Posts: 202
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Joined: Jul 2009
Ouch. I'd think the best that you can hope for is that the excessive voltage burned up one of the traces that goes from the batteries to the CPU. I'd try checking for continuity between each battery terminal and whatever pin they go to. Beyond that, you could try looking for damaged passive components (caps, resistors, etc.) but in all likelihood that much of an overvoltage probably killed some ICs.
Posts: 429
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Joined: Jul 2011
Recently I also got an HP 28S off of ebay and the seller had unknowingly included three 12V batteries from his garage door opener which really look surprisingly similar to the correct 1.5 V batteries. Not checking I inserted them right after opening the box and the calculator emitted some very strange sounds but doing nothing else. I guess I tried and tried for at least two minutes before even suspecting something wrong with the voltage. After inserting some of the spare 1.5 V cells I always keep for my 41's the 28S works flawlessly!
I hope yours will do, too, after only some minor surgery!!!
P.S. Have you checked the battery door to be correctly closed? That thing is a bit wobbly and might cause connection problems.