HP25 problem at revival



After many (many) years I decided to put my HP25 (bought 1/12/1975) at work again.

I left it because at the time I was told that it was not possible to find new battery pack.

I only now discovered the MoHPC following the advice of someone.

Now I replaced the batteries by new NiMH batteries.

After charging when running the display shows a full line of 0 and nothing modify this.

Any hope to solve the problem?



I'm sure others will know better and I don't own one of these fine cals but here is a reasonable guess: After so many (many;) years capacitors tend to go bad. In my experience, this can cause a lot of nasty failures. If no one else will come up with a better idea, replacing all capacitors is worth trying.



Most caps in HP handhelds are tantalums... it's rare for them to fail.

The problem is bad ACT chip, it was damaged by the charger being plugged in with a bad battery or no battery in place. The battery is the load for the charging circuit, if the battery is open and presents no load, the voltage inside the calculator rises to 10 volts and damages the integrated circuits.

The only source for the chip is from another 21,22,25,25C,29C or 67. The 27 is not compatible. The ACT is the 22 pin DIP at the bottom of the processor board.

Edited: 19 Feb 2006, 9:53 a.m.


Thanks for the information.

So there is no solution, unless I change the circuit.

Now the problem will be to find it.


Alain Delmotte


To find one, setup a saved search on ebay (look for a cheap woodstock - like a 21). The search will email you when it finds one. Bid, win, pay, receive, unpack, warm up the soldering iron...

Of couse, you could just get another 25 or a 25C.

Regarding batteries on woodstocks. I NEVER recharge batts in the machines. Use the external charger unit, or make one, or better yet remove the central plastic bar in the batt pack so it can accept AA rechargeables, and when they're low remove them and recharge outside the calculator.

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