Seiko 7A38 - by the numbers

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Forum Home > Bend it, Mend it - Mods & Wreckers > Inside the 7A38

Sir Alan
Member
Posts: 458

I thought I would start a thread on what's inside the 7A38, and how (and why) it works the way it does. I am no expert, but I thought I would post my thoughts and findings along with some pictures.


My first addition is how the crown/stem release mechanism works. This is pretty fundamental as without this knowledge, getting the movement out of the case to work on it will prove impossible without damage (and I should know!!).


Here is the back of the movement (fully assembled):




The crown can be pulled out (and pushed back in) to any one of three positions:


Position 1 - crown fully pushed in

 


The things to note in the picture above are that the crown stem is held in place by a small 'finger' - this is all that stops the crown from being completely pulled out of the movement.



Position 2 - crown pulled out to the first click (position)


Here you can quick set the day / date,  and by pushing one of the three buttons, reset the chrono, and 3pm, 9pm sub-dial hands



See how as the crown moves out, the 'finger' holding it in pivots and the plate it is part of moves from right to left (see the small dimple in this plate.

 


Position 3 - crown pulled out to the second click


Here you can turn the hour/minute hands to set the correct time (and day/date) and remove the crown/stem




See how the clutch assembly that the stem passes through has separated, and the gear at the end (furthest left) of the stem is now engaging with the gears that drive the main hour/minute hands.


 

See also how the 'finger' plate has moved even further to the left. What isn't obvious is that this plate (and the dimple) have become visible through the back plate of the movement (in the hole marked PUSH).



 

 

Pushing down gently (no force required) on the dimple causes the 'finger' to lift off the stem (only by a fraction), enough for it to stop retaining the stem, which can then be pulled out.

 

It's simple, but I thought it a good idea to see what's happening.


Note also the brass gear wheel that is partly showing through the hole above the PUSH hole (below the SEIKO and to the lower right of the jewel bearing).


Well, if you mistake this hole as the one to release the crown, then you run the risk of this:



See the missing tooth at the 2:45pm position :roll:

 

This killed a movement and had to be replaced (which I managed).  So, take care in there. ;)


April 1, 2012 at 1:47 PM Flag Quote & Reply

Seiko7A38
Site Owner
Posts: 13266

This topic could possibly be (re)subtitled 'DEEP inside the 7A38 movement - where wise men fear to tread', eh Simon ? ;)

Well done - a nice write-up and some super photos.

I'm not very good at multi-tasking myself - it's usually one or t'other ....  or.... but not usually both at the same time. 

I did have a try, last year, at documenting a (failed, as it happens) attempted repair of someone else's 7A38, in a thread on RLT.

See: http://www.thewatchforum.co.uk/index.php?showtopic=69868 - unfortunately, my embedded photos are now all (re)moved.

That watch had been 'messed with' before, and was probably suffering from either a displaced or broken jewel in the main plate. 

If it had been my own, I would likely have gone deeper into the movement, in an attempt to determine / rectify the root cause of the problem, but as it was someone else's, I decided that 'Discretion is the better part of valour', and returned it only partly repaired. 


Here's a couple of the better photos I took at the time:




April 4, 2012 at 11:05 AM Flag Quote & Reply

Sir Alan
Member
Posts: 458

It was the thread you refer to Paul that triggered my interest to a level where I decided I wanted to have a go myself. :P

--


April 5, 2012 at 5:31 AM Flag Quote & Reply

JSS
Member
Posts: 32

I love posts like these.  I need all the information I can get before I go poking around   er... start to service my own 7a38's.

Let's have some more Simon please.

April 5, 2012 at 6:27 AM Flag Quote & Reply

Sir Alan
Member
Posts: 458

Today I decided to strip down one of my latest purchases, a 7A38 7190 from Germany. I was very happy with this watch (given what I paid for it), but knew there were possible horrors waiting inside as the 2pm chrono pusher was very sticky and reluctant to work as it should.


So, here is the photo story of my afternoons work.


The case back off, and the movement on show:



note the stuck 2pm pusher, and the mark on the movement nearby, This closeup shows it better:



So, time to take things apart. Starting with the battery terminal:



Only two screws and its off. Then a further five screws and the back plate comes off:



and



This reveals the circuit:



and again, some evidence of trouble is apparent (see the discolouration of the circuit). In closeup we can see more:



Hmmmm. Not looking good. Looking at the underside of the backplate we can see rust:



OK, time to take the circuit off. This is usually easy (it simply lies over a number of plastic locating pins) - this time its reluctant to come off in the area where the discolouration is. I'm wary that removing it might damage the circuit (possibly cracking a track), but I've started so I'm going to finish. I carefully free the circuit revealling:



a clean circuit - phew!! 



And underneath:



and a closeup to the rust area - just a few flecks of rust:



Here's a closeup of the tiny gold plated regulator (the 0.26 second per day adjustment), and also something that I'd not paid any attention to before, the tiny little arm that makes contact with the circuit to tell the cpu at which position the crown is:



I've read some very informative posts by DaveS referring to this not making contact properly and causing problems.


Now lets focus on the area of rust, and the side view looking at the pusher clip:



That doesn't look good :(


So, it's off with the green spacer block (which is just a push fit and easy to remove) to reveal:



Hmmm, as suspected rusted pusher clips - but not just one, two (out of three).  In closeup:



and (not as bad):



The clips come off easily, showing the rust around the pins they sit on:



I clean this off with the tips of my tweezers, and trusty rodico:



urghhhh:



but a nice clean result:



Now, to my spares box:



and on with two new clips:



and



Doesn't that look better :)


Now, another lesson learned. You'll see that I did the dissasembly with the crown out. When I removed the green spacer block, this dropped out:



I found it (phew) and knew that it was part of the crown / stem clutch assembly. Note the fleck of rust attached to it. I've noticed that many of these tiny gears are magnetised - so rust is going to be drawn to it which can only lead to trouble. I removed the rust fleck and popped the crown / stem back in.


I then gave everything a good clean with rodico, and then back on with the green spacer block:



Back in with the battery gold plated contact and the regulator:



Back on with the circuit:



and the plates:



Then, the moment of truth ...... in with a battery and crown pushed fully in. Does the small second hand move?



Woo hoo. It's alive :D


Now the test of the pushers: YES


Reset the hands: YES


Self test: YES


8) 8) 8)


Here are my trusty tools:



Note that one pair of tweezers is made of brass. VERY useful when the magnetised parts want to stick to the normal tweezers!!!


And the screwdriver - a 1.2mm Bergeon does the trick nicely.


Finally, I cleaned the rust off the worst clip:



Imagine that in the movement. :roll:


April 6, 2012 at 2:08 PM Flag Quote & Reply

Seiko7A38
Site Owner
Posts: 13266

Superbly written up and photographed, Simon - Well done !   Keep this up, and be sending you my iffy 7A38's to work on ! 


I'm wary that removing it might damage the circuit (possibly cracking a track), but I've started so I'm going to finish.


You were lucky with that PCB - and possibly that the damage was caused by water ingress / rust, rather than leaking battery acid.

It's always nerve wracking, on the occasions when you have to peel the PCB away from the green plastic '710' insulator spacer. 

Needless to say I've had a few like that over the last couple of years. :( The area around the crystal (oscillator) and the battery negative terminal contact plate seem to be the most vulnerable to attack, resulting in crumbling laminate and track breakage. 


Here's a few (not particularly good) photos of the rebuild I did on my battery acid-damaged Orient J39725-70 'Panda' last November:


Clear evidence of staining on the back-plate - I'm assuming the previous owner had partly attempted to clean up:




The sight no budding tinkerer ever wants to find after removing the back-plate - a severely weakened battery acid damaged PCB:




The PCB was removed and sadly discarded - the area of laminate around the negative terminal was brittle and simply disintegrated. 


Here, the spacer has been washed off; negative battery terminal replaced; cleaned up properly with Rodico, and partly re-assembled:




Rebuilt with new (actually secondhand, IIRC) PCB and re-assembled. Some staining of the back-plate still evident:




April 7, 2012 at 7:16 AM Flag Quote & Reply

Seiko7A38
Site Owner
Posts: 13266

The area around the crystal (oscillator) and the battery negative terminal contact plate seem to be the most vulnerable to attack ....


Post subtitled 'Rogues Gallery'. :roll:

I don't know why I've bothered keeping these PCB's; I'll likely end up binning them, but here are 3 examples of battery acid damage:




The worst example of battery acid damage I've personally encountered was to the J9320 movement in my Orient J39908-70:




The old battery was rusted solidly in place. I simply replaced the entire movement with a spare rebuilt 7A38A movement, and left this to soak in WD40 for a few months. Hopefully I'll manage to salvage some usable components from it at some point. Roundtuit job #99. :roll:




Bad as it may look, believe it or not, that trashed J3920 movement of mine pales into insignificance compared to some.

I'd seen DaveS post these photos before in another earlier thread. Anybody read this recent thread on SCWF ?

http://www.thewatchsite.com/21-japanese-watch-discussion-forum/28536-how-long-long-enough.html



Now that's what you call a seriously toasted 7Axxx movement !!  

April 7, 2012 at 8:32 AM Flag Quote & Reply

Seiko7A38
Site Owner
Posts: 13266

Further to my previous 'Rogues Galley' photo, in the above post, Simon and I were chatting on the phone earlier today ....

about some of the unfortunate 'side effects' of apparently minor acid damage to PCBs. :(


Anyway, a little more on the unpleasant subject of battery acid damaged PCB's.

Here's a couple of photos of a 7A38A PCB (Seiko p/n 4001726) which I removed a while back (can't remember what from now).

It's not one that I'd consider throwing away (yet), as it's not competely U/S - but then again, it's far from being fully functional. :(


Here's how it appears under 'normal' lighting conditions. Not too bad, eh ?




But here's how it looks in the harsh light of day (the albeit brief exposure to UV has probably done the C-MOS IC no good at all):




As with a few partly dismantled, or suspect movements or parts, I'd bagged this PCB with a small Post-it note, which reads as follows:

Suspect faulty PCB. Minor acid damage near negative (-) contact and resistor. Time and Chrono' Self Test function both work O.K..

But Chrono' will not start (on pusher), nor can the 5/100 second register be re-set. Try on another movement (which I haven't done).

April 13, 2012 at 12:59 PM Flag Quote & Reply

Sir Alan
Member
Posts: 458

Here's what I found in my 7A38 7270 today:



and the pcb:



getting this off the 710 spacer block was 'tricky' - it started to crumble as soon as I touched it.


But, we got there (as best we could) to give this:



which doesn't look 'too' bad - I was worried about the resistor to the right of the quartz oscillator. :roll:


But I was more worried by the crack to the pcb itself :o



but I thought I might as well see if it was still alive (I really didn't expect much). And ... it fired up perfectly (well, 'almost').


As Paul has said above, damage of this nature does have some consequences. The main chrono second hand is reluctant to move - it judders under a self test but after a reset function it will move. I haven't stripped this down, though I'm sure it's not a mechanical issue. 


I will try swapping pcb's over at some point.

April 13, 2012 at 2:01 PM Flag Quote & Reply

Seiko7A38
Site Owner
Posts: 13266

Simon.

I hadn't realized (you didn't say, when we spoke on the phone) that you'd taken photos of the battery acid damage to your recent incoming 7A38-7270. Given the extent of the corrosion in the area of the negative battery connection and the fact that the PCB was fractured, I think you should consider yourself extremely lucky that it worked at all. That PCB looks worse (in that area) than the one I took out of my Orient J39725-70, which was as dead as the proverbial dodo. If I were you, and most functions are working, I wouldn't risk disturbing that PCB again. You might end up with even less functionality than you currently have, if you try swapping it over.  

April 13, 2012 at 8:32 PM Flag Quote & Reply

Sir Alan
Member
Posts: 458

I really liked the 7A38-7270 as it was in great condition. So, as a short term solution I swapped dial and hands with a known working movement and thought I'd tackle the non-moving chrono second hand another day.


Well, today turned out to be the day ;)


I wanted to find out if it was the movement (mechanical) or the circuit (electrical) that was causing the problem.


I assumed it was the circuit, due to the battery acid damage. So, I removed the circuit, which (as Paul predicted) caused futher damage:



and



Wow!!!


the -ve terminal is only being held on by that tiny track. :o


So I swapped in a known working 7A38 circuit, and everything worked. :)


And, as I needed a circuit for my 7A48-7000, and this one still looked like it had a slither of life left it it, I tried it out ........... and it worked. 8)


http://www.seiko7a38.com/apps/forums/topics/show/7614384-inside-the-7a48



April 22, 2012 at 6:07 PM Flag Quote & Reply

Seiko7A38
Site Owner
Posts: 13266

This thread on UhrForum.de is worth a look - google translates it fairly well: http://uhrforum.de/seiko-sports-100-7a38-7070-a-t110191

September 6, 2012 at 6:58 PM Flag Quote & Reply

Sir Alan
Member
Posts: 458

Nice link Paul. From the thread it looks like he managed to get the main bridge back on, something I put down a couple of months ago, ready for a second try.


Well, it can obviously be done .................


September 6, 2012 at 7:15 PM Flag Quote & Reply

Rob Benham
Member
Posts: 277

Thank you so much for this series of photos.  I was forwarned about the loose cog/clutch part on the stem, and I'm sure that saved me a LOT of grief. :D

January 28, 2014 at 8:54 PM Flag Quote & Reply

Seiko7A38
Site Owner
Posts: 13266

Sir Alan at April 1, 2012 at 1:47 PM

See also how the 'finger' plate has moved even further to the left. What isn't obvious is that this plate (and the dimple) have become visible through the back plate of the movement (in the hole marked PUSH).



 

 

Pushing down gently (no force required) on the dimple causes the 'finger' to lift off the stem (only by a fraction), enough for it to stop retaining the stem, which can then be pulled out.

 

It's simple, but I thought it a good idea to see what's happening.


Note also the brass gear wheel that is partly showing through the hole above the PUSH hole (below the SEIKO and to the lower right of the jewel bearing).


Well, if you mistake this hole as the one to release the crown, then you run the risk of this:



See the missing tooth at the 2:45pm position :roll:

 

This killed a movement and had to be replaced (which I managed).  So, take care in there. ;)



Please excuse the (partial) re-quote of Simon's original post, but ....

Hopefully this will serve as a further salutory warning to other would-be / 'newbie' tinkerers.

Subtitled: How NOT to release the crown stem on a 7A38 (or any other 7Axx) movement.


A fellow forum member kindly emailed me a link to a Japanese blog page: http://synchronar.blogspot.jp/2014/06/blog-post.html

I was already familiar with the blog, but had somehow managed to overlook this particular page which was posted in June 2014.

The article describes the construction of a 'military look' 7A38 Franken from a 7A38-706A and a 7A38-7270, which seems to be a popular mod. As I habitually do, I started right-clicking and saving the photos. Then I came to the 6th one down the page. :o

Google translates the Japanese caption as: Unplug straight crown while thrust this around the movement. :lol:




Guess which (wrong) hole in the movement back-plate that the probe is poked in ?  


January 24, 2015 at 4:15 AM Flag Quote & Reply

PHK
Member
Posts: 260

I saw that and was stunned. At least I know which hole to poke! :)


January 24, 2015 at 5:59 AM Flag Quote & Reply

Ferrari Formula
Member
Posts: 1

Hi  Is this movement the same as from a 7A38-7060??  Thanks

October 27, 2017 at 11:05 AM Flag Quote & Reply

Seiko7A38
Site Owner
Posts: 13266

Geert. All Seiko 7A38 models use the same 7A38A signed movement. The same movement (but re-branded - fitted with a differently signed back-plate) is used in many other watches, notably: Yema N8's (Shimauchi V906); Orient and Racer J39's (Orient J3920), Puma Y19's (JEPIC Corp.) and of course the Ferrari Cal. 531 used in the Cartier Ferrari Formulas (15J quartz day/ date version), which you are familiar with.

See also this thread: Ferrari Formula Chronograph movement swap advice


October 27, 2017 at 12:36 PM Flag Quote & Reply

FLX
Member
Posts: 6

I can also add to Sir Alans experience:

My 7A38-7060 lost a couple of hours over night. When I realized the next day I opened it. I was shocked when I saw the battery leakage. The battery terminal was rotten off and there was quite some rust around the battery.

When I took the PCB off I realized the same thing as Sir Alan. The battery contact was only held in place by the track. I decided that I couldn't destroy any more and used 2K-Glue to stabilze the PCB. After it was hard I used some rodico and alcohol to clean the PCB. After putting it back in and fitting a new battery terminal from my spares it was running again 8)

Picture of the PCB before gluing and cleaning:



July 3, 2019 at 3:09 PM Flag Quote & Reply

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