HP25 HOW CAN I TEST IT?

[Jorge]

I bought a HP25 at a flea market,the battery pack is missing the batteries and it has no adaptor or charger,how can I power it to see if it works.I'll apreciate your help. Thanks.

[Andrés C. Rodríguez (Argentina)]

All of the following is suggested without any warranty. A possible thing to do is to carefully replace the batteries on the battery pack (I assume you have a battery case, without the batteries) with standard alkaline AA batteries. Be cautious about the polarity ! To power it from an AC adapter, you will need a rather specific connector AND you need rechargeable batteries inside, to act both as a ripple filter and a voltage regulator. If you need other ways, one may be a REGULATED 2.5 Volt DC supply applied directly to the battery contacts on the calculator (check polarity carefully!) or apply a REGULATED 3.2 Volt DC supply on the AC adapter pins at the calculator back. In that last case, polarity should be not a concern, since there is a series diode inside. Last, you may remove the rubber feet, unscrew two screws and access the calculator interior, to power it from an outside source. Good luck, HP25s deserve to live forever!

[Andrés C. Rodríguez (Argentina)]

Please, see that if you use any external power supply, there will be acharging current at the battery terminals, so do not operate the calculator in that manner with regular batteries inside. The alkaline alternative may be safer and simpler that others, but in that case NEVER use an external supply. Alkalines will last more than the charge of the nicads, maybe on the range of 6 to 8 hours...