Battery pack for an HP-35


Hello all,

believe me or not, but I was given these past days two old HPs for free.

One is an HP-25, serial number 1803B... in very good cosmetic condition, as well as the leather-like pouch, with very few and minor scratches. It seems to have been used very few, as the keys still have the famous "rotate-and-click" behavior. Unfortunately, it has been left for years with the battery pack inside, and I found a huge leakage when openning it.
A good cleaning and fresh AA batteries did not bring it back to life. At least I now have a very nice paper weight...

The other I received just today is an HP-35, serial number 1302S..., in very good cosmetic condition too. Like the first one, it seems to have been unused the major part of its life, which is rather surprinsing when one knows the price of this calculator in '73!
Since there is no battery pack in it and the battery terminals don't show major evidences of battery leakage, I keep a tiny hope it's still functional.

My question is the following: since there's no marking in the battery compartment nor on the battery cover showing the polarities of the battery terminals, could someone tell me what they are, so that I could test the calc? Same for the 3 plugs on the back side, near the battery door.
In case of use, mine is like the third one starting from the left on this photo.

Any help much appreciated.

Kind regards.


Edited: 9 Mar 2010, 4:33 p.m.


Here ya go:



if that link didn't work on your computer any better than mine, try:

there's also another guy selling reproductions of the plastic frame and tabs in kit form on ebay.


Here is a link to an old posting RE: HP 45 restoration. Scroll down to the 9th photo for the labelling of the prongs.

HP 45 posting

Cheers, Geoff


On the HP 25, I'm assuming that you have put the new AA's into the original battery case. If so, be sure you have good contact at the closed end of the case, where the cells are (supposed to be) jumpered together (the original cells had a soldered on strip connecting them at this end). Without that strip, that spring in the end of the case is the sole connection. Perhaps it's corroded through.

Best regards, Hal


Hal suggestion is excellent, while there, sometimes the round soldered battery contacts of the logic pca are corroded.

I have a 29C sold as broken which appeared excellent. Never powered up and I could not find any corrosion damage. It turned out to be the negative battery contact on the logic PCA. This was a cold solder and the connection although appearing perfect was broken at the solder.

After remelting both of the logic pca contacts (they look like rivets) on the logic pca the calculator works and I haven't looked back since.


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