Battery terminals on the HP-35?


"Classic" afficionados --

I have temporary custody of an heirloom HP-35 from 1971 that is not for sale (no, it's not a "red dot" moedel). It has no battery pack or charger, but I am aware that it uses 3x 1.2V cells in series. I also have a copy of an article regarding the fabrication of battery packs for these units.

Looking at the opened battery compartment from the underside of the calc, which terminal is "+"? From a diagram in the article, it seems that the terminal nearer the informational backplate is the "+" terminal. (This would be consistent with the Spice-series units.)



So i don't know wich way it is inserted into the Calculator. But i hop this will help you:
the Contacts are not centered. If i place it contacts up on the desk and the contacts are more to the left, then its the top one wich is + .

regards, Harry


figured out that you are right. It is the contact closer to the lable (and to the centre of the calculator) wich is connected to +

Regards, Harry


If you have an ohm meter, you can check which battery contact is ground (negative). It is connected to the middle of the three pins in the charger plug.


Harry and David --

Thanks for the help. In fact, Figure 1 of the (still-available) article I mentioned Making New Battery Packs for the HP-35 Calculator shows the connections between the calculator terminals for the battery, adaptor plug, and the internal circuitry.

It seems as though either the calculator power switch is bad, or there is a break in the circuitry.
All the resistance tests are normal, except that I get "open circuit" between the battery terminals and between the center- and right adaptor terminals with the power switch closed.

It also seems that six screws must be removed in order to take the back off; the middle two are under the thick foil on which the instructions are printed. I would rather not remove the foil, without a replacement available.

Best regards,

-- Karl



Does your HP35 not turn on, even with good batteries, as indicated by your comment "It seems as though either the calculator power switch is bad ... "?

I have an old HP35 (from 1973) and its power switch barely works - I often have to slide it back and forth several times to get the machine turned on reliably. You might want to spray some contact cleaner down into the switch (using some precaution to not overspray everything else!). Others here may have alternate suggestions for a bad switch.


You can check a lot of the internal wiring with an ohmeter.

First remove the battery pack and switch the unit on. Next measure across the outer two posts in the charger port. They should show zero ohms becuase they are connected together by a strap when the charger is unplugged... one pin is connected to the battery and the other connects to internal circuitry. If the strap is not making contact the machine will not run on a battery, but should run on a known good charger.

If the machine runs on the charger and the strap is making good contact but will not run on a known good battery then you probably have (often invisible) corrosion on the battery contacts. The battery pack makes contact to the terminals along the SIDES of the tips of the gold contacts. Clean well with very fine sandpaper.

Next measure between the center charger port pin and the negative battery contact. This should always show a connection.

Next measure between one of the outer charger port pins and the positive battery contact. This should show a connection. If not then the power switch probably needs cleaning.

Classic series chargers are notorious for bad connections in their plug. Also well known for a bad main electrolytic filter capacitor. Either problem can cause the machine to not light up.

If you need to open the machine to clean the power switch you can do it without removing the label. You need to peel up the top two corners of the label about 1/2 inch to access the screws. I wet a razor blade with the temporary adhesive releaser called UnDu and slide it under the corners. Be careful not to slice the label or sharply fold it up. Often the label corners will stick back down. Sometimes you need to glue them to keep them down. I never reinstall the screws under the label so that future repairs do not risk further damage to the label.


David and Dave --

Thanks much for the "tech tips"! I'll try the contact cleaner, but will open the back only with permission from the owner.

BTW, Dave -- I attended UNR part-time during several terms in 1993-94.

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