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

[mfleming]

Hi Jeff,

Your stumbles sound quite familiar! I am curious about your source for zebra strips - manufactured to order? Those zebra connectors were widely used in HP calculator models and are a prime failure point in older machines. There have been some interesting substitutes made with flex PCBs. I and I'm sure others would be interested in your experience with them.

~Mark

[Jeff_Birt]

I was able to get the zebra strips made to order with some limitations. The pitch was 0.05mm, the supports (side insulation) was a fixed width. The Length, Width and Height were all made to order. I had some made for the RP-33 and also replacements for the TRS-80 Model 100 (and similar) LCDs. If one wanted to order thousands they could get anything made.

You can get 6 or so smaller strips for $50 with shipping. Larger quantities are of course much less per piece. I would be happy to pass on the contact information privately. I'm hesitant to post it publicly and perhaps have them possible be inundated with requests for 'one zebra strip for a particular piece of equipment'. Just thinking the name could be spread without details, but perhaps I'm being silly?

Getting the correct fit of the zebra strip in the shim to a bit of trial and error. It needed to be tight enough to hold the strip in place while inserting the PCB but not so tight that it prevented the strip from expanding laterally as it was compressed. Getting the height right is tricky. You know you want 10% compression of the strip and you can measure the gap but you have to guestimate how much the rest of the parts involved will flex.

[Csaba Tizedes]

(03-17-2022 08:17 PM)Jeff_Birt Wrote:  I'm hesitant to post it publicly

Do not hesitate. They are manufacturers. If they do not want produce their product, what they want on market?! Frankly speaking: I read your post for this company name or link as many people here...
UPDATE: Your video really well organised and useful. Thanks for sharing!


Csaba

[dmh]

I'm after HP 71b zebras, surely there's a lot of other people after them too?

We could do a combined order with someone reshipping?

Where is the manufacturer located?

Just a thought...

[dmh]

So will you be making and selling the RP-33? They are hard to find and I've only been able to get hold of an RP-8 but the VX-4 could do with more memory than the RP-8.

(03-17-2022 08:17 PM)Jeff_Birt Wrote:  I was able to get the zebra strips made to order with some limitations. The pitch was 0.05mm, the supports (side insulation) was a fixed width. The Length, Width and Height were all made to order. I had some made for the RP-33 and also replacements for the TRS-80 Model 100 (and similar) LCDs. If one wanted to order thousands they could get anything made.

You can get 6 or so smaller strips for $50 with shipping. Larger quantities are of course much less per piece. I would be happy to pass on the contact information privately. I'm hesitant to post it publicly and perhaps have them possible be inundated with requests for 'one zebra strip for a particular piece of equipment'. Just thinking the name could be spread without details, but perhaps I'm being silly?

Getting the correct fit of the zebra strip in the shim to a bit of trial and error. It needed to be tight enough to hold the strip in place while inserting the PCB but not so tight that it prevented the strip from expanding laterally as it was compressed. Getting the height right is tricky. You know you want 10% compression of the strip and you can measure the gap but you have to guestimate how much the rest of the parts involved will flex.

[rprosperi]

(03-20-2022 08:37 AM)dmh Wrote:  I'm after HP 71b zebras, surely there's a lot of other people after them too?

We could do a combined order with someone reshipping?

Where is the manufacturer located?

Just a thought...

I would definitely be interested in buying a few, to have on-hand, just in case.

[Dan C]

Can you reuse Zebrastripes form other devices?
If you have some units totally dead with these Zebrastripes in?

[Jeff_Birt]

(03-20-2022 02:09 PM)Dan C Wrote:  Can you reuse Zebrastripes form other devices?
If you have some units totally dead with these Zebrastripes in?

Not likely. They are all different sizes, custom made to the application.

[Jeff_Birt]

(03-20-2022 08:51 AM)dmh Wrote:  So will you be making and selling the RP-33? They are hard to find and I've only been able to get hold of an RP-8 but the VX-4 could do with more memory than the RP-8.

Yes, I will make some for sale. Hopefully the next version of the PCB will arrive late this week so I can test them. The RP-33 clone 'should' also work as an RP-8 for the FX-790/90 type machines too.

[dmh]

Great :-)

How do we get on the list?

Do you have an idea of the price?

Thanks, dmh

(03-21-2022 12:24 AM)Jeff_Birt Wrote:  

(03-20-2022 08:51 AM)dmh Wrote:  So will you be making and selling the RP-33? They are hard to find and I've only been able to get hold of an RP-8 but the VX-4 could do with more memory than the RP-8.

Yes, I will make some for sale. Hopefully the next version of the PCB will arrive late this week so I can test them. The RP-33 clone 'should' also work as an RP-8 for the FX-790/90 type machines too.

[Jeff_Birt]

(03-18-2022 06:34 AM)Csaba Tizedes Wrote:  Do not hesitate. They are manufacturers. If they do not want produce their product, what they want on market?! Frankly speaking: I read your post for this company name or link as many people here...
UPDATE: Your video really well organised and useful. Thanks for sharing!

Csaba

After thinking on this some more this forum seems a good place to share the manufacturer info whereas YouTube might not have been. Details are below. If you have an original zebra strip you are trying to replace it has likely not sprung back to the original non-compressed height after being compressed for decades. It took me two tries to get the RP-33 strips the correct height. For the M100 I got it the first time as I had some original strips to measure and could just get some just a hair taller.

Given manufacturing tolerances of the machines and strips what seems to work in one machine may be right on the edge and not work in others. I was able to test the RP-33s in multiple VX-4s I had and have sent one to a friend in Germany who has a larger collection of machines to try them in so when he gets it and can test it will provide more data. Same for the M100, sent some strips to friends to try out.

Getting a smaller number at first to check fitment is a good idea I think. When you get to the point of knowing what size will work and can then place a larger order you can get a much better price. Your total cost will include any trial batches which are a higher cost per unit plus the lower cost/unit of the higher quantity. When dividing the cost this is something to keep in mind.

More unsolicited advice: It would make sense I think to have one person be in charge of contacting the company for a joint project. I have done a large project with a friend in another country involving getting tooling produced to reproduce some obsolete connectors. Even with just he and I both working in tandem it could get confusing for us and the manufacturer if someone involved forgot to 'reply all'. Just FWIW.

The company is located in China and I have found them to be very reasonable to work with. If you guys get some strips made let us know how it worked out.

Company info:
http://www.cmaisilicone.com
contact@cmaisilicone.com

[Jeff_Birt]

(03-21-2022 04:28 AM)dmh Wrote:  Great :-)

How do we get on the list?

Do you have an idea of the price?

Thanks, dmh

I'll post an update when I'm ready to make them. The small batch of the updated PCB version should arrive this week if they work I'll order 50 or so PCBs. I'm guessing price will be <$30USD which should be enough to cover the development cost and parts, etc.

[dmh]

Great, thanks!

I'll take 2 if possible.

I'll await your update.

(03-23-2022 03:20 PM)Jeff_Birt Wrote:  

(03-21-2022 04:28 AM)dmh Wrote:  Great :-)

How do we get on the list?

Do you have an idea of the price?

Thanks, dmh

I'll post an update when I'm ready to make them. The small batch of the updated PCB version should arrive this week if they work I'll order 50 or so PCBs. I'm guessing price will be <$30USD which should be enough to cover the development cost and parts, etc.

[dmh]

So who has the part, tools and skill/experience to measure, specify and organise the HP 71b zebra? :-)

(03-23-2022 03:15 PM)Jeff_Birt Wrote:  

(03-18-2022 06:34 AM)Csaba Tizedes Wrote:  Do not hesitate. They are manufacturers. If they do not want produce their product, what they want on market?! Frankly speaking: I read your post for this company name or link as many people here...
UPDATE: Your video really well organised and useful. Thanks for sharing!

Csaba

After thinking on this some more this forum seems a good place to share the manufacturer info whereas YouTube might not have been. Details are below. If you have an original zebra strip you are trying to replace it has likely not sprung back to the original non-compressed height after being compressed for decades. It took me two tries to get the RP-33 strips the correct height. For the M100 I got it the first time as I had some original strips to measure and could just get some just a hair taller.

Given manufacturing tolerances of the machines and strips what seems to work in one machine may be right on the edge and not work in others. I was able to test the RP-33s in multiple VX-4s I had and have sent one to a friend in Germany who has a larger collection of machines to try them in so when he gets it and can test it will provide more data. Same for the M100, sent some strips to friends to try out.

Getting a smaller number at first to check fitment is a good idea I think. When you get to the point of knowing what size will work and can then place a larger order you can get a much better price. Your total cost will include any trial batches which are a higher cost per unit plus the lower cost/unit of the higher quantity. When dividing the cost this is something to keep in mind.

More unsolicited advice: It would make sense I think to have one person be in charge of contacting the company for a joint project. I have done a large project with a friend in another country involving getting tooling produced to reproduce some obsolete connectors. Even with just he and I both working in tandem it could get confusing for us and the manufacturer if someone involved forgot to 'reply all'. Just FWIW.

The company is located in China and I have found them to be very reasonable to work with. If you guys get some strips made let us know how it worked out.

Company info:
http://www.cmaisilicone.com
contact@cmaisilicone.com

[Jeff_Birt]

(03-23-2022 11:54 PM)dmh Wrote:  So who has the part, tools and skill/experience to measure, specify and organise the HP 71b zebra? :-)

I would offer to but I don't have an HP 71 which is sort of important

[Jeff_Birt]

A friend in Germany confirmed that the board design with a slightly incorrect contact pad layout worked fine in all of this Casio pocket computers that accept an RP-33. Rather than pitch those PCBs I built them and will offer them at a discount. There are only 8 of them left.

I did get a few samples of the V1.1 PCB with the corrected pad layout and they work fine too. I shrunk the outline on this version by 0.5mm all around so it is not as snug a fit as the V1 boards though the V1 boards all seem to fit/work fine. I should have more V1.1 PCBs next week so I can start building them.

The 'imperfect' V1.0 PCB version: $20
The 'fixed' V1.1 PCB version: $30

Shipping via USPS should be about $6.50 for 1-several boards. I'll get it set up on my website sometime next week after getting the new PCBs in and building a few to test them. For now I can send a PayPal invoice to your email address if you are interested.

[mfleming]

I can confirm the V1 board works with the Casio PB-2000C - just remember to press Reset then NewAll after turning on the calculator. Quite heady to have all that additional space!

Very neat looking board and zebras,
~Mark

[Jeff_Birt]

(04-18-2022 09:30 PM)mfleming Wrote:  I can confirm the V1 board works with the Casio PB-2000C - just remember to press Reset then NewAll after turning on the calculator. Quite heady to have all that additional space!

Very neat looking board and zebras,
~Mark

Thanks for confirming that! They 'should' work on any machine that could use the RP-33 but you never know what might be slightly different between models.

Just got the new PCBs in last evening so I'll be busy this weekend build some up and testing them.

[Nigel (UK)]

I’ve just received the RP-33 clone, V1.1, ordered from Tindie. It appears to work properly, both in a FX-880P and as an RP-8 in a FX-730P. Brilliant!

Nigel (UK)