Casio released the CA-503  Digital Invader Game calculator watch in around, I’m guessing, the mid to late 1980s – a number of years after they made the first watches with the same game in the  module.
Those watches – the CA-85/851, CA-86, CA-90/901 were the wrist version of the game Casio had popularised in their desktop calculators – MG-660, MG-770, MG-880, MG-890 around the same time.
So to re-release it, alongside the new range of calculator watches with two case buttons (one recessed) was testament to its popularity and longevity. If you’re in any doubt as to whether it was a popular game, you only need to google Casio digi-invaders and you’ll see versions made for phones and desktop computers by fans.
The CA-503 has the same case as the CA-501, CA-502 and CA-505 (and the 10-digit CA-601 and CA-602) but is so much better because…IT HAS THE GAME!!
This particular one was mint – NOS complete with the tag (no box or instructions but I can live without that). The only downside was the keypad, which had melted.
Watchmakers, like any manufacturer, have their hits and misses and Casio had their fair share of both (fortunately, mostly hits). Among the misses were the annoying and irreparable blue spot that is part of nearly every DB-1000 (the only thing that mars an otherwise amazing watch), the bleeding LCD that seems to affect a large number of their dual-layer  module watches, and melting keypads.
The most well-known of the melting keypads is on the DBX-100, a rare and quite outstanding watch that relies on its keypad for a good number of its functions. The melting issue unfortunately affects 100% of these, although I live in hope of one day seeing one that survived the rot. You’d think Casio would have learned from this in future models and it seemed they had until the CA-50x series was released.
I don’t know if they used the same rubber compounds as the DBX-100 but the result is the same. A gooey, unusable keypad that renders the watch virtually useless unless you can find a replacement.
Side note: Casio wasn’t the only watch manufacturer that had quality problems. I’d estimate around 60%-70% of Citizen digitals that you buy non-working these days can’t be resurrected, and don’t get me started on the Seiko G757!
The saving grace for the CA-50x range comes in the form of the stalwart CA-53W – a watch that, I believe, is still being made today. It’s keypad can be used to resurrect a CA-50x as long as you’re careful, and that’s what I did for this one.
Here’s the watch as it arrived (sans bracelet) – you can’t really tell the keypad is melted from the photos, but the black stuff next to it is where I turned it upside down to check the caseback.
Next, with the faceplate removed (which fortunately came off easily as the double-sided tape had degraded over time as well
Now the cleaning begins. You’ll need something reasonably blunt to scrape with, and lots of alcohol (the cleaning stuff, not the drinking stuff, although the latter could help)
Here’s the CA-53W keypad on the CA-503 faceplate – a great fit!
Still cleaning – this takes a LOOOOOOONG time
All done – parts clean and ready for reassembly. Note the two bits at the top left, they should be joined together. Time, old tape and the alcohol had caused them to separate.
The case had to be cleaned inside and out as the gooey rubber had found its way into every nook and cranny.
To make the donor keypad fit perfectly, you have to trim a tiny amount off the bottom. Fit is everything if you want it to work. Even still there is a lot of fiddling involved to get it aligned properly.
The conductive parts refitted – these I had to take from the same donor CA-53W because the two pieces had become separated and if there is not a tight contact between them, your keypad will not work. Ever.
Some double-sided tape (in tiny strips to reattach the faceplate, crystal and case together) and we’re done.
It’s a good couple of hours work to do this properly, testing everything works (and disassembling if things aren’t lined up properly) but the end result is well worth it!