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Where have my segments gone...

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digibloke

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Where have my segments gone...

Post12 Dec 2005, 12:16

Hey all. My PII took a bit of a knock at the weekend and has lost some segments (same segment on each number). I kind of figured that since the problem was caused by the watch being jarred that it was probably a connection problem rather than an IC problem. More out of curiosity than anything else I tried warming the module up with a hot air gun to see if I could get connectivity back. Hey presto it works fine - but only while the heat is being applied. (joy followed by huge disapointment as you can no doubt imagine :( ). On the bright side I guess this means the little transistor driver for the missing segments is still working?

I'm wondering if it may be a solder joint between the display and the module and so am considering popping off the display to remake the joints with silver epoxy.

Before I do this can anyone advise me as to how tricky this is? Am I risking killing my watch or is there anything else I should try first? Incidently I've got it hooked up to a battery at the moment and it's running and setting just fine. Grrrrr. stupid hobby.
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Lordpulsar

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Re: Where have my segments gone...

Post12 Dec 2005, 13:08

If its the same line of segments gone across the entire display, you have a loose connection on one of the driver chips situated directly above the display.. The best way to fix them is by using conductive epoxy and a needle tip... My advise is not to use the hair dryer on the module any more as it may start to remove the other drivers too !

Good luck !
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digibloke

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Re: Where have my segments gone...

Post12 Dec 2005, 13:46

Thanks for the quick reply. Uh oh. I was hoping that it wasn't going to be so fiddly - those things are tiny! Any idea how I find out which one is causing the problem? The segments which are out are the ones which make the top left vertical of a figure 8 (so a 9 looks like a 3). Any help in finding the culprit would be very much appreciated...
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Lordpulsar

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Re: Where have my segments gone...

Post12 Dec 2005, 15:02

The way I would go about it, would be to (very lightly) apply pressure on the ends of each driver to see which one had the dry joint.. If this didn't work, try a sewing needle and very gently press the nib under each end of the driver and try to make contact with the track and the driver connector...
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digibloke

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Re: Where have my segments gone...

Post12 Dec 2005, 15:18

Thanks very much for the tip. Sounds like a good way to progress as I really don't want to risk making things worse by fiddling about too much. I'll give it a go tonight - very carefully!

Best wishes DB.
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bruce wegmann

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Re: Where have my segments gone...

Post12 Dec 2005, 19:37

In engineering terminology, the seven segments of an LED display are lettered "A" through "G", "A" being the topmost, and progressing clockwise, so the upper left vertical segment is "F" [with "G" being the central horizontal one]. So your problem is with "F". The associated segment drivers are the row of seven tiny rectangular blocks directly above the display [the two on the far left, and displaced slightly lower, are digit drivers]. From right to left, they provide power to the A, B, D, C, E, F, and G segments, respectively [don't know why they reversed the positions of the C and D drivers, but they did; might simply have been the most straightforeward way to lay out the circuit]. Thus, the sixth one from the right is the troublemaker. Gentle pressure might re-establish contact . It is unlikely the defect is in the display, since all three F segments are out. A quick way to check the display contact is to short out the transistor [with a fine tweezer, touch the bottom and upper right contacts; if the segment returns, there is certainly no problem with the display itself...it also tells you there is power available on that line]. Removel of the bad transistor is purely mechanical; gripping top and bottom with a pair of heavy, blunt-nosed tweezers, just give it a slight twist. It should break free without applying any great amount of force [especially if one contact is already broken]. A replacement transistor can be extracted from a donor module in the same way. Conductive epoxy is by far the best method of re-attachment [any heating of the substrate may loosen adjacent components, and so do more harm than good]. Note: Drivers can only be replaced with ones of like type; you cannot put a segment driver in a digit position, and vice-versa. I presume one set of drivers is PNP, and the other NPN, and are thus not interchangeable in the circuit. Also, this repair, while not very complex, should not be attempted without some form of visual aid; it is too easy otherwise to get conductive epoxy where it does not belong, and so create new problems. I do these at 30x under a binocular microscope; with care, the fix is almost invisible.
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ledwatch

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Re: Where have my segments gone...

Post13 Dec 2005, 10:48

Tell me which segment is out [ 18:88 ] and I'll tell you which transistor is not making contact. I find conductive epoxy is often hard to work with around those trannies and if it doesnt repair the fault it will cause an access problem when it dries which makes further repairs difficult - what I do in this area for dry joints is to hold a dressmaking pin in a pair of small pliers by the head, then heat up the point with a small butane torch, quickly apply to the three transistor legs and you have a micro soldering iron which works well.
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digibloke

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Re: Where have my segments gone...

Post13 Dec 2005, 12:15

Thanks again for this wealth of information. It looks like Bruce is right as it seems to be the 2nd transistor from the left (just touching it lightly with a pin for a few seconds brings the segments back on - thanks for that tip Simon A).

Just have to decide between micro soldering iron "a la ledwatch" or silver epoxy now. DB
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azimuth_pl

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Re: Where have my segments gone...

Post04 Feb 2006, 14:05

If you don't have the time to check each driver then just take a look at my picture below:
Image
or read more at my website:
http://crazywatches.w.interia.pl/photo/ ... _tech.html
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coconutman351

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Re: Where have my segments gone...

Post13 Jul 2019, 09:57

Well it was a matter of time before one of my P2 LED segment row went out. Based on Bruce's letter reference, segment E does not turn on for every digit. It looks like the affected driver transistor is 3rd from the left. I also tested the LED display segment E by shorting across pad and the segment E lit up across the display. I placed a doner transistor in its place but that row of E segments just won't light up. Any ideas what could be the problem? I just tried to place the replacement transistor in its place and applied some pressure. Would the only way to test the replacement segment transistor is to use bonding epoxy to ensure proper conductivity? I know that would be ideal but i wanted to test it first before bonding it to the circuit board.

I have a photo of the module with the segment E transistor removed.
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charger105

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Re: Where have my segments gone...

Post15 Jul 2019, 11:08

Make sure you use the correct transistor type.
Segment drivers are different to digit drivers.
One is NPN, One is PNP.

Rgds.
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Re: Where have my segments gone...

Post17 Jul 2019, 22:53

Thank you Charger105, good to know. The donor transistors were from the segment drivers from an equivalent P2 non-working module. I suspect there are three conductors to connect, base, emitter and collector so I would need to be aware of its orientation when installing in. Any recommendations to a good conductive bonding epoxy?

Bruce, thanks I received you PM on fixing this module, I may just take you up on it as this requires some steady hands and a really good microscope to align the connecting points of the transistor to the PC board pads.
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Re: Where have my segments gone...

Post01 Aug 2019, 23:18

I have a similar issue. I figured out it is the 2nd and 4th from the right. When I use the tweezers to short them the segments light up. So that means they need to be replaced? With pressure they don’t come on.
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