Time to retire the HP 28S

Looks like it’s time to retire my trusty old HP 28S calculator that I’ve had since my second year of undergrad.

Popped a fresh set of batteries into the calculator (after it had been sitting idle and battery-less for a while). Calculator powered up just fine, but the Enter row and G-L row of keys weren’t working. Twisting the body of the calculator a bit will make the Enter key work, but not reliably. Also makes using the calculator kind of awkward.

While searching the webs for documentation on how to disassemble an HP 28S, I came across this Museum of HP Calculators forum post about disassembling one that was having problems with keys not working.

Definitely not a trivial process.

I found that by pushing down on the keypad near the back arrow button, all of the keys that weren’t working would work again.

It seems like the whole calculator is held together by an array of posts that are “welded” to the upper and lower halves of the case. When some of those posts break, the connection between the keyboard and the rest of the calculator gets flaky because there’s less pressure holding everything together.

From one of the posts on the forum thread:

heat stakes that had sheared off, primarily from the case back, not the mushroomed heads. I suspect it is the physical force from opening the case that stresses the heat stakes in the latch area that causes them to fail. Once that happens, there is a zipper effect to the left that eventually leads to failure of the smaller diameter stakes under the display that causes the classic loss of keyboard to logic board contact.

The forum thread also mentions replacing foam that helps press the keyboard contacts against the calculator’s logic board. After almost 30 years, I’m sure that’s also part of the problem with this calculator too.

Probably not worth the effort to try and fix this calculator. I’ll keep it around as a museum piece and for sentimental reasons.

Fortunately the other one I bought a few years ago still works.

2 Replies to “Time to retire the HP 28S”

  1. I got my HP-15C in 1984 or 1985. A LCD segment died a week from the warranty running out, so HP swapped the unit. I’ve been using it ever since (but certainly not taxing its capabilities like when I was in school).

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