How to safely clean battery corrosion from LED modules?

[Wade]

I recently acquired three NOS National Semiconductor watches from eBay, including the boxes they came in with original instructions. However, the batteries were also original NOS and at least two of the three modules have much caked on corrosion and salts as a result of the leaky 30+ year old batteries left installed. I can't determine the conditions of the third module yet because I'm having a hell of a time removing the screw-on case back. But I assume it also has extensive corrosion as well.

In one of the watches, the gases from the old batteries must have blown off the snap-on case back. In another watch with the screw-on case back (the one I can't open yet), the gases presumably blew off the red crystal from the case as it was rattling around in the box when I received it.

I'm going to remove the boards from these modules and clean them, but what is the best method to go about this? Any chemical cleaner suggestions?

I think I have a shot of getting at least one of these modules to work, the one marked "WM01N" on the sticker. The PCB for this one appears to be made of ceramic, and all the wire bonds, switches and quartz crystal appear to be protected with what appears to be a silicone sealant.

Much advice is appreciated and thanks in advance!

[Wade]

I forgot to post a pic!

http://img683.imageshack.us/img683/8783 ... 00003u.jpg

[Wade]

Here's another one showing the corrosion.

The silver toned "Exelar" watch came with a black leather band, but is dry and brittle almost beyond use. The gold toned watches came with awful brown bands made of leather grained rubber, but at least usable.

http://img41.imageshack.us/img41/7095/200912300005.jpg

[retroleds]

I'm going to remove the boards from these modules and clean them, but what is the best method to go about this? Any chemical cleaner suggestions?

"CLR"(Calcium/Lime/Rust) or "Lime-Away" on a Q-Tip work very well. Keep using a fresh Q-Tip and don't dip a used one back in the fluid(there will be lots of dissolved metals in it. Finish with a clean Q-Tip with a little clean water and let dry. Most common failure on your National Semi's is the quartz osscilator...apparently hundreds were shipped with fairly bogus crystals. I'd guesstimate 7 out of 10 just need a new crystal.

[Wade]

Thanks for the tips and I'll pick up some "CLR" today. If the quartz oscillators are bad, I should be able to solder in some new ones as I learned how to do this from you. Will follow up on my results.

Have a Happy New Year!

[Wade]

So far I got one watch to work. The corrosion from the old batteries completely ate up one of the battery pads and one of the switches. I looked around these forums and someone here suggested using brass shim stock for a new battery pad. So I did use .002" brass shim to make a new battery pad and switch and soldered them in. It wasn't easy, but the results were rewarding. This particular watch only displays the time and seconds (unless I'm doing something wrong). Also the LED display is made up of tiny dots - cool!

http://img22.imageshack.us/img22/1843/exelarled1.jpg

I started working on the second watch, but when I used the CLR it cleaned up the corrosion and a lot of the traces on the circuit board. I thought about using the same .002" brass shim to make new circuit traces, but is there an easier way?

[bruce wegmann]

You could use conductive silver expoy to "paint" on the circuit board over the missing sections of the traces, so long as it is clear where the original traces were. This is the sort of thing that is best accomplished under 20X or so of magnification, so, unless you have a good microscope, or a REAL steady hand, this might not work for you.

[Old Tom]

An easier alternative to Silver epoxy is SIlver PCB trace repair paint (I use Ecolit 430). This is a suspension of Silver in a conductive lacquer and applies just like any other paint. Things to remember are to clean the PCB as best you can, thoroughly shake the paint before use to disperse the Silver (it tends to clump when left) and work in a cool room to slow the drying time (very thin traces can dry very quickly before you stroke them out to good finish). These paints can carry more than enough current for a LED watch even in a single brushed on layer.

[retroleds]

3rd option: since the NS boards are very simple on the battery contact side(3-5 traces depending on model), you can consider simple point "A" to point "b" type connections using very thin wire, soldering or silver epoxying at the connection points. Then paint your wire(s) with nail polish for insulating purposes and you are off and running! I'd agree with Old Tom on using PCB trace paint/ink over silver epoxy - a pen style applicator works pretty good. The epoxy starts to "curd" and not stick very good within a few minutes of mixing so generally works better for single point of contact type repairs. Just my $0.02

[bruce wegmann]

Totally forgot about the conductive paint...definitely the better way to go. Nothing beats the silver epoxy for re-establishing connection points, though.

[Wade]

Update on my little project.... I did manage to get got all three watches to work! When I'm completely done I'll take photos. One of the watches still has a sticking set button. On another, the red crystal had popped off and I don't know yet how to install it back on the case.

Scanning these forums here have helped a lot. I used one of those sticky balls (and running under hot water) to finally unscrew the case back off one of the watches. The conductive silver paint also worked to paint back the traces on one of the modules! It's not cheap ($22 for a .5 oz bottle), but I suppose that's a lifetime supply for me. At first I was really worried as I painted one of the traces incorrectly. I had to start over and redo it, but the watch still didn't work (it would show a constant "0"). But then in another forum I saw a suggestion or replacing the trimmer (the old one was covered with corrosion) with 22pf capacitor. I did that and luckily the module works! I eventually may replace the capacitor with another salvaged trimmer, but if the watch is anywhere near accurate I may just leave it in. I also had to replace 2 of the 3 quartz crystals. My work probably would appear sloppily done to you guys, but the parts and circuits paths in these modules are small!

One thing I really learned is the "New Old Stock" definitely does not mean a mint new working watch.

[james_stan]

Sounds like you had some fun - encouraging to hear your success too.

Regards

James