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My HP-25C Died!!! <snif> - Printable Version

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My HP-25C Died!!! <snif> - Ian Primus - 12-14-2001

I was using my beloved HP-25C during my math class, and as I reached for the multiplication key during some long equation crunching, the display died. My first instinct was to replace the batteries. (I had temporarily soldered a AAA battery clip in there until I get a rechargeable pack, I have one on it's way.) So when I got home, I removed my electrical tape from the back (holding in the batteries) and replaced the AAA's with two fresh ones from the battery charger (I was using NiMH, they last longer), and when I turned the calc back on, it didn't do anything. I got more investigative today and took the calc apart, and removed my battery clip mod, and hooked up some AA's via aligator clips to the correct terminals. Still nothing. I went through and checked a couple diodes and resistors that had crummy solder joints, to no avail. I don't know much about HP calculator repair, but I am determined to get this working again. Since this calc was most likely sitting unused for several years before I got it (about a week ago), is there any chance that something may have dried out in there that was startled by my sudden use of it, or something that could have jiggled loose (I carried it in my pocket to prevent it from getting damaged in my backpack) Any ideas as to what could prevent the calc from turning on would be appreciated. Also, while I know this is not the place, and I will place an ad in the classifieds when I find out what part I need to replace, I may be in need of another broken calculator for parts. Any help would be appreciated. BTW - when my HP died, I went into RPN withdrawl, having to go back to my TI-83+, and I found the most wonderful peice of TI-83+ software on the net - http://www.chem.com.au/cp/software/ti83/rpn/ Reverse Polish Notation for the TI-83+! I love it! This site even has a link here for more info on RPN. Long live the HP (I hope...)

Ian Primus

Re: My HP-25C Died!!! <snif> - Andreas Stockburger (Germany) - 12-15-2001


sorry, but it could be possible that the 3.x Volts of the two AA-Batteries was to high for the electronics. It happend to my first woodstock also :-(

sorry no better answer.

Andreas Stockburger

Re: My HP-25C Died!!! <snif> - Ian Primus - 12-15-2001

I thought of that before I hooked up the batteries in the first place. I was using NiMH batteries, which are almost identical to the old Nicad cells in that the put out only 1.2 volts per cell. 1.2 + 1.2 = 2.4, and the calc was meant to run on 2.5 volts.

Ian Primus

Re: My HP-25C Died!!! <snif> - Frank - 12-15-2001

I repaired a 29c with similar symptoms a year or so ago by replacing a visibly slightly browned/cooked to high value resistor with a new one somewhere on the board that prevented some display voltage from getting there.

Re: My HP-25C Died!!! <snif> - Tony Duell - 12-16-2001

First of all, take the 25C apart. In case you've not done this yet, first remove the battery. Then take out the 2 feet at the display end of the machine and undo the screws under them.
Press on the charger pins to free that end of the case, then slide the (white) bottom case away from the display end of the machine to free the hook at the front edge. Separate that end of the case and take the case off.
First check all the contacts between the keyboard and the logic PCB. The Woodstocks feed Vss over those pins IIRC and bad connections will stop the machine working.
Then you should check the power supply voltages. Find the ACT chip, which is a 22 pin DIL device, normally at the front (non-display) end of the board. Pin 12 is ground (as is the -ve side of the battery, pin 1 is Vss (+6.2V), pin 2 is Vgg (-12V). Check those voltages with a known-good battery connected. If you have a 'scope then check them for ripple as well. If the voltages are right, then post again here and I'll suggest some other signals to check.