Where's my digit?

[digibloke]

Hey all. I got a lovely one button SS Hamilton QTC in the post this morning (the same module as a P2 which is why I'm posting here) . The module has a weird thing I've seen a couple of times where someone has deposited some resin or epoxy all over the front of it (my Omega TC1 has the same thing). Don't know why anyone would do this as it makes it really hard to get at anything and the trimmer switch is now glued fast.

It was as dead as a dodo with new batteries so I cleaned the contacts - still dead. I hooked it up to a battery and left it for about six hours and when I checked it with a magnet just now it was running. It sets fine but the 2nd digit is missing (of course it is otherwise it'd be too good to be true).

Question is which of the drivers (2 above and 2 below the display) give juice to the second digit?

I know we've done the missing segments thing to death but I can't find much info on the single digit transistors?

[bruce wegmann]

The digit drivers are the four transistors on the left end of the display, the top pair both vertical, and the bottom pair consists of one vertical at the lower left corner, and another horizontal along the lower edge, just to the left of the clock chip package. Assuming the "1" is digit one, driver 1=lower left, driver 2=lower right [horizontal one], driver 3=top left, driver 4= top right. Digit and segment drivers are not interchangeable; one is NPN, the other PNP [depending on whether the display is a common anode or cathode device], and so require opposite signal polarities to switch "on" or "off". Soldering [either to remove or replace] is not recommended [the heat may dislodge or weaken the connections, or damage adjoining components]. Conductive silver epoxy is the best technique here. You can safely put the module back under power immediately after installing the new part to make sure a good connection has been made [if not, you can remove the epoxy easily with a cotton swab soaked in acetone, and try again]. Ordinary pins and needles are too heavy and blunt to make good applicators; I use a very fine watch oiler, which, with its' tiny spatula-like tip, is nearly ideal. This repair should be done under magnification [at least 15-20X], to be sure no short circuits are created when applying the epoxy [the connection pads are very small, and close together], and great care should be taken not to over-apply [when the part is settled into position, the epoxy can be forced out from under the connections and form a conductive "bridge" that effectively shorts out the transistor]. Though the material sets up well after only five minutes, I always allow a days' curing time before calling the repair a success [it also gives the bonds time enough to reach full strength, so the watch can be handled normally]. Done properly, such a repair should last many years. The coating you refer to is actually a kind of silicone rubber, which was intended to protect the module from moisture [and perhaps the corrosive effects of leaking batteries]. I have seen this only on the early, large P2 circuit boards [the same kind that is also in most P1s], never on a P3. It apparently did not live up to its' design expectations, and was abandoned in favor of a simple light coating of matte-black paint. It can be removed by careful peeling [it does not "stick" to the module in the way a layer of paint would], but if you start to do this to get access to the part you want to repair, you might as well go all the way; a partially-peeled module just looks terrible...

[ledwatch]

But before plunging in and changing a driver transistor you should check to see if its faulty or you will be wasting a whole load of time. Check for dry joints and then test by applying 1.5volts to the base, chances are evens that the fault is in the logic chip....

[digibloke]

Thanks very much for the for the replies. I think that replacing the whole transistor is way beyond me (they always look sooooooooo much smaller once removed from the module...) but I have had some luck in the past with repairing dry joints on the segment drivers. I'll start by trying to find it's base under all the silicone gunk, test for dry solder joints then give the "1.5v from the middle of a module tester" a go to see if it's dead or not. Hope I can bring it back to life as it really is a super watch and It'd be great to keep the original module in it.

As an aside, I've never really been that interested in Hamiltons before but the finish on the case and band is really something. Mirror polished around the time screen and sides of the case, then dull (not brushed but kind of sandblasted?) on the rest of the strap and case. Hopefully this'll be my "wearer" for the summer (World Cup 2006 watch :D )

[digibloke]

Hurray my digit's back :D

It was a dry solder joint on the transistor - (so thanks for letting me know which one it was I had to find under all that silicone Bruce).

If someone had told me a year ago that I could be so pleased to see a little red number 9 then I'd have laughed at them.

DB.

[azimuth_pl]

For those of you who have not seen an early P2 (2800) module this picture might help to understand the above description by Bruce: