Spares for vintage calculators HP-25, HP-27?


Hello there

I would like to complete my two vintage calulators HP-25 and HP-27 and I would need some spare parts, if available:
1. AC adapter/charger preferably with EU plug
2. Battery pack (even just the case/housng without NiCd's)
3. Mounting screws 2pcs
4. Rubber pads (to cover the screws)
5. Spare "STO/N.D." key button for HP-27
6. Original printed manuals

Could anybody give me any help or hint about how/where to get this items?

Thank you very much.


HI, Samo.

Chances are you are going to succeed finding what you want and contacting the right people by posting your message here, at the classified ads. The 'for sale' and 'wanted' messages in this site are usually post there. Here, at this forum, people usually share their experiences and discuss repair and usage.


Luiz (Brazil)

Edited: 24 Feb 2012, 7:24 a.m.


A respectful suggestion: NEVER use them with an AC adapter. Don't even connect them to an AC adapter. Run them from rechargeable AA batteries instead; and recharge the batteries outside the calculator.

Sometime ago, I used to run my HP-25 from alkaline AAs, as the greater voltage caused no harm on my units; but nowadays I'm more cautious and would play it on the safe side using NiMH AA rechargeables. Again, don't recharge the batteries in the calculator (BTW, the internal charger was designed for NiCds from the 1970's, current batteries are not the same).

Using the AC adapter puts your calculator at the verge of unrecoverable risk. If the battery contacts fail, even for a split second, the greater voltage from the AC adapter will kill the chips inside, and there are no spares available. Batteries act as sort of regulator, absorbing the voltage excess, but that is the only protection.


Just read this archive about charging a HP-25 safely with a simple mod :

Captain Zener


The hp25 zener mod is excellent. I was wondering why no one had done something like that yet.

Since the original hp charging scheme was for NiCads, has anyone made a mod. to properly charge a pair of NiMH batteries in the unit, so the battery pack does not have to be disassembled for re-charging? (Does anyone make an aftermarket HP25 battery? Or, is it always a matter of putting new batteries in the original plastic shell?)

Is it practical to reuse the original charging cord with a modern charger e.g. something like a DuraCell minicharger? (I have not looked at the minicharger, no idea what the circuit is, or if it charges the two batteries individually or in series?) Or, is it practical to build a small modern (NiMH?) charger circuit into the original charger case?

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