HP-25 displays "Error" when trying to input numbers

[Harald]

After several years of continuous use my HP 25 seems to have died today. I've used it on a daily basis and since it was connected to the power cord I very rarely turned it off in the last couple of years. But today it had a surprise in store for me. Everything seems to work fine. The stack works fine, I can add, multiply, divide,subtract and probably lots more. The only problem is, entering numbers is quite tricky :) As soon as I press any of the number keys it displays "Error". The shifted functions on those keys do the same. But I can input numbers by pressing "Sigma +" for example. And after entering them most calculations appear to work fine to.
When I switch to programing mode, it looks like there is something in the program memory that doesn't belong there.

I have tried swapping the keyboard, but that didn't change anything.
Any ideas on what happened here?

Regards,
Harald

[Michael de Estrada]

The Woodstock series calculators have very fragile charging systems, and can be seriously damaged if operated with the AC charger without a good battery installed. If the batteries were worn out or not making good electrical contact with the calculator terminals, ICs can be damaged or ruined. In any event, the AC charger should only be used to charge batteries, preferably with the calculator turned off. Frankly, I'm surprised your calc has lasted this long turned on continuously with the AC adapter connected to it.

[Harald]

Well, I never connected it to the charger without a battery installed. Also I disconnected the calculator from the cord and carried it around when I needed to, and therefore replaced the battery every now and then when it was getting weak. I have tested the battery in my 29C now and it lasted for two hours. I am well aware of the woodstocks not being able to operate without a battery and I don't think in my case this is the problem. Nevertheless it most likely is on of the ICs that has been damaged, but the question is which one and why?
Thanks for your answer!