O&P), well worth a look for some of the close-up macro photos. 

May 23, 2013 at 2:28 PM Flag Quote & Reply

Site Owner
Posts: 12250

Here's a thread posted yesterday on SCWF showing the complete strip-down, clean and servicing of another 7A28-7040 / -7049:

August 2, 2013 at 2:23 AM Flag Quote & Reply

Sir Alan
Posts: 457

Right. I'm back (with this thread). I haven't forgotten it, in fact its been a bit of a nemesis .......

Last weekend I did a service on Woody's 7A28-7000 'Alien' which along with the pusher block being gunged up with araldite was also suffering from the stuck sub-dial hand syndrome (in this case it was the 9pm sub-dial).

This manifests itself as one (or more) of the sub-dial or main chrono hands either refusing to move, or pausing for a number of 'ticks' before moving. The latter is a pain because when you reset the chrono, the reluctant hand doesn't reset to zero, meaning you have to zero it. 

So, because I knew this would annoy the hell out of me, I decided to try and fix it, and I did. I didn't find anything obviously wrong (so no broken gears, duff coils etc.) and there wasn't even anything obviously clogging things up.

All I did was to remove the bridge (of the affected hand), remove and clean the gears, reassemble and oil (now this is something I haven't done before - well, not as a matter of course).

Now, its not a really big deal, but it did spur me on this weekend to sort out a couple (well three actually) of my 7A28's that were all suffering the same fate.

My gold sunburst dialed 7A28-7030 had a jittery 3pm sub-dial hand. 

see how the 1/10s hand is one mark off zero?

So, strip down time - I love taking the 7Axx apart.

First bridge off, the bridge I need to remove next is the one at 6pm. To proceed further the movement needs to come out of the case to allow me to get the 710 block and coil off.

here we go:

everything looks pretty clean, and there are no signs of what might be causing that sticking hand.

and with the two gears out.

I cleaned everything, carefully reassembled things and gave each bearing an oil (using Moebius Quartz oil).

Putting the bridges back on needs care. Its really easy to think all gears are in place, but the bridge won't sit down flat.

Don't think it will sit down if you tighten the screw!!

It means that either the gear is not in place in the lower bearing hole, or the upper hole.

It just takes patience.

Now I've got a fully functional watch:

As I was on a roll, I also attended to my other 7A28-7030, this one with a darker gold sunburst dial and a more orange main chrono and sub-dial hands.

This watch had a reluctant main chrono hand. So, less work than above (only the first bridge to remove), but when I was in there I did clean and oil all bearings.

And this one is back to 100%

(when I took this one apart I found it had no case back gasket!!)

Honestly :roll:

Tomorrow we're going back to the main bridge ..........


October 26, 2013 at 4:31 PM Flag Quote & Reply

Site Owner
Posts: 12250

Once more unto the breach, dear friends, once more. 

Glad to see you've given your thread a well-deserved bump, at last, Simon. Keep up the great work ! 

October 26, 2013 at 5:36 PM Flag Quote & Reply

Sir Alan
Posts: 457

Gaaaaaaahhhhhhhhhh :mad:

This main bridge is (so far) IMPOSSIBLE.

I have tried everything to get this damn thing to sit down on the 7 shafts (5 on one side, 2 on the other) and so far (obviously!!) I've failed.

I thought maybe oiling the bearings might help things to slide home, but nope.

Access from the side is extremely limited.

Access through the inspection holes is limited.

It must be possible, but not by me.

It really is super frustrating.


October 28, 2013 at 5:31 PM Flag Quote & Reply

Sir Alan
Posts: 457

Note to self.


October 28, 2013 at 5:37 PM Flag Quote & Reply

Posts: 508

Simon, honestly I do not believe I am more skilled than you in servicing the Seiko 7A movement (or any other watch movement for the matter), but I do agree the main bridge is frustrating, at times. But I do manage, after a little fiddling, to put it back in place, every time. What I usually do is to put the screws in place and screw them down just until they touch (but do not press) the bridge plate, and with a very thin nail tip I move the gears and the coil in place, turning them right and left until they get centered and the bridge plate just sits in place, one gear at a time. It usually takes me some 20 minutes, but eventually I do manage to fit it in place.


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October 30, 2013 at 5:27 AM Flag Quote & Reply

Sir Alan
Posts: 457

Hi George.

I think once I've done it once, it will all click (!!) into place.

I've tried what you suggest, both screwing the bridge down very lightly (this also helps if you accidentally invert the movement and everything falls off!!), and gently nudging the gear train.

I'll keep trying - probably this w/e. 

Thanks for the support ;)

October 30, 2013 at 9:03 AM Flag Quote & Reply

Posts: 508

Since today was a calm day, I made a video this morning reassembling the main bridge of my "guinea pig": 7A38-7060. It took a long time to upload, but finally is online. Here's the link:

"Reassembling the main bridge"

Sorry for the hands in the way a couple of times and for the movement off target, it was hard to check on the camera display the correct position of the movement, while working on it ;)


My personal Blog:

October 30, 2013 at 12:34 PM Flag Quote & Reply

Sir Alan
Posts: 457

George, you're a star 8)

I like the methodical way you tackle that, and in particular the gradual tightening of the two bridge screws.

I shall attempt this technique in due course (not 'till the w/e I think - I've had a couple of late nights and I want to be on top form for this!!)

Thank you.

October 30, 2013 at 1:57 PM Flag Quote & Reply

Posts: 508

Thanks Simon, I hope the video not only is helpful for you, but for all the community. 


My personal Blog:

October 30, 2013 at 2:35 PM Flag Quote & Reply

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