HP 34C displays random chars



I have an old HP-34C that has no more batteries, but it was working fine a couple of years ago with the transformer only. Now I'm trying to make it work and I have only random characters displayed on the screen. Any clues? I check for AC contacts, internal switchs contacts, and now what? Is that the logic that is broken? Or maybe too much humidity?

I would like to sell it to a collector. Is there people interested to buy it for hid/her own collection?

Thanks, and sorry for those too many questions.
Please reply to email address too.


Hi Patrice,
When you say you have tested the internal switches' contacts what do you mean? I assume you have not disassembled it. My guess would be there are internal contacts that need to be cleaned which would make opening it up necessary. Before you go that far though you can try putting an external 3Vdc supply on the battery contacts and see if you get normal operation. It is possible to use standard AA batteries but there is a risk of breaking the battery contacts if they are too long and put too much pressure on the contacts. Also keep in mind that this model is rather difficult to open and some or all of the internal components, inlcluding the chips, may be held in contact with the rest of the circuit with pressure - no soldering - so I don't know how difficult it would be to re-assemble. I've never done it myself so I have no direct experience. I'm sure someone else here can give you more practical advice. As far as its value, IMO it is valuable. It's sophisticated and refined. It went about as far as that technology could go to provide a reliable, useful and transparent set of features. It was the last LED designed by HP and benefited from nearly a decade of momentum in LED calculator design. It gracefully bridges the decade of the 70's with the decade of the 80's in the quest for the perfect math tool. Good luck getting it working again.


I'm glad to see something nice said about a Spice model, especially the 34C. I think of it as a 15C without complex numbers. And couldn't we all live with a little less complexity?!!!

I hope running without the battery isn't the cause of the anomaly - especially a permanent cause.



using it without the bats installed may be the problem.
AFAIK, the batteries are used as a 'buffer' to power peaks.
Another possibility would be simply corroded or loose contacts.

Somewhere on this site are pages which tell you to open, clean, and repair a series 30 calc.




the random display of characters, i.e. 'characters run across the display erratically' can be caused by a power supply with too much 'hum' or poor (no) filtering. this kind of power supply can cause the calculator to switch on and off with 120 or 100 instants per second (ac wall outlet frequency 60 or 50 Hz respectively).
best countermeasure is to reinstall the accumulators, even if you want to use the calc with the power supply.

cheers, hans


I don't want to worry you, but running a HP3xC or HP2xC calculator off the AC adapter with
no (good) battery pack installed is th surest way I know to ruin the memory chips. Without the battery pack, the output of the adapter is around 12V, and when the calculator is turned off, the backup circuitry (to supply a little power to the RAM to maintain its contents) then
supplies 12V or so to the RAM chip (designed to work at less than half that voltage). It tends to die, possibly killing other chips with it.


All the Spice and Woodstock calculators need GOOD batteries installed to function properly. They are very easily damaged (especially the "C" type Woodstock machines) if they are run without GOOD batteries insatlled and making GOOD contact with the power terminals.


Patrice -- assuming that you still have the display problems after correcting your power supply, the problem may be similar to one I had with my 34C. This was corrected by opening the case and moving/cleaning the display assembly pins. Opening the case the first time is both simple but a bit daunting. After removing the two screws in the battery compartment, you must firmly hold the calc upside down on a table and pull up firmly. At a certain point the plastic snap clip at the bottom of the calc will yield and disconnect. At that point you can access the display assembly and pull it out or move it back and forth to reestablish good contact. I pulled mine out and cleaned the contacts with Radio Shack "tuner cleaner" on a "Q-tip." You can also use isopropyl alcohol. Reinstalling the display assembly and closing up the case I found everything working OK. By the way, the battery terminal closest to the center of the calc is +.

Good luck!


I had the same problem with my HP 34C. It is now working fine after replacing the battery pack. I used the Radio Shack Replacement Pack, catalog no. 23-191, cost $5.99. Any similar battery should do. Just make sure they're nickel-cadmium, AA size, total 2.4 volts, 700 mah. Good luck. Hope it's nothing more serious with yours. I know I love my 34C!

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