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I have an HP7 that I acquired without a battery pack. I would like to test it to see if it actually works before investing in a new battery pack. Could someone tell me which battery terminal is plus and which is minus relative to the display end of the calculator?

I am assuming that three NiCAD batteries would provide about 3.6V, so I will try powering it at that voltage from a regulated bench supply.

I have a charger with it but it seems to be inoperative. When powered, I can't see DC between any of the three terminals. Is there some sort of an interlock that holds the supply in standby until it is plugged into the calculator ? After reading several warning posts I am reluctant to power the calculator without a battery pack installed.



From this very museum:


And from the battery POV :

From the plug POV:

Output 3.6-4.7 V DC @ 150-190MA calculator, and 4.2 V DC @ 50-60 MA battery charger. (Exact specs vary slightly with charger model but chargers are interchangeable.) A complete recharge takes 14 hours with the calculator on or off. The batteries are low when extra decimal points are displayed.

| o - o |
| + o |

Looking at the plug, the center terminal is marked negative. The left terminal is marked + and charges the batteries and the right unmarked terminal (also positive) supplies operating power.



Edited: 30 Apr 2012, 1:28 p.m.

Place the calc upside down on the table with the top facing away from you and the bottom nearest to you. This places the card reader away from you and the instruction label nearest you.

the top prong is -ve (furthest away from you)
the bottom prong is +ve (nearest to you)

I use a battery pack from radio shack that takes 3 nicad AA batteries. This rules out any bench supply mistakes and clip the leads to the appropriate terminals.

Cheers, Geoff

check out this old museum post. the 45 has the same battery set up as the 67.

hp 45 set up

You should be able to test the voltage on your charger when plugged into the wall socket. If you have no voltage on both sides, there is a good chance that the fuse has blown. Check for continuity across the fuse to confirm and then replace it or solder a small wire across it.

This from the archives explains how the charger for the HP 67 works.


Wow, what a great group, thanks everyone for your kind help.
I'll check it out tonight.

It's possible that there's a break inside one of the wires near the plug that fits into the calculator. I've had that happen on a charger and found the problem by sticking a pin into each side of the wire back from the plug in order to make a contact while testing for DC.