Re: Spares for vintage calculators HP-25, HP-27? Message #3 Posted by Andrés C. Rodríguez (Argentina) on 24 Feb 2012, 2:10 p.m., in response to message #1 by Samo
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.
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