A cure for 28S death


This is my first post in the forum. Hopefully I stumbled on something that others can use.

Recently the batteries died in my 28S. The symptom was that the buffer filled with N characters.

After I replaced the batteries the problem continued. I tried removing the batteries and letting the circuits discharge for a few minutes before trying again. It worked, or at least the N characters stopped streaming into the buffer. The new problem was that half the screen was blank, the other half was black, and none of the keys worked. After reading stuff on the web I resigned to the fact that my 28S was finally dead after twenty-five years. I removed the batteries and set it aside.

Today, a week later, I picked it up and was about to throw it in the trash. Then I thought what the heck, I'll try it one more time. I put the batteries in and . . . it worked!

I don't know what's going on inside but it seems that the deep discharge over the course of a week unstuck a bungled bit.

Hope that helps. Nice forum you have here.



Sounds like the remedy I tried which cured my 29C.


As unfortunately has been demonstrated here, a problem resolved by battery removal is often a sign of a problem that will be seen again.

HP did not plan or wish for these devices to still be operating 25 to 40 years after HP made them. In fairness, no company would. The HP Clamshells are the worst. They were purposely designed to be non-repairable. They, with every calculator HP made between 1985 and 2003, were intended to be throw-away machines.


I have a little pet explaination for this: the magic repairing drawer. Stick stuff in the drawer and wait a few months, then it's fixed. Sometimes it actually works.


I wonder why deep discharge works. My theory is that over the decades there is some doping migration in the silicon junctions. That migration affects parasitic capacitances in the micro.

Just thinking out loud.


In my idea the magic drawer can be explained:
Inside the calculator a capacitor has been soldered in parallel to the battery.
This capacitor takes over the battery during battery exchange to retain the memory.
The voltage across this capacitor also prevents the calculator from a reset when (fresh) batteries are inserted.
A reset can be forced by discharging this capacitor.
Discharging can be achieved by properly shorten the battery connections or by given the capacitor some time (could by weeks!) to discharge by itself.

The 'death' behaviour of the calculator is propably caused by lockup of the CPU due to a temporary to low supply voltage.
Only a reset can release it from this state.


Thanks, Kees. That makes sense.

For the record my 28S locked up again. It filled the buffer with N characters and now won't respond to any keys.

Time to shop HP for a new model.



Please, do not throw it in the trash. At least not now. Chances are you are going to find another HP28S with a missing battery cover or with a bad LCD, I mean, leave it as it is for future part donation.

Even if it is not exactly for your own calculators, you see?


Luiz (Brazil)


You might have a mechanical problem with the keyboard inputting 'N' characters until the buffer fills up. Have you tried twisting the case?




I will try twisting the case. If that doesn't work then I guess I will part it out instead of throwing it away.

In the meantime I bought a new HP 35S. It should arrive this evening. It is more calculator than I need in my post-college life but I just love using HP and RPN.


Before it really die, take care to dump the rom, so you can resurrect it in go28s (see google-play) on an android device :)


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