I get remotes a lot with batteries probably left in them from the 90s. A lot of times the batteries will leak or rust. The leakage can be neutralized with vinegar and the rust can either removed by hand or electrolysis.
Unless the leakage is very light, it has usually seeped onto the board of the remote, so it is good to open it up to properly handle it. I find a guitar pick or credit card works well at opening these remotes.
Attached are photos of two recent remotes, with the first being a backlit remote and the second a newer CLD-D504+ remote (these are a little harder to open).
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Remote_1.jpg [ 222.05 KiB | Viewed 680 times ]
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Remote_2.jpg [ 304.81 KiB | Viewed 680 times ]