One problem that some of these old computer have is the limited or no availability of the original spec batteries for them.
The Avocet 20 I still have on one of my bikes still works, but the original mercury based battery is not made anymore. There is an equivalent that fits that you can get from places like Radio Shack, but I noticed that those replacements do not last nowhere near as long as the old ones did. Lucky if they even last a whole season.