Sometime in high school in 1979 or 1980, I bought my beloved HP41CV. The first programmable calculator with alphanumeric display and, of course, reverse Polish notation. It’s got Wikipedia entries and has become a collection item, fetching multiple $100s on eBay.
Since leaving Belgium in 1989, I faithfully moved the calculator with me but forgot to take out the batteries. Even when I put new batteries in, the display did not come to life. Then I resorted to the place with all answers: Google and YouTube. And I found this great YouTube video titled “HP 41CV calculator repair of bad battery contacts.”
My HP41CV had exactly the same problem: two of the four battery contacts had been eroded and were down to black plastic. (Clearly a design flaw of under designing the copper!) Anyway, I figured a good project for Christmas Eve morning: I dusted off my soldering iron, cut 10cm of speaker wire and disassembled it into tiny copper wires, and soldered them on top of the bad battery contacts. Et Voila!
(I should admit that the soldering didn’t really work that easily; it took me 3 times with some Flemish cursing before it finally worked.)