Greetings Programs!

A project log for HBKD-81

An open, replacement case design for a wedge style home computer

dave-collinsDave Collins 04/28/2023 at 05:150 Comments
Hello There!

With the test fit out of the way, a few weeks back I finished off the design drawings for the top and back panel for use with a rapid turn laser cut / fabrication service and I think we all agree the results are breathtaking.  I went right to work finishing up the inside of the case by adding a new DC-DC converter, a ESP32 for a ZXWESPI and a bit more board work to finish off some of the loose ends that were hanging around from the restoration and now I have a fairly satisfying end result:

I swear to everything that is holy, that is a very nice looking ZX-81 and I am fantastically pleased with it. It looks like something out of everybody's favorite 1980's film about laser bikes.  Typing on the thing hearkens back to entering basic programs into my C64 from the ring bound users guide, and I love it.

Just for fun however:

So how did we get here:

It started out as a simple Christmas gift from a friend from across the pond:

It needed some heavy restoration work and I love that sort of thing, its kind of cathartic In a way.  Initially I wanted this to be a fun side project, I was going to hand fabricate the mods using strip board and find or build a replacement case and that would be the end of it.  Then i saw iNibleSloth's totally new ZX81 build and I fell in love.

You can follow along with the early stages of that restoration in my hackaday "pages" blog.   I finally finished the restoration a couple of months later, and started work on the keyboard.  I'd hit kind of a snag at this point however, for start's the cases out there were not really what I was looking for.  iNimbleSloth's keyboard, in fact most keyboards for the ZX81, lack a space bar, offset shift keys and other nice things that I really wanted.  I also wanted a case like the KD-81 from Suntronics: 

The only trouble was, even with the best of google searches; and asking everybody's favorite language model -- you can't really find one of these things (let alone pictures of the inside.)   I decided with this kind of fabrication, I was going to need some help and I took a shot and asked my sponsor at PCBWay would they like to participate in making something fantastic, and they agreed!

The fabrication build review:

The HBKD-81 is a set of panels and PCB's for the PT-10 industrial sloped console enclosure.  They end up looking an awful lot like the inspiration, and I love them.   To get all of the fabrication done I used the sponsor of this project  PCBWay makes fantastic rapid turn PCB's for prototyping and end products alike.  Additionally, they do injection molding, sheet metal & acrylic fabrication, and 3d printing.  For this project I used PCB Fabrication services for the keyboard, 3D printing services for the test fit for the top panel, and in this part of the project  I did acrylic panels which PCBWay cut and drilled and the results are very nice:

It took roughly 8 days, that worked out to  be about a week and a half from sending the design, weekends, a small delay for engineering questions (this is standard, more about that later), and shipping.  I was never out of the loop on anything, when they did actually reach out the representative was very kind and asked insightful targeted questions and I was confident it would be handled perfectly, and it was.  The only issue was with my design files, There was a hole that was not clearly marked as a hole. The kind people handing the issue on the other end supposed it was a hole and reached out to ask if that was correct.  I answered the question and production continued; it was very easy.

The pictures make clear what words can not.  The fit is perfect.  Nothing was cracked scratched or damaged in shipping.  It came packaged along with the other panel and a small PCB I built for testing.  Everything was sublimely easy. I was as always, very impressed with the quality, speed and price.  Even if PCBWay's costs are a bit higher than the competition having something done right the first time, with no scratches, within tolerances; is worth its weight in gold if you ask me.

Sponsorship is one of the only ways I can keep up this kind of pace.  And so for that I am eternally thank full for the care and support PCBWay has shown my projects.

Internal build:

I tried very hard to get the internals right.  I really wanted it to come out as clean as possible, and I knew I wasn't going to use the expansion port, so I built a simple back panel that just covers the back:

As you can see I went with a standard barrel jack here, If i could change one thing about the build I would have gone with all rounded panel mounts.  Cutting the small rectangular openings was slightly nerve-raking! I probably spent the most time trying to make it perfect and it really took a lot of slow, deliberate work. The power port still ended up a bit wonky with a little vinyl bezel sticker to cover up my crimes from the craft knife.  The rounded hole for the video port was much simpler as I could simply drill a hole to size.

On the inside I point to point wired the power and video sections.  The modern ULA replacement outputs a clean 1v PtP 75 ohm signal.  To resolve the DC offset I installed a simple electrolytic cap and wired the video line directly.   I built a small proto-board up, with the zxwespi-iot-mi version, and finally installed a 3A bog standard DC-DC Buck converter for cool, clean power.   The only issue I ran into here was I needed to install an extra bypass cap at the output of the module to bring the ripple down even further to eliminate some high frequency interference in the video signal.

The video and tape port patches for the ZXWESPI were made directly to the top of the board.  This was made simple by the fact that all of the ports had been salvaged by a previous owner.    I did have to tone out an alternate location for the video patch, but as there are multiple via locations on the top which are gigantic by 2023 standards this was no problem at all.  Here's the best bit. due to the positioning of the ESP-WOOM module at the front of the case, when the module accesses the network or loads a file it looks like this:

Complications, solutions, and conclusion (how can I get one):

I did at the very end run into some issues with some of the sockets needing a re-flow from the backside.   As I was working on principle photography for this very write-up I ran into RAM type Video issues.  looking at the back of the board I discovered more cracked traces under the ram chip.  The biggest issue is the board's finishing is de-laminating on the bottom side, and is aggressively salt water corroded on the top.   This lends itself to its own unique set of problems but as I am writing this I have been running hi-res invaders demo loop and it has yet to crash, or have any issues.

This about wraps up this project.  I am very pleased with the results but the more I prod at this old board the more I grow concerned I may damage something.  And so, I think it best to move on and enjoy it for what it is :).  I will definitely be re-visiting the PT-10.  I still want to build a off the shelf  retro terminal (ESP32 or Pi-PICO based.  I haven't put much thought into what it might be.  For now I will be re-focusing on the next generation of HB6809 upgrades and looking to that.

As far as getting your hands on your very own HBKD-81 you can certainly try to build your own.  Over the next few days I will be releasing all of the source material I used.  I don't think I will be making them for re-sale at this point.   The main reason being, even though its a low cost build, its still firefly spendy.   I'd say probably in for about $150-$200 building one of your own.  I don't like the idea of asking over $100 for something I designed with no engineering experience.    Additionally, I probably over spent on some of the parts to get them quickly so that  I could meet personally set deadlines for projects.  Obviously there's no such thing as no, for crazy money, you are always welcome to reach out and we can discuss it but this was a lot of work, and I work another full time job.  I don't know if I have the time or energy to build another one, and still work on the other things that make me happy.  I do however have a few extra keyboard PCB's i don't need extras of.  If anybody who's serious wants one I'll throw it in a USPS box for just the cost of shipping.

Thanks for sticking around to the end on this one, its been an amazing trip start to finish. Please leave a note or a comment if you have any questions.   I'll post an update when the project files are all uploaded, I really hope others can have the joy of completing one of these things building your own keyboard case is a huge thrill.