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Forum Home > Parts Info, Tech Tips and Tinkering > Re the 7A38-728A with time function failure

Rob Benham
Member
Posts: 281


It arrived in a box that would have been big enough had the band been laid flat.  It wasn't, but at least it was tied at the back.  At first it seemed in incredibly good condition, but I see the otherwise new looking crystal has what looks like a windscreen stone chip over the lower right tachy minutes counter. (Only as big as the 0 of the 10 minutes ) Shame, just not bad enough to change the crystal, but it would have to be declared for an honest sale.

Inside it looks like new.  The self-test runs the top two small counters and the sweep second hand and everything zero's.  The rest seems to be dead.

Edit: I've just suddenly realized I can't remember if the bottom seconds hand ran during the self test, so further reporting of symptoms is a bit pointless until I'm home.

First hint of hope was that the crown wheel travel from first click to fully in seemed oddly short.  Furthermore, the release lever was just visible when the crown was fully in.  I wondered if it was a 'bogus part' and perhaps just not returning far enough to switch the watch on.  I hit upon undoing the crown from the shaft a few turns to give it added length.  Mmmm, Turned as hard as I dare and it doesn't budge.  (just how are they undone safely?  The shaft was being held with surgeon's forceps)  I was being impatient, anxious to know - when I get back I'll try it out of the case.  Now I need to know what is common to the main hands and the bottom dial. (It certainly does not run when crown pushed in.)

That one I missed last night would have been handy for diagnostic substitution.

http://www.ebay.com/itm/Seiko-Quartz-Chronograph-Tachymeter-Japan-7A38-728A-Wrist-Watch-E-2347-/370977149817


January 19, 2014 at 2:15 AM Flag Quote & Reply

Seiko7A38
Site Owner
Posts: 14137

First of all, Rob, the constant seconds sub-dial (at 6 o'clock) is not part of the chronograph function, so that hand doesn't rotate during the 'hold in for two seconds' self test. It should carry on ticking away merrily independently.


It's possible that the time function has given up the ghost due to 'old sticky oil' syndrome - and the movement needs a good clean and service. You could try the old trick of leaving the watch somewhere slightly warmer (e,g. airing cupboard) and see if it starts up again.


Alternatively, take the movement out of the case, refit the crown and stem (which will also confirm any suspicions you had about their positioning) and the battery, and try gently nudging / helping the constant seconds hand around - using something like a wooden cocktail stick; see if it twitches - as if it's trying to move under its own steam at any point.


Then again, if the time function is still completely dead, it might (if you're lucky) be an electrical fault. Easiest way to eliminate these is to swap the PCB and the H.M.S. coil (in turn) for known good ones.


January 19, 2014 at 4:03 AM Flag Quote & Reply

Rob Benham
Member
Posts: 281

I'll be home and hopefully in the land of the rested by Tuesday.  I hope I can resolve this easily or it might be put in a box of good intentions - awaiting duplicating my Texas den in England. :roll: 

January 20, 2014 at 12:42 AM Flag Quote & Reply

Rob Benham
Member
Posts: 281

While I await spare for the 7T32-7A99 - the brown dial one, as I call it, I've come back to this mixture of good and bad.

It doesn't bode well, since even with a new battery, the constant second hand and the main hands are dead.  BUT, looking through that hole, the one follks have been tempted to poke the crown release pin into, (thinks, when will I do the same.  :/ ) I can see a brass cog wheel.  It is twitching.  Best guess, a one-second twitch.  It goes back between pulses so that there is no progess. 

I haven't heated the watch yet, but will do.  The irony is, it's freeeeeeeeeezing in south Texas today.  Showing 5 degrees C on the patio. 

I just had to have a look inside.  My mini vice/movement holder isn't here yet, so working with the movement back in the watch.  I've reached a point where I need HELP!

.

I've poked the screws into a sheet of paper to show the positions - but is there any need?  Not counting the battery connector, they all look the same.  Can I rely on this in the future?

Having enjoyed watching the changing of one of the pillars, I was very nervous about the torque needed to turn the screws.  They're all out, but must they go back in dry?

I was disappointed to find I couldn't see much more with the PCB in the way, and drew the line there. I'll look back at the forums, but it takes me time to find things as I've not started making proper references yet.  I will, I promise.

Does the team think pairing off the PCB is the next step?  (after all, the watch didn't run in 25 C )  If so, is there any major point(s) that must be observed.  I've read about UV light on the chip and am trained in electronics, so I'll not schock it, but I'm mindful of spoiling this particular circuit, since everything looks so totally new!  It's bewildering to think this watch has ever been used when looking inside.

I'm quite willing to go back a step and put the back plate on again, but I did try the constant second thing but there was not a hint of pulsing at that point. 

I'm wondering if there's another cog with a broken tooth that's just teasing at the one I can see.  Shivers . . . I am nowhere near ready to remove the main bridge - but it's in my nature to want to try.  :roll:   I'll wait for a $10 practice one for that.

January 24, 2014 at 6:13 PM Flag Quote & Reply

Rob Benham
Member
Posts: 281

One thing I've noticed that belies the mint interior is the fact that it has a bogus sweep second hand.  I assume it does.  There is no ornate teardrop shape near the end.  Also, it's not perfectly aligned with the zero.  Is there a known vulnerability to the mechanism when changing that?  I'm mindful the test works and the hands can be set, so I'm hoping that it isn't a problem apart from the fact it doesn't look nearly as nice. 

I notice the wires coming from the winding have a blob of clear epoxy? or some-such on the PCB.  It makes scoping the contacts even harder.  I haven't yet worked out how to fault-find when you have a @%^$ great plate of metal over the PCB. :(   I don't dare power it up with contacts not pressed home.  Unloaded pulses can do unpleasant things to chips. 

January 24, 2014 at 8:41 PM Flag Quote & Reply

Seiko7A38
Site Owner
Posts: 14137

Rob. The sweep second hand on your 7A28-728A is correct for that model.

The 7A38-728x range all have dials and main hands without lume, so why would the sweep second hand need a lume pip ?

There's plenty of reference material on the forum, and I'm pretty sure I've already sent you a link to this thread (by PM):

 http://www.seiko7a38.com/apps/forums/topics/show/8099991-another-little-7a38-mystery-finally-solved-at-last-


Regarding electrical testing. If in doubt, refer to the manuals. 

http://service.seiko.com.au/i/seiko/documents/technicalguides/7A28A.pdf 7A28A Technical guide (note 28, not 38) pages 9-11.


January 25, 2014 at 4:26 AM Flag Quote & Reply

GeorgeClarkson
Member
Posts: 508

Hi Rob,

You are exactly where I was a year ago, restoring my father's Seiko 7A34-7010. I was afraid to open it, but I wanted to fix it, without bringing it to a watchmaker.

Anyway, may I suggest you take a look at the videos I did of the tear down of my 7A48-7000, the movement of which, apart from the moon phase complication, is almost identical to any other 7AXX movement?

I made a complete tear down video, divided in several parts, being the first one this:

http://www.youtube.com/watch?v=TE5OwXnyBEM

Sorry for the music in the first two videos, I was listening to the radio while working on the watch and Youtube deleted it, so I put the first music they suggested... :P

Anyway, the tear down per se starts with video 3 (without music...):

http://www.youtube.com/watch?v=zff1PhC_lIY

I tried to be as precise as possible, but remember I am no professional watchmaker, jsut a hobby collector like anyone else here ;)

--

My personal Blog: www.onlyvintagewatches.com/blog

January 25, 2014 at 5:04 AM Flag Quote & Reply

Rob Benham
Member
Posts: 281

Thanks, Paul.  That's good news.  The problem I have now is one of those failing memories for new things.  Challenges like this can only do good, but as I've said, this is a much bigger site than I'd imagined, and having to repatriate myself to the UK is taking 99% of my brain power. :roll:

Thanks garacs1, that has given me just what I need to see if there's any contamination on the contact side of the PCB.  I doubt there is, since the rest is like new. 

Right now, I'm doing my usual thing of logging every screw position.  I've alwas done that, but it takes time poking them into cards.  Another thing, are the coils interchangeable?  I will check the resistance of mine later today, but swapping would be one way.

Given that one can't run the watch with the back off, has anyone pulsed the coils with a small voltage?  It could tell a lot, though I'd have to guess at the voltage.  I'd intended to scope the coils but the dangers of putting fine wires in there is too great.  I could be done, but not easily.  Those blobs of resin make it much harder.

I need to do some honest work now, but tonight I'll see if I can get to a stage where I can see the gear-train work (or not).  I'm still mindful of Paul's comments about the drying up of oils.  Hard to tell what the condition of that might be when the watch is so dirty on the outside but so perfect inside.

Is there a time when I can spin the entire gear-train without power?  I don't dare apply such a force anywhere without appropriate knowledge.

Oh, that brings me to a point about Paul's movement holders.  I agree they're very important, but from time to time, it would be nifty to be able to see the other side - on a screen if possible.  With mini cameras so good and cheap these days, has anyone built such a setup?  I suppose a pair of mirrors would be the more practical way to go, but that means light and an open lower end.  A new ballgame.


January 25, 2014 at 1:01 PM Flag Quote & Reply

GeorgeClarkson
Member
Posts: 508

Rob, the coils are different in pairs:



As per checking their functionality, refer to this image:



Both images were taken from the PDF scan of the manual, which you can find HERE.


--

My personal Blog: www.onlyvintagewatches.com/blog

January 25, 2014 at 4:42 PM Flag Quote & Reply

GeorgeClarkson
Member
Posts: 508

If I were you, I would not mess around the coils trying to feed them power through an external source, since we do not really know how many volts they are to receive from the battery, which is 1,5V, but it is used to power the whole movement... One could say 1,5V / 4, which is 0,375V per coil, but not counting the IC, the dispersion thorugh the circuit block, the quartz...

Just check the image I posted above, you find the correct values for testing the coils, with a normal multimeter. This should be more than enough to understand if a coil is working, or not. Clean the contacts on the circuit block, taking care not to damage them, and you should be good.

As per your question:


Is there a time when I can spin the entire gear-train without power? I don't dare apply such a force anywhere without appropriate knowledge.


You can turn all the gears removing the movement from the case, removing the circuit block, and pulling the crown to the second position, thus turning it. Hour and minute hands should turn, all the other smaller hands no.

--

My personal Blog: www.onlyvintagewatches.com/blog

January 25, 2014 at 5:05 PM Flag Quote & Reply

Rob Benham
Member
Posts: 281

Ah, a timely reply.  I'd just finished measuring the coils, and the readings and difference pans out nicely.  Thanks for that.

I've got a pair c 2.08k and the other two are c 2.6k measured with a Fluke

I looked for some movement when dabbing the connections using an old AVO 8 that I lugged to the States years ago.  Not enough to even make it twitch.  What's worrying now is that I can't see the 1 second twitch when it's put back together. 

With the PCB removed, I was able to easily turn the aforementioned cog and wonder if I've got to a point where there's a tooth missing or some such.  I'd surely still see somethng moving if that was the case . . . erm, wouldn't I?

I'll have another look this afternoon and must be sure it's not just the 'crown fully in switch'   Is that a technical term? ;)  but will then be hunting for a PCB/Chip to try in it.  It would seem a shame, because every molecule of that layer looks new.

Oh, one thing.  With the crown in, there's a little strip of steel that pops up in the same hole as the cogs.  I have no idea what its function is but it's quite rigid when touched with a needle.

I'll have a look at the manual over a cuppa. 

January 25, 2014 at 5:06 PM Flag Quote & Reply

Rob Benham
Member
Posts: 281

I think our posts crossed -

Well, I am indeed very concerned about the chip, so don't want to zap it.  As far as the coils are concerned, two volts across 2k ohms is one milliamp - worst possible case, so I doubt one could zap it, but I'm not going to risk it.  I have spent a lot of time a while back testing 50 microamp movements by just using the battery via the AVO.  Delicate stuff, but as mentioned it didn't pulse the time coil at all.   

My guess was .8 of a volt on the coil and to my astonishment, there is a Test Point (top of the row of three) with .78 volts fluctuating - I'd guess on the second. :/  Mmmm . . . Lucky guess?  Until I scope it I'll no know the true nature of the voltage. 

What I am seeing is full battery voltage of 1.55 on ALL the other TPs.  Since all but 3 are accessed through the metal one has to be very, very careful not to touch the back plate.  But then, watchmakers are, aren't they. :D  I have no probes anything like needed, but using laquered wire might avoid a mishap. 

My guess is, the voltage is always on the coil and the chip grounds the other end of the winding on demand.  I set about starting the stopwatch and trying to catch the accumulated minutes as it moved.  The pulse is very short so I'm not surprised I couldn't see it on a meter.  I don't have a recording device and I'm not sure if I can set my old Tektronix to fire on a negative pulse.  I'll try.

I feel what I should be seeing is a tick pulse on coil A's TP.  I know my curiosity will make me test that reasoning on my own watch. :o

January 25, 2014 at 7:31 PM Flag Quote & Reply

Rob Benham
Member
Posts: 281

Well, that's Sunday morning gone.  I felt sure I'd know more within an hour, but I don't.  Well, not much, anyway.  I'll risk teaching mother to suck eggs and just blog in the hope it will help someone. 

.

The test points (TPs) most give the running voltage of the battery.  But as I said above, the top one of the row of three on the bottom left edge (as the 7A38 is usually orientated)  does show a clock pulse.  Each part of the squarewave is 3ms duration.  It's solid and shows a higher voltage than the fluke.  In fact, virtually the full battery voltage. 

EDIT:  Last night I found that one of the two TPs near the row of three, switches on and off with the crown position.  That was exactly what I wanted to know in the first place, but had swapped the stem over before I found it. :/

.

Since the stopwatch runs, I set that going and lookd for a pulse on what I thought might be its TP.  Bat volts only.  I searche around as I wondered if I'd got the wrong points.  Nothing. I got out another 7A38 and checked.  It was exactly the same. :mad:   The TPs must all be showing the line voltage apart from one.  What a barmy set of TPs.  Then I turned the scope to AC and turned up the wick.  I fitted a local ground lead to help me get rid of noise.  Now I could see a c 1 sec pulse just about everywhere.  Not surprising, that kind of thing happens, but on the time coil there was a definite oposite direction pulse.  Very, very low voltage, and almost certainly the kick back from the coil's back EMF.  Okay, something to say it was working.  Got back to the duff watch.

.

It was much the same except the back EMF pulse on the time coil  was even smaller, and I thought, not so even.  (This is where I need a head-banging emoticon.)  So, all in all it was inconclusive - but probably the absense of a pulse on the switching side of the coil.

     Check the contact surfaces on the PCB again and start looking for a PCB I can risk for swapout.  Might as well have done that in the first place, but if you'd seen me fault-finding on my Oldmobile van thread you'd know it's in my nature to want to know what's going on.  I learned a lot, and found rodent had eaten a lot of insulator off wiring right up to the main computer.  So, worth the two weeks of curious prying.

.

But now I'm in the macro world, and it's a big but, I watch spellbound on this forum as folk fettle watchs.  That's easy, I think, but then get over to the bench.  Mmmm . . . I'd be really embarassed to let anyone see my antics trying to fettle micro kit.

So, I need a swapout PCB and any other hobby electronics bod to confirm my amature logic is sound to this point.

Oh, one other thing.  The very diagram in garacs link I'd like to look at again, simply keeps vanishing.  Never known it before.  It's 8 or 9 I think, but the one showing the function and position of the coils.  Even the list of chapters on the left turns white.  Most odd.


January 26, 2014 at 3:01 PM Flag Quote & Reply

Rob Benham
Member
Posts: 281

Well, I'm totally wrong about the Test Points.   Following the circuit lines reveals they will in no way reveal the signal going to the coils apart from the 'noise' on the lines.  Drat - it would have been such a handy diagnostic tool.  Another point is, these TPs are very frail.  The copper print spans a hole in the PCB and sharpening one of my scope probes left indentations I could see from the other side.  They seem to have no function, but again, spoiling a mint circuit, even microscopically, goes against the grain.  Mint, yes, but I'm begining to think it has to be the fault.

Basically back at square one, with substituion the only way forward.  The gear train moves freely and I can't see any missing teeth, but that's not to say there's not something wrong somewhere - I have years of experience with electronics, but I'm a total newbie on the watch fettling side.

This could be a very attractive watch, so I'm mindful I must get it right.  I take it the cicuits are hen's teeth now?

January 28, 2014 at 11:42 AM Flag Quote & Reply

Rob Benham
Member
Posts: 281

I forgot to mention.  Paul, that page count 8 - 9 are the very ones that seem to vanish.  Suddely this morning, they're there, so will try to print out. 

I see the 'clutch lever' is what I was referring to - and at last I can see the picture of the step rotor.

I also feel this business about voltage on the coils might be more complex.  c 2k ohms would be a lot of current for a tiny part of that chip to meter out.  I fear it might be sending a pulse of medium frequecy and into an inductive load would mean the current was much less.  All things I must know to have a true understanding of the rotational generation. 

However, this may all be academic, as I'm really struggling to work on the more  simple parts and very much doubt I could cope with the minutiae.

January 28, 2014 at 12:24 PM Flag Quote & Reply

Seiko7A38
Site Owner
Posts: 14137

Oh, one thing. With the crown in, there's a little strip of steel that pops up in the same hole as the cogs. I have no idea what its function is but it's quite rigid when touched with a needle.

I'll have another look this afternoon and must be sure it's not just the 'crown fully in switch' Is that a technical term?

I see the 'clutch lever' is what I was referring to - and at last I can see the picture of the step rotor.


I wasn't sure whether you were referring to this (in your various posts) - the setting lever / switch:




But if it was, Rob - you may want to remember this link:

http://www.seiko7a38.com/apps/forums/topics/show/9397929-problems-with-stem-and-resetting-chrono-hands


January 28, 2014 at 12:42 PM Flag Quote & Reply

Rob Benham
Member
Posts: 281

Thanks, Paul.  Another smashing photo.


I've found the 'crown switch' as I'll call it, and one can clearly see the contact wipe marks on the PCB.  I imagine on an old watch might well track brass/gold over the gap.  But the thing I think is the 'clutch lever' can just be seen in the gap below the three 'DON'T POKE IN HERE' cogs.

I tried your link above again, and this time I could see page 8 and 9.   I got the info I needed, before it vanished again.  Never known a PDF to do that.


Now, and odd thing has just occurred to me.  Back to the 'crown switch'.  The 'wipe mark carries on PAST the contact point where it would make no contact at all.  I'd be excited about this, but as mentioned, one can see the voltage switch on, on one of the Test Points just above it.  I've also tried another stem and not quite pushing it home etc.   Another PCB is the logical way I suppose.  Do I risk a working watch?  The safe way would be to put this circuit in the good watch.  Food for thought. 

January 28, 2014 at 2:01 PM Flag Quote & Reply

Seiko7A38
Site Owner
Posts: 14137

Another PCB is the logical way I suppose.


It would have been a bloody sight easier, Rob ! :lol:

It's also the way I go about it - I keep a 'gold board' PCB and set of known good working coils solely for eliminating electrical faults. ;)

'When in doubt - swap it out'. 

January 28, 2014 at 2:05 PM Flag Quote & Reply

Rob Benham
Member
Posts: 281

Nice to have.  Worst scenario, are new circuits still available?

January 28, 2014 at 7:42 PM Flag Quote & Reply

Seiko7A38
Site Owner
Posts: 14137

Not any more, unfortunately Rob. :( I managed to grab a couple in the past, but I've not seen any for sale for a couple of years now.

January 28, 2014 at 8:13 PM Flag Quote & Reply

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