Building a Casio RP-33 clone, working with Zebra strips

[Jeff_Birt]

Hi all,

I recently made a clone of the Casio RP-33, 32K SRAM expansion which was used in the VX-4 and many other machines. This was an interesting project as it was a chance to learn how to work with zebra strips. Even though it is just a RAM and cap on a small PCB I still found clever ways to stuff it up a few times.

I first laid out a version using an SOP chip package and then realized it would be too thick. Casio used an SOP but used a paper thin PCB and spaced the PCB up to squeeze it in there. In an effort to make it a simple build I switched to a TSOP package by borrowing a TSOP package from another Eagle library and managed to get one with the wrong pinout. D 'Oh! First batch of PCBs were scrapped.

The first batch of zebra strips I got were 2.4mm tall which was just enough to only make contact if you pressed down on the PCB a bit. I then got some that were 2.8mm tall which worked fine. The tricky parts are measuring the height in which the zebra strip will sit and then trying to estimate the flexure of the RAM module PCB and computer's PCB. Ideally you want a 10% compression in height on the zebra strip.

When trying to see if the RP-33 Redux would also work as in RP-8 for older machines I noticed that the module opening in these RP-8 machines is a tad smaller, no problem just shave 0.5mm off the PCB perimeter.

I had the strangest problem with Eagle on the RP-33 board. I noticed that on the PCBs I had made the zebra contact pads were 0.5mm too wide end to end. When I went into Eagle to edit that part I found it was correct. No matter how I changed the footprint it would not show up correctly on the PCB. I made a change and decided not to save it and got the prompt about it not being saved but the path was not to the Eagle library folder, rather it was to the project folder where I had once saved a copy of that part.

When I selected the part for editing it was pulling from the file in the project folder but when it rendered the PCB it was pulling from the Eagle library folder. I deleted the copy from the project folder and was then able to edit the version in the Eagle library and get the PCB generated correctly. Whew! The 3rd rev of the PCBs.

I did a video about this project. I did find out about the footprint error after the video though so I would not use the Eagle files I posted on GitHub at this time. I'll update them after receiving the new PCBs and making sure they work.

In short working with zebra strips is not that difficult it is harder to find someone to make them for you in small batches.

https://youtu.be/DSXBF3-MclE