HP29C strange behavior

Hi everybody
I have just received two HP29C, and one seems to have some life in it
I'm powering the unit with an external battery pack
as the unit is switched on, the Errormessage appears

it blinks once, and then ANY key pressed produces the following output

Switching to program mode is working

but then again, pressing any key (in prgm mode) will produce the following output

Further pressing will increase the program line counter, but the rest of the display shows what you see in the picture
Has ever happend to someone else ?
Thanks in advance for help, and have everyone a nice weekend, Alberto


Hi Alberto,

The cause could be one shorted key in the keyboard. Did you try swapping the keyboard between your two units? Or with another Woodstock?


Joel Setton


Thank you very much for your suggestion, actually I had already tried, and with the other keyboard the unit doesn't even power on. I may have a HP25 spare keyboard, but I believe pinout is different, I'll keep you posted, take care Alberto


The cause is most likely a bad ACT or ROM. The only way to troubleshoot is to replace all the chips one at a time with known good parts, starting with the ACT.

FWIW, all Woodstock keyboards are the same pinout. The only difference is the 27 which lacks the second toggle switch. So, putting a 27 keyboard on a 29C logic board would work but it would be stuck in prgm mode.


Oddly enough, the HP-21 wires the keyboard columns differently than the other Woodstock models, but this is done on the logic board, and the keyboard is, as you say, identical.

This was a bit of a surprise when I first was trying to get the HP-21 working in Nonpareil.


The code you are seeing in program mode is an illegal program step with the hex code FF. This suggests to me that probably one of the RAM chips has gone bad.

Chip failures in Woodstock series calculators are often the result of having the external power adapter plugged in with no battery pack installed, a bad battery pack, or the pack making poor/no electrical contact for mechanical reasons or due to contact corrosion.


That's also my guess, since the previous owner has admitted that he has tested the unit powering it without the battery pack (that was dead anyway) installed. The second unit is behaving like this :

the models have close serial numbers, the interior if the model that has a fixed number on the display is :

while the interior of the unit with the "random zeroes" is like this :

really I wouldn't know what to take from one unit to transplant to the other ... thanks in advance for any suggestion ! take care and have a great weekend, Alberto


The RAM chips are the 16-pin ceramic-packaged ICs. I'm not completely convinced that I would transplant anything, given the both units act like they have bad chips. :-(


Thanks for the suggestion, I have cleaned both mother boards, and transplanted the "violet" chips onto the motherboard hosting the "withe chips". This unit is now fully working, and it retains data too ! The other seems dead ... I'll keep working on this in the next days. Take care and have a nice weekend Alberto


Great!!! Congratulations. At least for me, there's this sense I get when I hold and use and display a newly restored and functioning HP that a little piece of the universe is better again. Good job adding to that piece a litle bit more.


Good work!


Thank you guys, that now brings me back to the other one. I'll load some pics later today. The unit now working is the one that originally had the "white" chips. When turned on, showed always the some number. Since this is the older, I started reworking on the other one, the one that showed some highlighted zeroes on the display. Took away all the chips, the powersupply board, cleaned throughly in and out and polished to get the board to shine conditions. (well, you'll see the pics). To my surprise, when I have resoldered all the components, except the ram chips which I have installed on the other unit, this was not even turning on anymore. My guess is that the powerunit, has some bad contacts. Differently fomr the other, which has the power-involved-components directly mounted on the board, this has the components soldered on a mini board which is then connected to the mainboard. Since the machine was showing something before, I'd like to bring it back at least to that condition and then start some investigation. Does anyone has ever tried to feed power to the machine bypassing those components ? Take care Alberto


What part number do your calculators have for the ROM/anode driver? That's the 18-pin chip under the display. I expect they would be 1818-0378 or 1818-0431. I've been trying to get a 1818-0378 (or any other than the 1818-0431) to dump the ROM.


Eric-- I have one with an 0432, but it's clearly a replacement on a dead board. Does that even make sense? Do you want it?


Unless I've gotten my part numbers confused, the 1818-0432 is the ROM/anode driver for a 19C, and if one was installed by mistake in a 29C, that would certainly keep the 29C from working.

I don't specifically need a broken 29C, but in general I am interested in getting broken units of any pre-Saturn models to have extra parts for reverse-engineering purposes.

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