My very schizophrenic 29C


Hello all,

I apologise this is a new post about my 29C but this is quite awkward.

Today, I decided to work with my 'recovered' 29C. Remember from that 'Error' & strange numbers in the display while pressing keys glitch?

Well, today, I started up my 29C only to be greeted with a rerun. Again with the Error greeting followed by the quirky numbers when I pressed a key. Well, about four hours ago, I removed the battery pack. A few minutes ago, (Four hours later), after reinserting the battery pack, my HP-29C seems to be behaving itself.

I am befuddled and perplexed as to what is happening here?

Any thoughts? I appreciate your insights.

Thanks again and please forgive the repeat of my original issue.

Edited: 1 Apr 2012, 10:55 p.m.




I once tried - and gave up - repairing a Woodstock with a closer behavior. It worked pretty fine for some time, then it freezes, then the display runs for itself with a lot of flashing segments, then it goes back to normal... Bad contact, cold soldering, broken trail, defective IC, whatever.

When it got to my hands it was nonoperative with some signs of battery leakage. I removed all components, cleaned the mainboard (a lot of residual cooper oxide above the IC's closer to the batteries contacts) and soldered all of them back. It worked right in the first attempt and I was glad about it. Then I closed the case, set the batteries in place, tested it for a while then the described problems appeared. I opened it about two or three times after that, but I got to no conclusion at all. My guesses are still: bad contact, cold soldering, broken trail, defective IC, whatever. Then... I gave up.


Luiz (Brazil)


I'm not making any suggestions. I tried.


would you like to sell it?


Ya know. I would in a minute if I could get a 29C to replace it and treat the new one like a museum piece--never taking it out of the house.

Edited: 2 Apr 2012, 2:19 p.m.


Matt if you wish to have a 24h and 100% working calculator, don't forget the advice of a friend, here on the board, a magician with this kind of jewels! Think about that, I'm pretty sure he can solve the problem. as I understand whether you like adventures and trials you prefere to don't open yourself the calculator.
I can bet that the problem should be simply due to a bad contact of the display on the PB, you wrote that it fell down inside a tote bag if I'm not wrong. maybe it needs just to be opened (woodstocks are up to me the simplest to do, for this pourpose)and to check it!

Have you well checked the contacts in the battery comp, are they clean?
If you see even a little presence of green colour, I'm sure that the PCB needs a vinegar shower (it's not a joke!) and maybe it can help to solve definively your problem. I met the same issue with a HP21 yers ago and I "repaired" it this way...

But, don't forget please italian islands!

Edited: 2 Apr 2012, 9:23 a.m.


Hi there. No, that's not so far off. Elsewhere in this forum, I read about the vinegar bath operation. In lieu of complete disassembly, is it possible to just take a Q-tip with vinegar and swab the battery contacts? Yes, it's not the complete surgery but, would this be step be beneficial until I could get the help needed for the other option of disassembly and maintenance?


Hi Matt, to take the bath anyhow, the calculator must be "undressed" before, and left then openedfor a while, better for a day to completeley dry before to test it; but ofcourse before to take this action I suggest to look carefully inside it , better if with a magnifier, to check if the PCB is dirty or not! Absolutely don't be afraid to open it for screws under the feet, no labels to be replaced with risk of a damage,like in the classic models....., try please'll win a 100% workin' HP29!

Edited: 2 Apr 2012, 4:09 p.m.


Well, I tried the least-daredevil approach. I took the distilled vinegar and cotton-swabbed the battery terminals. It seems Bonnie (No, I don't name my calcs. That's HP's name) is behaving.

Edited: 2 Apr 2012, 5:13 p.m.

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