Faint LED's - easy fix?

[LEDluvr]

Ok, I apologize in advance if this sounds horribly 'newbie'....

I recently bought another T.I. model 401 as a back up for my current one.
When I opened it up it had dead, somewhat leaky batteries still in it. (They had left a haze over the battery cells and battery contacts as opposed to green gunk.) I really didn't expect it to work but after a cursory wipe with rubbing alcohol and putting in a fresh pair of batteries - whala! - it came to life!

The problem is the display is really dim compared to my other 401. After pressing the command buttons a few times the display will be a little brighter but still not nearly as bright as my other T.I. I noticed that my brighter watch has Engergizer 301's in it and I put in a pair of Maxell SR1130 SW in the dim-performing watch. Looking at a battery chart in rewolf's / xevious' other thread, it appears the 301's correspond with SR1142's and not the SR1130's as I originally thought.
Before I buy a pair of SR1142's, what's the difference between that and the SR130's? Is my solution really this simple?

[azimuth_pl]

the batteries are probably alkaline and silver oxide but there shouldn't be any difference. they only differ with their longevity.

[LEDluvr]

Hmmm so it's not the batteries. Any tips azimuth_pl (or anyone else) for getting a dim display to work brighter?
I was thinking of trying the "hold and cold treatment' mentioned in another thread but I'm not sure if it would be applicable to this problem?

[rewolf]

... I noticed that my brighter watch has Engergizer 301's in it and I put in a pair of Maxell SR1130 SW in the dim-performing watch. Looking at a battery chart in rewolf's / xevious' other thread, it appears the 301's correspond with SR1142's and not the SR1130's as I originally thought.
Before I buy a pair of SR1142's, what's the difference between that and the SR130's? Is my solution really this simple?

The only difference is size (and capacity). There should be no percievable difference with new batteries.
Energizer 301 are 4.2mm high, SR1130 only 3.0mm. SR1142 are 4.2mm (with the 3- and 4-digit SRs, the last 2 digits are simply height in 1/10mm).

In the long term, Silver Oxide is better for LEDs because they retain their 1.5V until the end. Alkalines decay from 1.5V down to 1.0V from the very first day, so the watch gets dimmer by time. The nominal capacity (mAh) isn't much different, but the voltage delivered.
Note that the Silver Oxides come in 2 flavours: low drain and high drain. High drain is obviously better for LEDs - it usually has "SW" ending, low drain "W" (Maxell code).

The Energizer 301 are low drain, corresponding high drain would be Energizer 386 or Maxell SR43W.
General rule for LED: take the biggest battery that fits, and use Silver Oxide high drain type. Unfortunately, these are also the most expensive...

[LEDluvr]

Thanks rewolf. The watch with the low drain Energizer 301's is definitely much brighter than the watch with the high drain Maxell SW's. So the batteries aren't the culprit in this case. (And I'll stick to high drain silver oxide in the future.)

I'm going through all the contacts (battery, pushers) and giving them a good scrape and cleaning. If that doesn't do it I guess I'll try the 'hot n' cold technique'to brighten up the LEDs unless anyone has a better idea?

[rewolf]

Maybe the LED have been degraded by years of exposure to UV radiation in a shop window, or high temperatures by the sun always shining on it.

[LEDluvr]

Thanks for the info rewolf.

[LEDluvr]

Thanks for the info rewolf.

[retroleds]

It could also be the quartz crystal....some of them start draining the batteries really quick, even though they are still keeping decent time. And clean the magnifiers over the LEDs with a little isopropyl(rubbing) alcohol) - that sticky crap they used on the module front seems to migrate across the plastic.

[LEDluvr]

Thanks Ed. I'd like to give the crystal swap a try, where can I buy one with the correct frequency? Flea Bay I imagine, any better sources? (I only need 1 at this point).

Cleaning the magnifying bubbles is good idea I hadn't thought about but his particular model doesn't have the magnifying bubbles. Which has brought me to another question...

My first TI of this model said 'model 401" on the case back. The 'back up' watch I recently bought, and this is the dim lighting one, has 'model 101' stamped on the back. The modules look identical to eachother and they do not have magnifying bubbles but they do have a colon between the hour and minutes and they do have a light sensor. I noticed most (if not all?) the other models don't have the colon or light sensor but do have magnifying bubbles. I'm guessing one of the case backs does not belong with the watch. Here is a pic - is this really a 101 or a 401 model?

[rewolf]

If the watch has a light sensor, that could be the culprit.
I don't quite believe that a defective crystal would cause a dim display - at least not with fresh batteries. As long as the watch keeps good time (and doesn't drain batteries fast) time I'd rather keep the crystal.

[Old Tom]

I would go with Rewolf on this- the photo sensor in the TI watches is probably a Cadmium Sulphide photocell which go low resistance in light (less than 1K ohm) and high resistance in dark (greater than 1M ohm). When low (viz in bright sunlight) the IC turns up the current to the display and it appears brighter (and conversely in the dark the display is run at lower current & thus dim). The normal fault condition for aged CaS photocells is to go high resistance (and thereby dim the display)- to test this bridge the photocell terminals with a 1K resistor which should restore the brightness if the photocell is the culprint. The other fault is for one end of the photocell to have broken off from the circuit board (a very high resistance in effect)- prod it to see if it makes any difference to the display. Replacing a defective photocell is difficult as parts are not available- usual repair is to solder a 1K SMD resistor in place of the cell and always run at full brightness (seen this in quite a few Pulsars).

[LEDluvr]

Thank for the tips Old Tom n' rewolf.
I'm new at this kind of repair and Tom you mentioned to "bridge the terminals of the photocell with a 1K resistor". Where can I buy this resistor, at a regular electronics shop? Should I ask for the store for a "1K electrical resistor" by name?

First I'll take a good look and see if the problem is an end of the photocell that broke off from the circuit board. That would be an easy fix. If I have to solder a 1K SMD resistor in place of the cell should I try to remove the cell or just solder the resistor on the back of the module and connect it to where the photocell goes? Having it run at full brightness is no problem for me - batteries be damned!