Re: HP 29C Over voltage? Repairs? Some more ideas Message #5 Posted by Andrés C. Rodríguez on 12 Jan 2000, 8:58 p.m., in response to message #4 by John
I am just guessing now, but I don't think that reverse polarity from batteries will be enough to fry the calculator... On the other hand, I don't think either that there are any reverse-polarity protection, since it is not possible to happen with the HP original battery pack. I am sorry I do not have my old HP25 anymore, so I am unable to open it and take a look to the circuit. If there *is* something burnt, let's hope it's a bipolar transistor or diode (discrete components) near the switching power supply, that may be fixed... But, again, you will need some extra help from other people, or looking at another machine. Now, I'm just thinking that a significant difference between my 25 and the 25C or 29C models is that the batteries are supposed to keep a quiescent current flowing to the continuous memory chips (to preserve contents when the calculator is off). So the chips will be receiving the battery voltage when the power supply circuitry is off. However (again) I don't believe that the difference between 2.5 V and 3 V will fry the chips, but reverse polarity may do the harm there. I hope someone will be of more help than I.
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