Just bought a HP-67 calculator at an auction

[rprosperi]

Yeah, OK, "all" was too harsh... There's no question that many fewer TI-59s had the roller failure than HP's (though I've seen a TI gummy wheel repair kit, so not all), suggesting perhaps TI used more than 1 material for them. My typical luck was a unit that was bad. I also recall more than a few fellow students in the late 70's having card readers that failed in their 59s while still only 1-2 years old, though clearly not from the same type of failure.

As there seem to be some original units still working, environmental conditions over extended periods sure seem to be a likely suspect.

I'll read the article SlideRule posted, maybe there's some hints.

For the units you guys assert are still working and original, how do you know they're original? Have you owned them all along?