Anyone build a ZVC (Zeprom Voltage Converter)


I've examined my Zeprom Voltage Converter closely, and it appears that it would not be too hard to make a copy of it. The components are all 'standard issue', there is just one IC, a voltage up-converter, which can be had from multiple manufacturers today for a few dollars. The small circuit board would have to be reproduced, there are kits that can be used to build custom PCBs. Perhaps the hardest thing to come by is the empty HP-41 module and contacts, but the least expensive modules (maybe $10 on ebay?) could be used to salvage the parts.

I wanted to know if anyone has built, or heard of anyone else building, a clone of the ZVC. I'm going to try it, and would certainly like to learn from any others that have attempted it.

Thanks! Dan


Hi Dan,

Though I haven't attempt to build such a ZVC clone, it really looks like a very interesting project.

Of course, the first step would be a reverse engineering on the original one to obtain detailed schematics.

Next, confirm all the required components are available on current market.

Re-desing the PCB, and go for a short sample production run.

One drawback may be the fact that there are very few ZEPROM's out there and, consequently, very few potential users.

Anyhow, you can count with my help and support if you consider it valuable to your project.

Best wishes.



Diego, to have this offer of help from such an esteemed designer and builder of things HP-41 is valuable indeed!

I have completed the first step, I have a schematic. There is one part though, that I have not yet figured out what it is. It is a 'can' shaped device, like a transistor, but with only two leads. It is labelled on the top with '221K'. The rest of the parts are common diodes, some 1% tolerance resistors, a few capacitors, and the heart, an MC34603 voltage up-converter IC.

Step number two is almost complete. I have verified parts availability for all but the 'mystery component'.

I'm starting to put together a digi-key order for the parts, but am waiting until I have some time to research the mystery part and get it identified. I should be able to make a PCB with one of the homemade PCB kits from Radio Shack, using a double sided copper clad board. Then I need to obtain a junk HP-41 module to complete it. At the rate I'll be able to work on it, it might take a few months, but I'll eventually get it done.

Any help would certainly be appreciate. Any improvements to my plan are welcome.




Hi Dan, and thanks for your kind words

So it looks like you've got a nice part of the job already done... Great!... ;-)

Can you send me some close-up pics of the "mistery" component? in order to see if I can identify it. The shecmatics may also be helpful as it can show what this part is supposed to do.

Feel free to mail me with any commensts or ideas you may want to share.




Diego, Allen, Muhammad, and all,

Here is some additional information, including photos. I have loaded high resolution pictures of the ZVC at The original pictures are high resolution; to see the original resolution, select a picture by clicking on it, click the 'All sizes' button above the picture, and then select 'Original'. You can also download the original picture, it's 2592 x 1944 pixels.

The mystery component is labeled 221K on the top, and is to the lower left of the large 470 uF can capacitor laying on its side. The mystery component might be a capacitor, but it's appearance is different from most capacitors, and in an email communication I had with Christoph Klug, he made reference to a 'coil' (inductor?) that Zengrange had experienced failures with in early versions. But this does not look like a standard inductor either.

And yes, Diego, the voltage upconverter is "MC34063", and you suggested. I made a typographical error in my previous post. Perhaps the "mystery component" then is an inductor.

From the hand-drawn schmematic that I have, the function of the ZVC appears simply to pass Vcc from the HP-41 onto the V+ line to the ZEPROM module 'cleanly' (through a diode) unless the switch (S1) is activated, in which case the voltage upconverter supplies the necessary 12 volts to the ZEPROM V+ line for programming. An interesting part of the design is the parallel arrangement of three 1-ohm resistors to help produce what may be some sort of reference voltage for the upconverter chip. I'll reproduce the schematic I have into something that I can post.

It certainly may be possible to build a much simpler circuit today. If voltage upconverters have gotten simpler to design circuits around, perhaps all the 'peripheral' components may not be needed?

The PCB is two-layer, front and backside. It is not copper though, I don't know what the metal is, but it is also used as the contact material for the ZEPROM to attach to.

Thanks for everyone's help!



one thought - Matthias & Co, could you check your ZVC and check if you have the same components? Maybe in one of the other existing ZVC's the 'mystery' component is better labeled.

Great project btw, following intensely here!




Hi there,

Thanks for the great close-up pics Dan, positively it's a 220 microH ferrite core inductor (coil=inductor).

Regarding the PCB, the outer platting could be tin or some silver alloy. However copper is what the tracks are mede of, as bare copper trends to create an oxide layer (non conductive) on its surface, its fairly common to place a more stable metal deposit in order to increase contact reliability.

Best wishes


Edited: 12 July 2008, 7:46 p.m.


I concur. An inductor. From the likes it is probably a 12V or 15V upconverter to generate the programming voltage for the EPROM in the module.

You know I have found a SRAM part that is also has a built in EEPROM that copies the RAM contents at power up and power down from EEPROM to SRAM and vice versa. It works exactly like a SRAM except it is non-volatile.

If the EPROM can be replaced with this part you will not even need a converter to program it.

Something to look into. A module that is non-volatile SRAM with a lot of potential. I do not know what is on the ZEPROM module or that other thing clonix41 can somehow benefit from this SRAM part.

Let me know what you think after looking at the website.



Certainly a device using non-volatile RAM in a ROM-like fashion is of great interest...

This is what I thought back in 2004 when developed my NoVRAM module... ;-))

Hope you enjoy the specs, currently several dozen users can take advantage of the powerful (and highly expensive) HEPAX module thanks to this, without worrying about loss of data due to power failure or module extraction... and for a fraction of the HEPAX module price.

Cheers from the Canary Islans.



... it is a most fantastic device indeed. I only have ZEPROMs now for educational purposes (to play with the various programming devices available) and collecting purposes. For everyday use I use the Clonix (especialy now that it has 512k for some MCODE functions/play.

For my endeavors in MCODE (very amateurish and clumsy) I use the NOVRAM, as I think the HEPAX is quite an exceptional modul/MLDL. Even better, Nov/Nov32 uses less power as well.

in ogni caso, I can wholeheartedly recommend the devices.

Which should not deter us the least to build a ZVC!!! I think Dan has a fantastic idea here and I am following with great interest.




221K sounds like a Cap. ?


Edit: According to the Cap Conversion table at 221K -> 220pF +/- 10%


...and the heart, an MC34603 voltage up-converter IC.

Hi Dan,

Could it be a "MC34063" instead?

If this was the case, chances are that your "mistery" component is nothing but a simple inductor of 220 microH...

Hope this help.




If you can send me pictures (high res) of the two side of the PCB & components I can decipher the circuit.

I can tell you what is what from the package.

This sure looks like a very interesting project.

Or if you have any specifications, I could help build a totally different compatible one with better performance. Parts have come a long way since this original design.

Is it two layer or multi-layer PCB?


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