HP 29C Over voltage? Repairs?


This may have been an extremely lame mistake on my part... Tested my old HP 29C, using two "AA" batteries. It worked great! Pulled batt and came back two hours later, now I get no response. Two "AA" batts equal 3V, HP 29C case says 2.5V. 1) Did I fry some internal component? Fuse? 2) Is the situation terminal? 3) Is it repairable?



As per my experience some 20 years ago with an HP25 (quite similar to the HP29C), the use of two alkaline batteries caused no problems. And I have read some similar stories from other people in this forum. It is true that the original batteries limit the supply voltage to 2.5 volt (2.6 perhaps when fully charged), but it seems that the 3.0 volt from alkalines are acceptable enough. In these models, the NiCd batteries were used both as filter and regulator when the AC adapter is in use. Do not use the HP29C with the AC adapter if rechargeable batteries are not in place (and in working condition), nor attempt to recharge standard cells.


By the way, there are no fuses, and if components were damaged, repairs will be very difficult, due to the use of custom parts in these machines. Warning!: If the AC adapter is used without rechargeable batteries in place, voltages as high as 8 volt will be applied at the internal components (prepared for about 2.5 Volt), and that surely may damage them. But if the unit was operated on alkalines only, it is very possible that the components were not damaged. You may check for dirty or unreliable contacts, broken pieces, etc. If the calculator does not work, you may find some information about repair shops here at the Museum; personally I am unable to endorse or criticize any of them.


Andres, Thank you for your quick posting to my problem. I do not have charger and that's why I was using standard "AA" cells to check the unit. All contacts are clean, batteries new.... Any idea on the absolute worse case of internal damage if polarity was reversed? John


I am just guessing now, but I don't think that reverse polarity from batteries will be enough to fry the calculator... On the other hand, I don't think either that there are any reverse-polarity protection, since it is not possible to happen with the HP original battery pack. I am sorry I do not have my old HP25 anymore, so I am unable to open it and take a look to the circuit. If there *is* something burnt, let's hope it's a bipolar transistor or diode (discrete components) near the switching power supply, that may be fixed... But, again, you will need some extra help from other people, or looking at another machine. Now, I'm just thinking that a significant difference between my 25 and the 25C or 29C models is that the batteries are supposed to keep a quiescent current flowing to the continuous memory chips (to preserve contents when the calculator is off). So the chips will be receiving the battery voltage when the power supply circuitry is off. However (again) I don't believe that the difference between 2.5 V and 3 V will fry the chips, but reverse polarity may do the harm there. I hope someone will be of more help than I.


Guessing again, perhaps too much, but... Usually, the circuitry associated with applying a standby voltage to CMOS RAM chips uses some diodes to let the chip work with the standard operating voltage when the machine is on, or with the (lower) standby voltage when the machine is off. If these diodes exist in the particular case of the HP29C, they may help prevented reverse voltage damage... it depends on the actual circuit configuration. Another component that may be involved is some sort of capacitor (perhaps an electrolytic or tantalum type), placed there to store an amount of charge that allow for the batteries to be swapped without losing memory contents. The capacitor may be a good thing to check, if not for other reasons, just because it is simple to measure and to replace if needed. Reverse polarity, or just the years may have affected it... Again and again, these are rather unsupported ideas, I hope that someone else may have more accurate suggestions to offer


Thanks a ton for the time you put into my troubles. The unit is in new physical condition (20+ yrs later) so I do not feel this is a repair I will try at home. It may just go back in the garage for another dozen years... Thanks again.



I have some experience with repairing the 29C and 25C. The problem with them is that the ICs that have the constant memory are connected directly to the battery and don't go through a regulator like most of the electronics does. If you put excess voltage on the battery circuit you WILL burn out the ICs. The same ICs have part of the operating system ROM in them so you can lose that too. I don't *think* two alkalines batteries will harm the ICs since they have to be able to withstanding the voltage of a fully charged NiCad and that easily approachs 1.55 volts per cell. Remember that the memory ICs are connected to power even when the calculator is turned off. Trying to run the calculator off of the charger without the batteries WILL burn them out since the chargers can put out over 15 volts without the battery load. That's what usually happens to the 2xC models. It's fatal and it's permanet. I no longer work on the 2xC models because it's hopeless. If you reverse the batteries it's also very possible that they will also burn out the memory ICs.

There are no reverse voltage protection diodes, regulators, fuses or other protective devices in the memory circuits. They memory ICs are custom made hybrid parts from HP. There are NO replacements for them, even from HP. The only possible replacements are similar model calculators but it seems that they always have the same problem.


Joe -- Thank you for the reply. I do not have the charger anymore and that is why I was testing with the two "AA" cells. From what I have read in the last two days, I believe there was an operator error and that the unit is dead. Is there a market for good looking, but dead HP 29C's? Be of good cheer -- John

Possibly Related Threads...
Thread Author Replies Views Last Post
  HP 41CV Repairs William Mabotja 3 420 11-20-2013, 04:45 PM
Last Post: Mike Powell
  HP-29c keyboard dead 2nd row problem Xavier A. (Brazil) 6 560 07-17-2012, 07:09 AM
Last Post: Xavier A. (Brazil)
  HP 34 and HP 41 Repairs Needed matthewx 4 408 07-16-2012, 09:12 PM
Last Post: matthewx
  HP-29C Counterparts Matt Agajanian 2 273 05-12-2012, 06:44 PM
Last Post: Matt Agajanian
  My 29C's even more bizarre behaviour Matt Agajanian 5 486 04-25-2012, 05:46 PM
Last Post: Matt Agajanian
  29C/25C question. Matt Agajanian 7 490 04-03-2012, 12:03 AM
Last Post: Matt Agajanian
  My very schizophrenic 29C Matt Agajanian 9 623 04-02-2012, 05:12 PM
Last Post: Matt Agajanian
  Help!! My 29C may need a doctor. Matt Agajanian 5 441 03-31-2012, 04:23 AM
Last Post: Ignazio Cara (Italy)
  Legible version of 29C Curve Fitting program Matt Agajanian 6 521 03-21-2012, 07:46 PM
Last Post: Matt Agajanian
  Sleek approach to Register Exchange on HP-29C Matt Agajanian 8 561 03-16-2012, 01:04 PM
Last Post: Marcus von Cube, Germany

Forum Jump: