Hi there, I've noticed that mostly analogue watches are discussed on this forum so hopefully posting about a digital watch is okay.
I bought this M929 from a thrift store for $3 the other day in a non-working condition. After opening it for the first time, it was evident that a battery had leaked at some point - there was quite a bit of corrosive material everywhere. After giving it quite the clean using vinegar for the batteries crap and isopropyl alcohol for everything else, the backlight was again functioning, however, the LCD was not.
There appears to be some sort of potentiometer on the back of the main module, although this looks exactly the same as the screws that held in a back metal plate so I stupidly rotated it for a while before I realised it wasn't a screw. I've roughly rotated it back using a reference photo I took before I dismantled it (below) but the best I can get is a very dim set of numbers that flash and are only visible if directly under a light and viewed from an extreme angle - and even then, it does this randomly. I'm not sure if the pot is even for the LCD, though. I've also checked all the other components on the board with a multimeter and there seems to be continuity at least, although measuring the pot leads to no usable number of ohms.
Here's a bunch of photos I took. Hopefully, they can be helpful. Any help with the repair will be greatly appreciated.
Here is what the watch looked like on first opening. You can see green/blue muck everywhere.
Here you can see the extent of the corrosion on the buttons. This was even holding some of them in place. Vinegar made short work of it, however.
A close up of the main module before disassembly.
The flipside of the module. Corrosion was bad here too
More disassembly and after cleaning
Otherside. Note the pot at the bottom
LCD, reflector and contact transfer things
Close up of the LCD
The plastic casing cleaned. There is still a bit of corrosion on those contacts but that is as much as I could remove.
Reassembled after cleaning. Accidentally added some scratches from the screwdriver slipping, though. Whoops.
The corrosion had actually eaten into the watches metal. Luckily the buttons were fine after being cleaned.