HP29C documentation and data


Hello, folks;

I'm taking the chance to try to repair two HP29C (unfortunately it seems their ACT are bad) and I decided to collect as much data as possible about them. I'll contact the owner and ask for a few more time with them so I can go further. Have a look at the image below (low resolution):

I scanned the mainboard and the power supply unit. I tried to "draw" the power supply in an electronic design SW but I did not achieve good results. So I decided to refine the original scanned image and generate the drawing over it. It seems I have a better look, now. I know it's not the final art I want but I'd like to ask you , please, check it and add your feedback so I can improve it. Original scanned images and track-only are saved.

I also removed the transformer and I measured its "coils" so it's now easier to "see" what's going on. I am transferring the "free-hand" drawings to an appropriate layout so I will add its final art later.

I measured the output DC voltages and I'll add them in a final drawing. I also tried to see (and record) the waveforms, but I have an old audio-lab, vacuum tube oscilloscope that is far from being able to read more than 100 kHz (if more than this, image is fuzzy and out of focus), so I cannot add this information.

I scanned the mainboard without any IC and the power supply, just a few components, both sides. This way I'll be able to generate final schematics.

I'll let you know as soon as it's ready and I'll post the information here. Please, Dave, is it O.K. to do it? I mean, will it be an issue if HP does not allow? Please, let me know.

Best regards.

Luiz (Brazil)


Hi, folks;

this is to add the image of the transformer coils.

As many of you may have already noticed, I'm using an old OrCad version (DOS based) to draw the schematics. No big deal, ahn?

Color names refer to the colors used in "painted" wire (isolating foil). The "COPPER" means transparent foil.

Spice's transformer has a different coil arrangement, as I have checked. I intend doing this sort or "cataloged information" for the HP25/25C. What do you think?

Best regards.

Luiz (Brazil)

Edited: 18 Aug 2003, 6:42 p.m.


Hi folks;

after collecting a few more data, there are three questions I want to ask. If anyone can help with answers...

1 - I saw at least three different codes for ACT chips:

1820-1741 (I saw some HP25, a few HP25C and one HP22 with this one)

1820-1523 (one HP25, two HP22 and at least two HP25C with this one)

1820-1596 (three HP29C)

Are they all interchangeable?

2 - The HP29C's power supply has two transistors and I did not remove any of them till now. Does anybody have the specs/ID for them? This information would allow me to let them in place instead of removing to identify them.

3 - Is there any relation between the 8-pin pinout used in RAM/ROM chips in the HP29C, the HP97/67, Spices and the HP41? I see that pin names for Ø1, Ø2, DATA, SYNC and Is/Ia are the same used in the HP41 and the HP97 ROM/RAM. Are they kept in the HP29C chips? If so, that would allow a better identified schematics.

Thank you for any information.

Luiz (Brazil)


As far as I know, all the woodstock ACT chips are compatible (except the HP27).

I have never had to take apart the HP29 power supply. I did have one with the on-main-board supply with a bad tantalum filter cap. One DC votage looked fine on a meter, but on a scope it was a square wave.


Hello, David;

as always, thank you for the technical info.

The fact is that I just want to catalog info, and I'd like to know who are those fellows. If I have the chance, I'd test the transformer with external frequency generator and get its characteristics. But I have no generator and no applicable scope, so I thought about disassembling it and collect physical characteristics and # of turns for each coil. Again, the calculators are not mine and I'll not risk damaging their P.S. units.

As I have new data I'm posting. I intend building an article about this particular "adventure" and I'd like others to read and add whatever needed.

I'll post a message about this later.

Best regards and thanks again.

Luiz (Brazil)


I don't see any unoccupied pads or vias for connections to the rest of the calculator. I assume wires were tack-soldered to pads - which pads are used for power input and output?


Hi, Ellis;

I used an input VCC of 2.4V (two rechargeable NiCad's) connected in C1 terminals (the electrolytic capacitor). All connecting points are shown below:

I removed all IC's and measured all voltages with no load connected to the output terminals. There are four connecting points that I found interesting. The ones I marked in blue are single connections to a paralell LC arrangement; a ressonant circuit, maybe. I did not follow the other two ones, shown in green, but the images will help figuring it out: all tracks are shown as all IC's are removed. If needed, I can mark where and which IC's are there.

Hope this helps for now.

Luiz (Brazil)

Edited: 19 Aug 2003, 4:18 p.m.


I don't have a 29C, I just want to try to analyze the power supply circuit. Your posts make it easy to do without any hardware! Is that power supply PCB "surface mounted" on the main PCB? No wires, just solder? If so, we could call it a "ball grid array" (BGA). Pretty advanced for mid-1970's!

The parallel LC circuit leads go to two pins (13 and 14) on the ACT chip, I imagine they are the resonant tank for the oscillator that generates the clocks for everything.


Hello, Ellis;

your analysis is correct. It's a BGA (I didn't know the term) and the connecting points shown are the only ones between the power circuit and the mainboard, no extra wiring. And there is also an isolating foil between them both.

After having a brief look at an HP21 schematic circuit, the only things conneted to pins 13 and 14 (ACT) are two capacitors.

Thank you, Ellis.

Luiz (Brazil)


Hi Luiz,

That schematic is wrong. Pins 13 and 14 are connected to an inductor and capacitor in parallel, not two capacitors.

The values are 150 microhenries and 330 pF, giving an oscillator frequency of about 715 kHz. (Which is then divided by 4 to give a clock of about 180 kHz (5.5 microsecond clock period).

- Michael


Hello, Michael;

thank you very much. I thought it was a suspicious arrangement, but I do not have an HP21 to check. I have seen many Woodstocks with inductors that are capsulated as an ordinary, tubular shaped resistor with colored rings to identify them (mH value?). Maybe the author of the schematic concluded it was another sort of capacitor, to justify the use of two of them in parallel, who knows.

Thank you for the additional info. I'll update my schematics with the values you post.

Best regards.

Luiz (Brazil)

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