HP Forum Archive 20

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HP 28S saw 36V versus 4.5V, damage and repair? Message #1 Posted by Joshua Keena on 10 Sept 2011, 10:10 a.m. I have an HP 28S that due to a battery mixup (human error) saw 36V versus 4.5V. Any insight into damage done and possible repair recommendations? The calculator is now non-functional. I was able to non-destructively take the case apart to isolate the CPU (logic) board.

Re: HP 28S saw 36V versus 4.5V, damage and repair? Message #2 Posted by Lincoln R. on 10 Sept 2011, 11:50 a.m., in response to message #1 by Joshua Keena 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.

Re: HP 28S saw 36V versus 4.5V, damage and repair? Message #3 Posted by Joshua Keena on 10 Sept 2011, 1:15 p.m., in response to message #2 by Lincoln R. Thanks, I'll try that.

Re: HP 28S saw 36V versus 4.5V, damage and repair? Message #4 Posted by Alexander Oestert on 10 Sept 2011, 2:01 p.m., in response to message #1 by Joshua Keena 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.

Re: HP 28S saw 36V versus 4.5V, damage and repair? Message #5 Posted by Joshua Keena on 10 Sept 2011, 10:58 p.m., in response to message #4 by Alexander Oestert Well that is a terrific outcome. I am very glad yours suffered no apparent damage, and your story gives me some hope. I'm actually without a battery door. I was working on fabricating a replacement when the unfortunate incident occurred. I sincerely appreciate you sharing, and here's hoping mine fares as well as yours.

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