These older HP’s have a cult like following.Well it's not really obsolete, but as others have mentioned, my HP-12C. I bought that calculator in college. I was in a Corporate Finance class and was using a Texas Instruments calculator and it took forever to arrive at the answer. Some chick had a 12C and was getting the answer in seconds, that was all to took for me to order one.
Theres even a guy who can repair the lcd screens running an online store.
I just sold my 11c complete with the faux leather slip case,manual and box for more than new. Still had the receipt from the university book store in the box. It was mandatory to buy one for the engineering school classes.
Most of what’s posted here is not obsolete, just old, maybe a bit nostalgic.
Now if someone posts they are using a whale oil lamp, that would be a better example.

