treasured HP-29c displaying jiberish

I have a 29c which I bought new in the late 70's. When the last
battery pack for it finally died, I read on a forum (I think this one) that you could just use AA's, put into the battery pack.
I tried this and it worked great for a couple of weeks or so. Then, suddenly one day, it started displaying ridiculous results to keyboard entries (it usually displays 1500 when I hit CLX, and other number keys give random large positive and negative numbers. Occasionally it will display properly, but not very often.

Any ideas? Will cleaning any of the circuit boards help?
Thanks in advance for any help.


I have a 29c which I bought new in the late 70's.

Me too. And back then I also have used AA's which went well all the times. But I've read somewhere that the surplus voltage can be harmful to the calc. Also there's a risk of inadvertently charging the batteries with the adapter. Since then I've never used AA's anymore. I even charge the NiCd's or NiMh's (both work fine) always outside the calc, for I've read that the charging circuit is critical to the rechargables in the old times. If you can get them, 500 or 600 mAh NiCd are the best choice to use.

I cut the middle bar from the battery pack, fixed the springs with a piece of double sided tape and am now able to replace cells very easily.

Occasionally it will display properly, but not very often.

I do hope it's not too late. Return to rechargables immediately.

O! And don't ever connect the calc to the mains without rechargables in it. That's definitaly harmful.

make sure that the batteries make good contact with the terminals
inside the battery compartment.

These terminals are very unreliable.

I have an HP-21 and I have been using it with alcaline batteries for the last year or so and I only have problems every time I change the batteries and have to make sure that they make good contact.




I read this article of katie long time ago and it was very helpful with an old HP-25
HP Articles forum (nº267) by Katie Wasserman.
I had also an HP-29C with the same problem as yours and, although it was not tested with that model, I tried this solution. ¡¡¡ IT WORKS !!!

I think that it is worth you try it.




I think you have an intermittent electrical connection in your calculator. Data is getting scrambled while traversing the buss.
Maybe you have some damage, but if the calc ever behaves normally you can basically rule this out and the actual logic is okay.

If you are really keen I can fix it for you...

Don W


A lot of funky HP problems can be fixed by cleaning the circuit boards with 91%+ isopropyl alcohol. Old solder flux, etc (often invisible) seems to the problem. Many machines had a least a few parts hand soldered at the factory and they never seemed to clean the flux on them, but even "clean" boards can have problems.


Thanks Don for your response,
My 29C is working normally now, aside from the fact that I have to reposition the battery pack occasionally to get it to come on. I think your intermittent connection theory is right on the money. It seems to be the battery contact buttons on the circuit board. I was thinking maybe re-tinning them with some fresh solder would help. What do you think?


Ah! That's right! Thye had "rivets", didn't they?

(I remember an old friends 33E...)

Yes, for sure. But I'd try lightly sanding them with wet and dry emery paper, first. Tht's probably all you need to do.

You may have acracked track or the rivet may not be making contact with the PCB track it's attached to...



I now clean Woodstock contacts by placing a good sized drop of De-Oxit contact cleaner on the contact and then use a wire brush attachment in a Dremel tool. It cleans the contact very well and does not remove as much metal as sandpaper.

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