35s Modification


I recall some discussion in the past regarding modification of the 35s. Given the relative ease of disassembly, good keys, and an attractive form factor is anyone up for using the case and keys to make a new machine?

All I'm proposing here is a new LCD and PCB with perhaps a more capable processor, more memory and USB/some form of IO.

I think this should be kept as simple as possible and would estimate a very nice mod kit to accomplish this could easily run $50.

Anyone interested?


I'd buy a mod kit for 50 bucks to add I/O


I'm in for $50.


Me too. If I like the mod I'll probably get another.


I'm in, also for more then US$ 50,00. Depending on the updates I'm getting.

Is there any chance to get also a "hp standard" IrDA connector, mainly for print output to the beloved 82240 printer?

Thank you anyway for the efforts.



How about putting the same into a 30b? It's smaller and has a full row of keys below of the display, so soft keys are easily done with a reasonable LCD.

Just my 0.02 Euros.


It was Jeff.O who did some early work on this "Phoenix 35sx" project back in 2007 and posted photos, got sample LCDs, and even did a DXF of the required board size and holes etc

IIRC it wasn't trivial to disassemble and required drilling out a bunch of heat stakes. If so, that means reassembly of a new PCB would not be trivial either. So it looked pretty messy from a physical hardware point of view.


Edited: 21 Apr 2010, 5:44 p.m. after one or more responses were posted


IIRC it wasn't trivial to disassemble and required drilling out a bunch of heat stakes. If so, that means reassembly of a new PCB would not be trivial either.

Looking at Jeff O.'s post linked by Reth, the assembly looks similar to the HP-50g which I have taken apart and the small screws are easily put back into the shaved-off stakes and hold very well.

From the pictures the process doesn't look that bad, certainly much easier than a pioneer for example. I'll pick up another 35s and a 30b to document the dissection and put together some CAD drawings. My suspicion is that replacement of the PCB and LCD will be reasonable enough to be worthwhile.

This obviously doesn't need to be too high-end. I think most people would be happy with something powerful enough to emulate saturn. Even 1MB of memory would be extreme in this application.... And a nice high contrast reflective LCD should complete the picture.

I'll post progress as soon as I can.



I prepared some CAD drawings that I can send to you if they would be helpful. Front and back of the PCB, in dxf format.



Jeff, that would be fantastic. At the very least we can define design constraints from your work. From your experience with assembly/disassembly of the units does my drop-in replacement sound feasible with at least moderate skill?



I think I have an e-mail address for you, I will look around and send you the files. If I can't find an e-address I will let you know.

It was not difficult to disassemble the 35s as documented in my post of a couple years ago. So I would say yes, your drop-in replacement certainly sounds feasible.

Good luck.




I'm in too.
Here is the link Dave is talking about

Edited: 20 Apr 2010, 9:09 p.m.


I'm in. (Obviously?)


On a related note, has anyone developed a schematic of the HP35s?

There appear two be two major integrated circuits under the epoxy 'blobs'. I assume these are the processor and a memory chip.

Please let me know if there are any details out there.



I don't have a schematic, but what's under there is a GeneralPlus microcontroller (based on a 6502 core) with masked ROM, and a 32KB SRAM. Except as a one-off hack, it's not practical to try to reuse the PCB for anything, as it is much less expensive to make a replacement PCB.


Would be interested to but need some pictures first.


Grüezi Matthias,

got your site purged?


Grüzi as well.
Yes, I cleanded it up two days ago. Sorry for teh delay.
I got a happy daughter six weeks ago and still have other things to do than collecting things.



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