problem with HP29 display



please advise. My HP29C calculator display has flashing zero's when switch on. Can it be repaired??? Is it possible to replace the battery pack with regcargeables, recomendations please.

I also have a HP45 which requires a battery pack. can any provide the exact spec of rechargeables that I can buy to replace the origional batteries?

Your assistance, ideas will be highly appreciated.

Thanking You



Do NOT run your HP 29C with an AC adapter if rechargeable batteries IN GOOD WORKING CONDITION (including clean contacts) are not installed. DOING SO WILL DAMAGE YOUR MACHINE BEYOND REPAIR.

Rebuilt battery packs with NiCd AA batteries are OK. In the meantime, it is possible, at your own risk, to try with regular alkaline AA cells. The voltage will be somehow higher than NiCd, indeed, but the difference may be within tolerance.

(I have done it with a non-Continouos Memory HP25, I suggest you may check with someone who had operated HP25C or HP29C with alkalines before going ahead).

If you use alkalines, never use the AC adapter with them!


You are absolutely right Andres.

I do not know what the "flashing display" indicates. Is it a flag on the 29c, or an error diagnostic?

Norman, I personally would recommend just replacing your old NiCad batteries with new NiCads-- it's a simple easy and cheap repair, sure-fire and guaranteed.

Alkalines could rupture if used in conjunction with a charger/power pack. So if you ever need to use alkalines in a calc, it is my habit to take a piece of cork and stopper the charge port. Just a way of reminding me not to be stupid.

Alkalines/dry cells/primary-type ZnCl batteries all put out 1.5v nominal PER CELL, while NiCads and NiMH are 1.2v/cell. Not a great deal of difference, but my personal rule is that you should not act as if alkalines and NiCads are equivalent if there will be more than 3 in series (an excess of > .9v). Most devices have regulation circuitry, true: but most regulation circuitry consists of converting excess voltage to heat, not great for reliable small devices.

Truthfully, I would be skittish about alkalines in the 25c in particular, mainly because this is VERY EARLY C-MOS, and I think it is more picky about the voltages it tolerates. I would try to keep it to NiCads.

I do not know about the 29c's circuitry, but would guess it is somewhat more robust and tolerant than the 25c. Just my personal impression. Nevertheless, Andres is right to warn you to NEVER run it from the power-pack without the rechargable batteries in place (they act as a "soft" buffer to the power from the pack).

There are many places that can rebuild a battery pack for you if you have a bad one. Luckily, you can even buy solder-tabbed AA cells on the Net, if you want to try it yourself. The packs are usually a combination of three or so AA NiCad cells, on most calcs. It is three on the 45, for instance. Opening up the battery pack carefully is probably the hardest part of the job. You can do it!

Don't solder directly onto a NiCad UNLESS it has solder tabs to solder to. If you ruin the venting system under the cap by melting it, the NiCad is never going to charge well, and might (if overcharged) burst. Katie Wasserman pointed out in an earlier post that Digi-Key sells some pretabbed NiCads, and I think that is likely an excellent and reliable source.

If you are wondering about NiMH, I have YET to see REAL decent data that NiMH, when charged by a NiCad-based charger with a NiCad based method, delivers more power overall or lasts more cycles. It has much less of a "memory" problem, as you have heard, but charging it is an exothermic process (produces heat) where NiCads are endothermic, and NiMH loses its charge as it sits idle at a 30-40% higher rate than NiCad will (all batteries lose charge when sitting, but NiCad is STILL better than NiMH in this regard, and I think this has some virtue in a calculator).

If NiMH is charged in a NiMH charger, you DO get more power capacity, and you DO get more cycles, but since HP chargers weren't originally designed for NiMH, the way to use them is to convert your calc battery pack to allow you to change batteries in it easily, then keepp a set of NiMH in your charger, and a set in the calc, and switch them out RATHER THAN trying to charge them IN THE CALC.

By "converting your pack to allow you to change batteries easily", I mean you can, for instance, use the NiMH cells with an industrial cap (a bit shorter than the button-topped consumer variety), and use a spring-metal contact in each side of the pack, and "seal" the pack after each change with scotch tape. It works, and even someday, can be easily reversed to a plain ol' NiCad pack again.

Be picky. Power is the only thing a calculator needs to do its work-- and on such a diet, the best you can do to keep your calculator healthy is provide it with the RIGHT stuff...



A flashing, abnormal display on an HP-29C most likely means a ruined chip. You may also see a flashing or abnormal display if you try to run the calculator from the wall adapter without a good battery pack installed and making good contact but in that case, you probably ruined a chip or two already, with the overvoltage. (The HP-29C is very fragile; I have several in my parts box with missing chips for which no replacements exist.)

Regarding alkalines, I've heard horror stories about the 3.2V output voltage of a pair of new alkalines being sufficient to ruin the early C-MOS circuitry. A pair of slightly used alkalines (<1.5V per cell) should not cause any problems, since fully charged NiCd cells can put out that kind of voltage as well.

As for the HP-45, if you have an old, dead battery pack, it is very easy to rebuild. If you're in North America or other parts of the world where Radio Shack/Tandy stores exist, you can buy a battery pack (RS part number: 23-170) that consists of three "AA" NiCd cells already soldered together. You can open the plastic housing of the old battery pack, take out the dead cells, cut off the connector pads (and clean them if necessary). Next, remove the plastic outer wrapping from the new pack, and solder the connector pads onto its solder tabs (watch the polarity!) If you did everything right, you can easily insert the new cells into the old battery pack's plastic housing, and seal the housing with a small (so that you can open it again if you have to!) amount of glue.

Alternatively, there as businesses (e.g., Battery Plus here in Canada) that rebuild old battery packs for a modest fee.



Hi Victor,

Can you give some info on Battery Plus in Canada?




Other than the fact that there's an outlet here in Ottawa at the Carlingwood Shopping Centre, and that they have a really silly Web site (flashy Macromedia stuff, no substance), I can't give you much. In particular, I have no idea if they exist in Montreal or not, sorry.


Possibly Related Threads...
Thread Author Replies Views Last Post
  HP-41c display problem Ignacio Sánchez 6 1,207 09-04-2013, 10:52 AM
Last Post: Ignacio Sánchez Reig
  Can the display for a HP32Sii be replaced with the display from another model? Bruce Larrabee 8 1,456 10-10-2012, 10:05 PM
Last Post: Luiz C. Vieira (Brazil)
  HP41 Display Problem Fouad M. Kaadou 3 830 06-29-2012, 07:46 AM
Last Post: Fouad M. Kaadou
  Sharp PC 1211 display change -- problem overnight -- suggestions? Gene Wright 12 1,617 05-16-2012, 03:31 AM
Last Post: Bart (UK)
  HP 34C display problem james summers 11 1,744 04-05-2012, 06:07 PM
Last Post: james summers
  HP 41C display problem Alexander Oestert 4 764 08-11-2011, 07:44 AM
Last Post: Raymond Del Tondo
  34C display problem Daniel CLEMENT 8 1,137 07-11-2011, 09:45 PM
Last Post: Michael de Estrada
  Hp-42s display problem aj04062 9 1,335 02-27-2011, 08:36 AM
Last Post: aj04062
  16C display problem John Robinson 3 675 06-07-2010, 10:26 PM
Last Post: Vieira, Luiz C. (Brazil)
  16 segment Led display would mimic a HP-41 display? Giuseppe Marullo 3 838 11-12-2009, 08:08 PM
Last Post: db (martinez, ca.)

Forum Jump: